Ring pattern in images with Newtonian telescope

1.51 R8RO #... · view views
Padraig Farrell: Hi R8RO, I feel your frustration, I had to go through similar processes using asi 294 Mc pro. I noticed your using a flat panel directly on top of the OTA. I stopped doing this and face the OTA at a white wall and allow my sketch pad to illuminate the wall. In my situation, a ring pattern were coming from the flats. With the panel at full power, it wasn’t noticeable, but at the lowest level it caught my eye when I looked at it from an angle. Don’t know if this any benefit in your situation …but from what I’ve been reading so far , I think your not going to let this bet you. I think you going to crack this problem. Padraig ( my flats: 50% histogram, 4 second + ) Thanks for your input. I have tried several different methods of flats. LED panel, Sky flats, Pointed to a white surface, Using my projector and projector screen, all give the same results so I just use the one that is most convenient, led panel. I may try to cut the focusing tube down a bit and see if that could be a problem (I doubt it). Like

1.51

#...

Padraig Farrell:
Hi R8RO,

I feel your frustration,
I had to go through similar processes using asi 294 Mc pro.

I noticed your using a flat panel directly on top of the OTA.
I stopped doing this and face the OTA at a white wall and allow my sketch pad to illuminate the wall.

In my situation, a ring pattern were coming from the flats. With the panel at full power, it wasn’t noticeable, but at the lowest level it caught my eye when I looked at it from an angle.

Don’t know if this any benefit in your situation …but from what I’ve been reading so far , I think your not going to let this bet you.
I think you going to crack this problem.

Padraig

( my flats: 50% histogram, 4 second + )

Thanks for your input. I have tried several different methods of flats. LED panel, Sky flats, Pointed to a white surface, Using my projector and projector screen, all give the same results so I just use the one that is most convenient, led panel. I may try to cut the focusing tube down a bit and see if that could be a problem (I doubt it).

0.00 Tim Ellison #... · view views · 1 like
Hi, Sorry to read that you haven't yet got to the root of the problem. It looks like it is a tough one to crack! I thought I would chip in again if I may, as a follow up to my comments a few days ago. I think I might be able to add a bit to your thinking. Firstly an apology. I hadn't previously read through the conversation sufficiently thoroughly and hadn't spotted the earlier mentions of the possibility of things tilting or shifting. I had thought I was the first to mention it, but I wasn't. Apologies for missing that! My telescope is similar to yours - Sky-Watcher 200P rather than your 200PDS. As I mentioned in my earlier post I had a similar problem, which took me a long time to figure out. In fact I haven't really figured it out I don't think, but seem to have fixed it "by accident". What I reckon happens with my kit is as follows. The flat frames contain a very subtle circular structure which is at or near the centre of the frame. This isn't visible when looking at the image on screen, but it is possible to devise processing techniques to amplify it and make it visible. Having done this I know the ring structure is there. And having thought about it quite a bit, I don't believe that this ring structure is a problem as such, but is something which SHOULD be present. I think it is probably related to the presence of the secondary mirror structure in the centre of the telescope tube. I believe - but haven't been able to prove - that a similar ring structure is present in the light frames. Because it is very subtle, and there is always a lot of other stuff going on in the light frames, I haven't been able to find a way to make it visible, but I think it must be there. When things are working correctly, I believe that the ring structures in all the frames will be precisely aligned, and during calibration those in the lights and those in the flats will cancel each other out and as a result the effect will disappear. However if anything moves between the taking of the lights and the flats, the rings will appear in slightly different positions in the two sets and will no longer align, and so will no longer cancel out during calibration. In this case with my images they gave rise to an effect which looked quite similar to what you are getting. Based on the above, I believe that "something shifting" is a strong candidate for being the cause of your problem. In my case the camera was able to rock slightly in the focuser and it appears it was this which was causing the problem. The rocking was only very slight - but it was enough. When I beefed up the camera support arrangements - which I did with the aim of getting rid of a differential flexure issue - the problem went away completely. Others have suggested that movement of either the primary or secondary mirror are possible alternative causes. I haven't seen either of those causing a problem with my system, but if either mirror is able to shift, then I imagine that might produce a similar effect, for similar reasons. You say you have made sure that your primary isn't moving, but that perhaps there might be slight movement of the secondary. Based on my own experience I think that rocking of the camera as you rotate to the flat-shooting position is the most likely cause, and ought to be a primary candidate for investigation. I would recommend looking quite hard at that. I hope the above is helpful. Good luck! Tim Like

0.00

Tim Ellison

#...

· 1 like

Hi,

Sorry to read that you haven't yet got to the root of the problem. It looks like it is a tough one to crack!

I thought I would chip in again if I may, as a follow up to my comments a few days ago. I think I might be able to add a bit to your thinking.

Firstly an apology. I hadn't previously read through the conversation sufficiently thoroughly and hadn't spotted the earlier mentions of the possibility of things tilting or shifting. I had thought I was the first to mention it, but I wasn't. Apologies for missing that!

My telescope is similar to yours - Sky-Watcher 200P rather than your 200PDS. As I mentioned in my earlier post I had a similar problem, which took me a long time to figure out. In fact I haven't really figured it out I don't think, but seem to have fixed it "by accident".

What I reckon happens with my kit is as follows. The flat frames contain a very subtle circular structure which is at or near the centre of the frame. This isn't visible when looking at the image on screen, but it is possible to devise processing techniques to amplify it and make it visible. Having done this I know the ring structure is there. And having thought about it quite a bit, I don't believe that this ring structure is a problem as such, but is something which SHOULD be present. I think it is probably related to the presence of the secondary mirror structure in the centre of the telescope tube. I believe - but haven't been able to prove - that a similar ring structure is present in the light frames. Because it is very subtle, and there is always a lot of other stuff going on in the light frames, I haven't been able to find a way to make it visible, but I think it must be there. When things are working correctly, I believe that the ring structures in all the frames will be precisely aligned, and during calibration those in the lights and those in the flats will cancel each other out and as a result the effect will disappear. However if anything moves between the taking of the lights and the flats, the rings will appear in slightly different positions in the two sets and will no longer align, and so will no longer cancel out during calibration. In this case with my images they gave rise to an effect which looked quite similar to what you are getting.

Based on the above, I believe that "something shifting" is a strong candidate for being the cause of your problem. In my case the camera was able to rock slightly in the focuser and it appears it was this which was causing the problem. The rocking was only very slight - but it was enough. When I beefed up the camera support arrangements - which I did with the aim of getting rid of a differential flexure issue - the problem went away completely. Others have suggested that movement of either the primary or secondary mirror are possible alternative causes. I haven't seen either of those causing a problem with my system, but if either mirror is able to shift, then I imagine that might produce a similar effect, for similar reasons. You say you have made sure that your primary isn't moving, but that perhaps there might be slight movement of the secondary. Based on my own experience I think that rocking of the camera as you rotate to the flat-shooting position is the most likely cause, and ought to be a primary candidate for investigation. I would recommend looking quite hard at that.

I hope the above is helpful.

Good luck!

Tim

1.51 R8RO #... · view views
Tim Ellison: Hi, Sorry to read that you haven't yet got to the root of the problem. It looks like it is a tough one to crack! I thought I would chip in again if I may, as a follow up to my comments a few days ago. I think I might be able to add a bit to your thinking. Firstly an apology. I hadn't previously read through the conversation sufficiently thoroughly and hadn't spotted the earlier mentions of the possibility of things tilting or shifting. I had thought I was the first to mention it, but I wasn't. Apologies for missing that! My telescope is similar to yours - Sky-Watcher 200P rather than your 200PDS. As I mentioned in my earlier post I had a similar problem, which took me a long time to figure out. In fact I haven't really figured it out I don't think, but seem to have fixed it "by accident". What I reckon happens with my kit is as follows. The flat frames contain a very subtle circular structure which is at or near the centre of the frame. This isn't visible when looking at the image on screen, but it is possible to devise processing techniques to amplify it and make it visible. Having done this I know the ring structure is there. And having thought about it quite a bit, I don't believe that this ring structure is a problem as such, but is something which SHOULD be present. I think it is probably related to the presence of the secondary mirror structure in the centre of the telescope tube. I believe - but haven't been able to prove - that a similar ring structure is present in the light frames. Because it is very subtle, and there is always a lot of other stuff going on in the light frames, I haven't been able to find a way to make it visible, but I think it must be there. When things are working correctly, I believe that the ring structures in all the frames will be precisely aligned, and during calibration those in the lights and those in the flats will cancel each other out and as a result the effect will disappear. However if anything moves between the taking of the lights and the flats, the rings will appear in slightly different positions in the two sets and will no longer align, and so will no longer cancel out during calibration. In this case with my images they gave rise to an effect which looked quite similar to what you are getting. Based on the above, I believe that "something shifting" is a strong candidate for being the cause of your problem. In my case the camera was able to rock slightly in the focuser and it appears it was this which was causing the problem. The rocking was only very slight - but it was enough. When I beefed up the camera support arrangements - which I did with the aim of getting rid of a differential flexure issue - the problem went away completely. Others have suggested that movement of either the primary or secondary mirror are possible alternative causes. I haven't seen either of those causing a problem with my system, but if either mirror is able to shift, then I imagine that might produce a similar effect, for similar reasons. You say you have made sure that your primary isn't moving, but that perhaps there might be slight movement of the secondary. Based on my own experience I think that rocking of the camera as you rotate to the flat-shooting position is the most likely cause, and ought to be a primary candidate for investigation. I would recommend looking quite hard at that. I hope the above is helpful. Good luck! Tim Thanks for your input Tim! I have also come to the conclusion that this might just be something to live with and can be contributed to the nature of Newtonians. What worries me though is the fact that the circle is so visible in my lights. You can definitely tell that there is a ring in the lights and this doesn't show up on other peoples images that I have compared with who have the same scope. One thought to this would be the nature of the TSGPU which pushes focus out by 20mm that may affect the dimensions of the ring (But then again the ring is also visible using my Baader MPCC MkIII). Another thing that threw me off is that the night before my latest successful attempt I did everything the same (took flats right before the sequence at the same direction of the target) and the ring was still there. So apparently it is not consistent. I feel like there is definitely slack or room for movement in the focuser tube, especially now that my camera sits pretty far out. Do you have any resources for stiffening this up? Getting real tired of this issue and I feel like I am running my head against a wall. At least I have narrowed it down to the tube as I get the same results regardless of what camera/CC combination I use. Any and all input is always welcome! P.S Don't worry about not reading through, I probably wouldn't either with all the responses that I've gotten. Edited ... Like

1.51

R8RO

#...

Tim Ellison:
Hi,

Sorry to read that you haven't yet got to the root of the problem. It looks like it is a tough one to crack!

I thought I would chip in again if I may, as a follow up to my comments a few days ago. I think I might be able to add a bit to your thinking.

Firstly an apology. I hadn't previously read through the conversation sufficiently thoroughly and hadn't spotted the earlier mentions of the possibility of things tilting or shifting. I had thought I was the first to mention it, but I wasn't. Apologies for missing that!

My telescope is similar to yours - Sky-Watcher 200P rather than your 200PDS. As I mentioned in my earlier post I had a similar problem, which took me a long time to figure out. In fact I haven't really figured it out I don't think, but seem to have fixed it "by accident".

What I reckon happens with my kit is as follows. The flat frames contain a very subtle circular structure which is at or near the centre of the frame. This isn't visible when looking at the image on screen, but it is possible to devise processing techniques to amplify it and make it visible. Having done this I know the ring structure is there. And having thought about it quite a bit, I don't believe that this ring structure is a problem as such, but is something which SHOULD be present. I think it is probably related to the presence of the secondary mirror structure in the centre of the telescope tube. I believe - but haven't been able to prove - that a similar ring structure is present in the light frames. Because it is very subtle, and there is always a lot of other stuff going on in the light frames, I haven't been able to find a way to make it visible, but I think it must be there. When things are working correctly, I believe that the ring structures in all the frames will be precisely aligned, and during calibration those in the lights and those in the flats will cancel each other out and as a result the effect will disappear. However if anything moves between the taking of the lights and the flats, the rings will appear in slightly different positions in the two sets and will no longer align, and so will no longer cancel out during calibration. In this case with my images they gave rise to an effect which looked quite similar to what you are getting.

Based on the above, I believe that "something shifting" is a strong candidate for being the cause of your problem. In my case the camera was able to rock slightly in the focuser and it appears it was this which was causing the problem. The rocking was only very slight - but it was enough. When I beefed up the camera support arrangements - which I did with the aim of getting rid of a differential flexure issue - the problem went away completely. Others have suggested that movement of either the primary or secondary mirror are possible alternative causes. I haven't seen either of those causing a problem with my system, but if either mirror is able to shift, then I imagine that might produce a similar effect, for similar reasons. You say you have made sure that your primary isn't moving, but that perhaps there might be slight movement of the secondary. Based on my own experience I think that rocking of the camera as you rotate to the flat-shooting position is the most likely cause, and ought to be a primary candidate for investigation. I would recommend looking quite hard at that.

I hope the above is helpful.

Good luck!

Tim

Thanks for your input Tim!

I have also come to the conclusion that this might just be something to live with and can be contributed to the nature of Newtonians. What worries me though is the fact that the circle is so visible in my lights. You can definitely tell that there is a ring in the lights and this doesn't show up on other peoples images that I have compared with who have the same scope.

One thought to this would be the nature of the TSGPU which pushes focus out by 20mm that may affect the dimensions of the ring (But then again the ring is also visible using my Baader MPCC MkIII).

Another thing that threw me off is that the night before my latest successful attempt I did everything the same (took flats right before the sequence at the same direction of the target) and the ring was still there. So apparently it is not consistent.

I feel like there is definitely slack or room for movement in the focuser tube, especially now that my camera sits pretty far out. Do you have any resources for stiffening this up?

Getting real tired of this issue and I feel like I am running my head against a wall. At least I have narrowed it down to the tube as I get the same results regardless of what camera/CC combination I use.

Any and all input is always welcome!

P.S Don't worry about not reading through, I probably wouldn't either with all the responses that I've gotten.

Edited ...

5.26 Lorenzo Siciliano #... · view views · 4 likes
Ok, I was listening to this thread until now and now I want to give my contribution. I have the same, exact, identical issue. I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame. I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack... I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed... But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube. My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot. Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider. Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!! Here is the before and the after. Ring pattern before and after The image is only STF stretched in PI. I used this spider: #TSSPI284 from Teleskop-Service. Hope this helps. Clear skies. Lorenzo Edited ... Like

5.26

Lorenzo Siciliano

#...

· 4 likes

Ok, I was listening to this thread until now and now I want to give my contribution.
I have the same, exact, identical issue.
I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame.
I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack...
I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed...
But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube.
My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot.
Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider.
Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!!
Here is the before and the after.

Ring pattern before and after

The image is only STF stretched in PI.
I used this spider: #TSSPI284 from Teleskop-Service.

Hope this helps.
Clear skies.
Lorenzo

Edited ...

1.51 R8RO #... · view views
Lorenzo Siciliano: Ok, I was listening to this thread until now and now I want to give my contribution. I have the same, exact, identical issue. I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame. I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack... I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed... But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube. My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot. Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider. Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!! Here is the before and the after. Ring pattern before and after The image is only STF stretched in PI. I used this spider: #TSSPI284 from Teleskop-Service. Hope this helps. Clear skies. Lorenzo Finally! Some concrete evidence of something that actually seemed to have worked! Is the ring still present in your flats/lights? How do you take your flats? When did you do this and has it continued working? I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it! THANK YOU Lorenzo! Like

1.51

R8RO

#...

Lorenzo Siciliano:
Ok, I was listening to this thread until now and now I want to give my contribution.
I have the same, exact, identical issue.
I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame.
I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack...
I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed...
But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube.
My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot.
Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider.
Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!!
Here is the before and the after.

Ring pattern before and after

The image is only STF stretched in PI.
I used this spider: #TSSPI284 from Teleskop-Service.

Hope this helps.
Clear skies.
Lorenzo

Finally! Some concrete evidence of something that actually seemed to have worked! Is the ring still present in your flats/lights? How do you take your flats? When did you do this and has it continued working?

I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it!

THANK YOU Lorenzo!

5.26 Lorenzo Siciliano #... · view views · 3 likes
Lorenzo Siciliano: Ok, I was listening to this thread until now and now I want to give my contribution. I have the same, exact, identical issue. I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame. I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack... I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed... But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube. My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot. Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider. Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!! Here is the before and the after. Ring pattern before and after The image is only STF stretched in PI. I used this spider: #TSSPI284 from Teleskop-Service. Hope this helps. Clear skies. Lorenzo Finally! Some concrete evidence of something that actually seemed to have worked! Is the ring still present in your flats/lights? How do you take your flats? When did you do this and has it continued working? I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it! THANK YOU Lorenzo! The ring is simply the effect of the shadow of the secondary mirror. In other terms, you (and me, and others) are imaging the entrance pupil of the telescope. It appears like a ring because something in the telescope is gradually and slowly shifting during the shooting session, while when you shoot your flats the position of the telescope is substantially fixed and nothing shifts anymore. When you stack your calibrated frames, the position of the shadow of the secondary mirror is shifted from one frame to another, and the net result is the "crater". So, yes, in my flats the shadow in the center of the picture is present, and so in my lights. But if nothing moves, it calibrates out easily. Until now the spider substitution has worked very well and in a very repeatable way. I took a lot of before and after shots and the difference is very apparent. But, as you know, your mileage may vary. It worked for me, but I'm very confident that this is the solution. Ciao. Lorenzo Edited ... Like

5.26

Lorenzo Siciliano

#...

· 3 likes

Lorenzo Siciliano:
Ok, I was listening to this thread until now and now I want to give my contribution.
I have the same, exact, identical issue.
I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame.
I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack...
I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed...
But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube.
My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot.
Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider.
Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!!
Here is the before and the after.

Ring pattern before and after

The image is only STF stretched in PI.
I used this spider: #TSSPI284 from Teleskop-Service.

Hope this helps.
Clear skies.
Lorenzo

Finally! Some concrete evidence of something that actually seemed to have worked! Is the ring still present in your flats/lights? How do you take your flats? When did you do this and has it continued working?

I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it!

THANK YOU Lorenzo!

The ring is simply the effect of the shadow of the secondary mirror. In other terms, you (and me, and others) are imaging the entrance pupil of the telescope. It appears like a ring because something in the telescope is gradually and slowly shifting during the shooting session, while when you shoot your flats the position of the telescope is substantially fixed and nothing shifts anymore.
When you stack your calibrated frames, the position of the shadow of the secondary mirror is shifted from one frame to another, and the net result is the "crater".
So, yes, in my flats the shadow in the center of the picture is present, and so in my lights. But if nothing moves, it calibrates out easily.
Until now the spider substitution has worked very well and in a very repeatable way. I took a lot of before and after shots and the difference is very apparent. But, as you know, your mileage may vary. It worked for me, but I'm very confident that this is the solution.
Ciao.
Lorenzo

Edited ...

7.46 andrea tasselli #... · view views
Lorenzo Siciliano: I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it! Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight? Like

7.46

andrea tasselli

#...

Lorenzo Siciliano:
I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it!

Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight?

5.26 Lorenzo Siciliano #... · view views
andrea tasselli: Lorenzo Siciliano: I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it! Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight? Yes, it's worth a try. But I have seen the spider of a Quattro 200... the vanes are insanely thin... They are so thin that you can bend them with your breath... And yes, I'll double checking everything in order to find what's moving in the OTA. In my case, the only thing that worked was the spider substitution. Ciao. Lorenzo Like

5.26

Lorenzo Siciliano

#...

andrea tasselli:
Lorenzo Siciliano:
I've been eyeing up the https://www.teleskop-express.de/shop/product_info.php/info/p11914_Artesky-stabile-Fangspiegelspinne-fuer-Skywatcher-200-mm-Newton-Teleskope.html looking for a reason to try it out, maybe this is it!

Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight?

Yes, it's worth a try. But I have seen the spider of a Quattro 200... the vanes are insanely thin... They are so thin that you can bend them with your breath...
And yes, I'll double checking everything in order to find what's moving in the OTA. In my case, the only thing that worked was the spider substitution.
Ciao.
Lorenzo

1.51 R8RO #... · view views
andrea tasselli: Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight? Lorenzo Siciliano: Yes, it's worth a try. But I have seen the spider of a Quattro 200... the vanes are insanely thin... They are so thin that you can bend them with your breath... And yes, I'll double checking everything in order to find what's moving in the OTA. In my case, the only thing that worked was the spider substitution. Ciao. Lorenzo I tried Lorenzo's methodology by putting my laser collimator into the focuser and shaking things about one part at a time to se what moved the most. The primary is rock solid and the focuser require a decent amount of pressure to budge. The spider however was rather loose. I tightened all the screws and squared up the spider vanes but I think this is just a case of a too flimsy spider and bad quality control by Skywatcher as it was still somewhat loose. I will try to do a sequence before I pull the trigger on a more sturdy spider to see if the tweaks improved anything. Lorenzo Siciliano: The ring is simply the effect of the shadow of the secondary mirror. What I find odd here is that the people who I've compared to don't have this ring in their lights/flats. If it is just the nature of a Newtonian wouldn't it be visible in everyone's lights and flats? Like

1.51

R8RO

#...

andrea tasselli:
Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight?

Lorenzo Siciliano:
Yes, it's worth a try. But I have seen the spider of a Quattro 200... the vanes are insanely thin... They are so thin that you can bend them with your breath...
And yes, I'll double checking everything in order to find what's moving in the OTA. In my case, the only thing that worked was the spider substitution.
Ciao.
Lorenzo

I tried Lorenzo's methodology by putting my laser collimator into the focuser and shaking things about one part at a time to se what moved the most. The primary is rock solid and the focuser require a decent amount of pressure to budge. The spider however was rather loose.

I tightened all the screws and squared up the spider vanes but I think this is just a case of a too flimsy spider and bad quality control by Skywatcher as it was still somewhat loose. I will try to do a sequence before I pull the trigger on a more sturdy spider to see if the tweaks improved anything.

Lorenzo Siciliano:
The ring is simply the effect of the shadow of the secondary mirror.

What I find odd here is that the people who I've compared to don't have this ring in their lights/flats. If it is just the nature of a Newtonian wouldn't it be visible in everyone's lights and flats?

7.75 kuechlew #... · view views
andrea tasselli: Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight? Lorenzo Siciliano: Yes, it's worth a try. But I have seen the spider of a Quattro 200... the vanes are insanely thin... They are so thin that you can bend them with your breath... And yes, I'll double checking everything in order to find what's moving in the OTA. In my case, the only thing that worked was the spider substitution. Ciao. Lorenzo I tried Lorenzo's methodology by putting my laser collimator into the focuser and shaking things about one part at a time to se what moved the most. The primary is rock solid and the focuser require a decent amount of pressure to budge. The spider however was rather loose. I tightened all the screws and squared up the spider vanes but I think this is just a case of a too flimsy spider and bad quality control by Skywatcher as it was still somewhat loose. I will try to do a sequence before I pull the trigger on a more sturdy spider to see if the tweaks improved anything. Lorenzo Siciliano: The ring is simply the effect of the shadow of the secondary mirror. What I find odd here is that the people who I've compared to don't have this ring in their lights/flats. If it is just the nature of a Newtonian wouldn't it be visible in everyone's lights and flats? This may just be a matter of sample variation. Maybe the spider is ok for 90+x% of the users but out of range for a few samples only? I'm glad you found the issue, this thread turned into a real thriller. I came close to suggest to produce a hollywood blockbuster movie out of it. And the result even fits to the geometry of the issue - a perfect circle slightly offset from the centre. Sounds like the description of a secondary mirror ... Clear skies Wolfgang Like

7.75

kuechlew

#...

andrea tasselli:
Wouldn't it be the case that you'd just need to tighten up the spider itself? I mean, the thing costs a good fraction of the entire telescope! And are you sure that the mirror doesn't flop because of the screws aren't tight?

Lorenzo Siciliano:
Yes, it's worth a try. But I have seen the spider of a Quattro 200... the vanes are insanely thin... They are so thin that you can bend them with your breath...
And yes, I'll double checking everything in order to find what's moving in the OTA. In my case, the only thing that worked was the spider substitution.
Ciao.
Lorenzo

I tried Lorenzo's methodology by putting my laser collimator into the focuser and shaking things about one part at a time to se what moved the most. The primary is rock solid and the focuser require a decent amount of pressure to budge. The spider however was rather loose.

I tightened all the screws and squared up the spider vanes but I think this is just a case of a too flimsy spider and bad quality control by Skywatcher as it was still somewhat loose. I will try to do a sequence before I pull the trigger on a more sturdy spider to see if the tweaks improved anything.

Lorenzo Siciliano:
The ring is simply the effect of the shadow of the secondary mirror.

What I find odd here is that the people who I've compared to don't have this ring in their lights/flats. If it is just the nature of a Newtonian wouldn't it be visible in everyone's lights and flats?

This may just be a matter of sample variation. Maybe the spider is ok for 90+x% of the users but out of range for a few samples only?

I'm glad you found the issue, this thread turned into a real thriller. I came close to suggest to produce a hollywood blockbuster movie out of it.
And the result even fits to the geometry of the issue - a perfect circle slightly offset from the centre. Sounds like the description of a secondary mirror ...

Clear skies
Wolfgang

1.51 R8RO #... · view views
I came close to suggest to produce a hollywood blockbuster movie out of it. Thanks Wolfgang! This has been my life for the past few months. Hunting down an invisible issue with what seemed like no end in sight. Really hope this is the solution I've been looking for! Like

11.01 Andy Wray #... · view views · 1 like
I would also just like to suggest making sure that the 3 adjustment screws on the secondary are all tightened down once you have collimated. Like

5.26 Lorenzo Siciliano #... · view views
And the result even fits to the geometry of the issue - a perfect circle slightly offset from the centre. Sounds like the description of a secondary mirror ... Clear skies Wolfgang Based on my investigations, I'm pretty sure that what we are seeing is the secondary mirror signature. Why do others newtonian owners not see that shadow? I don't know. Maybe is there, but in some cases is too weak to be seen... This for sure needs further investigation... What I've found could be the solution of the issue? Maybe yes. What I can say is that, until now, this worked for me. Ciao. Lorenzo Like

5.26

Lorenzo Siciliano

#...

And the result even fits to the geometry of the issue - a perfect circle slightly offset from the centre. Sounds like the description of a secondary mirror ...

Clear skies
Wolfgang

Based on my investigations, I'm pretty sure that what we are seeing is the secondary mirror signature.
Why do others newtonian owners not see that shadow?
I don't know. Maybe is there, but in some cases is too weak to be seen... This for sure needs further investigation...
What I've found could be the solution of the issue?
Maybe yes. What I can say is that, until now, this worked for me.
Ciao.
Lorenzo

7.46 andrea tasselli #... · view views · 1 like
Lorenzo Siciliano: Why do others newtonian owners not see that shadow? I don't know. Maybe is there, but in some cases is too weak to be seen... This for sure needs further investigation... I own 3 newtons and never had such an issue. Yes, it was long time ago for 2 of them but the latest 6" f/4 does not show any sign of it and this is just a standard TS Photon used straight away from the box. So, maybe, as you say, it's a feature of a poorly designed secondary spider. Like

7.46

andrea tasselli

#...

· 1 like

Lorenzo Siciliano:
Why do others newtonian owners not see that shadow?
I don't know. Maybe is there, but in some cases is too weak to be seen... This for sure needs further investigation...

I own 3 newtons and never had such an issue. Yes, it was long time ago for 2 of them but the latest 6" f/4 does not show any sign of it and this is just a standard TS Photon used straight away from the box. So, maybe, as you say, it's a feature of a poorly designed secondary spider.

9.90 Bogdan Borz #... · view views
Lorenzo Siciliano: Ok, I was listening to this thread until now and now I want to give my contribution. I have the same, exact, identical issue. I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame. I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack... I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed... But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube. My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot. Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider. Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!! Here is the before and the after. Ring pattern before and after The image is only STF stretched in PI. I used this spider: #TSSPI284 from Teleskop-Service. Hope this helps. Clear skies. Lorenzo Hi Lorenzo, Thanks for this information! I've been having the same issue and the same ring. I have thought of reflections from the border of the secondary, but as I've read about people who have painted their secondary around to no effect I kind of eliminated it from the checklist. This is clearly reflected light. Since it disappeared once you changed the secondary holder it has to be related to the secondary. But if the secondary moves or shifts during the night, this should lead to miscollimation and I haven't observed this on my subs. I saw an improvement with the change from the GPU corrector to the Maxfield corrector from TS (4 elements and long CC vs. 3 elements and short CC inside the tube). Second, the ring is absent when shooting narrowband (very very faint with the O3). Third the ring is absent when shooting under dark skies. I only observed it under my light polluted Bortle 8. So it is stray light and maybe less reflected from the shorter, 3 element CC. Anyway, it is not an issue under medium polluted or dark skies. Probably this explains why lots of imagers don't see it. The guys from TS did not observe it (and it was someone shooting under Bortle 5). Maybe the secondary shifts around its axis slightly (the optical center stays almost constant) and this generates the stray light while keeping collimation. Thanks again for the info. CS! Bogdan Like

9.90

Bogdan Borz

#...

Lorenzo Siciliano:
Ok, I was listening to this thread until now and now I want to give my contribution.
I have the same, exact, identical issue.
I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame.
I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack...
I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed...
But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube.
My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot.
Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider.
Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!!
Here is the before and the after.

Ring pattern before and after

The image is only STF stretched in PI.
I used this spider: #TSSPI284 from Teleskop-Service.

Hope this helps.
Clear skies.
Lorenzo

Hi Lorenzo,

Thanks for this information! I've been having the same issue and the same ring. I have thought of reflections from the border of the secondary, but as I've read about people who have painted their secondary around to no effect I kind of eliminated it from the checklist.

This is clearly reflected light. Since it disappeared once you changed the secondary holder it has to be related to the secondary. But if the secondary moves or shifts during the night, this should lead to miscollimation and I haven't observed this on my subs.

I saw an improvement with the change from the GPU corrector to the Maxfield corrector from TS (4 elements and long CC vs. 3 elements and short CC inside the tube). Second, the ring is absent when shooting narrowband (very very faint with the O3). Third the ring is absent when shooting under dark skies. I only observed it under my light polluted Bortle 8. So it is stray light and maybe less reflected from the shorter, 3 element CC.

Anyway, it is not an issue under medium polluted or dark skies. Probably this explains why lots of imagers don't see it. The guys from TS did not observe it (and it was someone shooting under Bortle 5). Maybe the secondary shifts around its axis slightly (the optical center stays almost constant) and this generates the stray light while keeping collimation.

Thanks again for the info.

CS!

Bogdan

7.75 kuechlew #... · view views
Bogdan Borz: Lorenzo Siciliano: Ok, I was listening to this thread until now and now I want to give my contribution. I have the same, exact, identical issue. I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame. I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack... I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed... But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube. My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot. Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider. Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!! Here is the before and the after. Ring pattern before and after The image is only STF stretched in PI. I used this spider: #TSSPI284 from Teleskop-Service. Hope this helps. Clear skies. Lorenzo Hi Lorenzo, Thanks for this information! I've been having the same issue and the same ring. I have thought of reflections from the border of the secondary, but as I've read about people who have painted their secondary around to no effect I kind of eliminated it from the checklist. This is clearly reflected light. Since it disappeared once you changed the secondary holder it has to be related to the secondary. But if the secondary moves or shifts during the night, this should lead to miscollimation and I haven't observed this on my subs. I saw an improvement with the change from the GPU corrector to the Maxfield corrector from TS (4 elements and long CC vs. 3 elements and short CC inside the tube). Second, the ring is absent when shooting narrowband (very very faint with the O3). Third the ring is absent when shooting under dark skies. I only observed it under my light polluted Bortle 8. So it is stray light and maybe less reflected from the shorter, 3 element CC. Anyway, it is not an issue under medium polluted or dark skies. Probably this explains why lots of imagers don't see it. The guys from TS did not observe it (and it was someone shooting under Bortle 5). Maybe the secondary shifts around its axis slightly (the optical center stays almost constant) and this generates the stray light while keeping collimation. Thanks again for the info. CS! Bogdan Interesting, if LP plays a role the issue should disappear or at least get tamed down a lot when shooting narrowband. Anyone to try out if this is the case? Alternatively someone who doesn't have the issue could expose him/herself to a Bortle 7+ zone, but that's a bit far-fetched ... Clear skies Wolfgang Like

7.75

kuechlew

#...

Bogdan Borz:
Lorenzo Siciliano:
Ok, I was listening to this thread until now and now I want to give my contribution.
I have the same, exact, identical issue.
I own a skywatcher Quattro 250, and I ran in the same issue, the "crater" at the centre of the frame.
I tried,as you @R8RO , everything: I flocked all the flockable parts of the optical tube, of the coma corrector (a gpu), of the mirror cell. I took flats in every possible way and position: sky flat, flat box, led tracing pads, t-shirt, white and grey walls, cloudy sky, but no avail... that crater was present in every stack...
I dismantled several times the whole optical assembly in search of something, but everything was ok, or it seemed...
But in all those attempts I noticed that, when I tried to collimate the optics with a laser, something was moving in the tube.
My first thinking was to the primary mirror: it's heavy, bulky, prone to moving around, but it was well seated in its cell. Next the focuser, but I had to apply a heavy force to replicate the movement of the laser dot.
Well, and if... secondary mirror? So I decided to pull the trigger and I purchased a new and more robust spider.
Well, believe it or not, despite the residual gradients, the problem is almost gone away!!!!
Here is the before and the after.

Ring pattern before and after

The image is only STF stretched in PI.
I used this spider: #TSSPI284 from Teleskop-Service.

Hope this helps.
Clear skies.
Lorenzo

Hi Lorenzo,

Thanks for this information! I've been having the same issue and the same ring. I have thought of reflections from the border of the secondary, but as I've read about people who have painted their secondary around to no effect I kind of eliminated it from the checklist.

This is clearly reflected light. Since it disappeared once you changed the secondary holder it has to be related to the secondary. But if the secondary moves or shifts during the night, this should lead to miscollimation and I haven't observed this on my subs.

I saw an improvement with the change from the GPU corrector to the Maxfield corrector from TS (4 elements and long CC vs. 3 elements and short CC inside the tube). Second, the ring is absent when shooting narrowband (very very faint with the O3). Third the ring is absent when shooting under dark skies. I only observed it under my light polluted Bortle 8. So it is stray light and maybe less reflected from the shorter, 3 element CC.

Anyway, it is not an issue under medium polluted or dark skies. Probably this explains why lots of imagers don't see it. The guys from TS did not observe it (and it was someone shooting under Bortle 5). Maybe the secondary shifts around its axis slightly (the optical center stays almost constant) and this generates the stray light while keeping collimation.

Thanks again for the info.

CS!

Bogdan

Interesting, if LP plays a role the issue should disappear or at least get tamed down a lot when shooting narrowband. Anyone to try out if this is the case?
Alternatively someone who doesn't have the issue could expose him/herself to a Bortle 7+ zone, but that's a bit far-fetched ...

Clear skies
Wolfgang

1.51 R8RO #... · view views · 1 like
LP plays a role the issue I have found this to be the case. I live in a bortle 7/8 city with my observatory on my balcony with plenty of local light pollution. When shooting narrowband the issue is almost non existent, some ringing in Oiii but that is the closest to broadband of the narrowband filters. So yes, Light pollution definitely plays a roll. Was hoping to do some test tonight but the weather had different plans. Will report back as soon as I have results from my spider teak up. Like

1.51

R8RO

#...

· 1 like

LP plays a role the issue

I have found this to be the case. I live in a bortle 7/8 city with my observatory on my balcony with plenty of local light pollution. When shooting narrowband the issue is almost non existent, some ringing in Oiii but that is the closest to broadband of the narrowband filters. So yes, Light pollution definitely plays a roll. Was hoping to do some test tonight but the weather had different plans. Will report back as soon as I have results from my spider teak up.

7.75 kuechlew #... · view views
LP plays a role the issue I have found this to be the case. I live in a bortle 7/8 city with my observatory on my balcony with plenty of local light pollution. When shooting narrowband the issue is almost non existent, some ringing in Oiii but that is the closest to broadband of the narrowband filters. So yes, Light pollution definitely plays a roll. Was hoping to do some test tonight but the weather had different plans. Will report back as soon as I have results from my spider teak up. I'm not using Newts - and with any thread like this my Newt-phobia increases day by day - but in my bortle 5 zone I have very good experiences with an Astronomik UHC-E light pollution filter. Far from cheap but worth every penny. No idea how well it will work in bortle 7+ but if you're desperate about the issue it may be worth a try. I keep my fingers crossed for you that tightening up the spider will fix the issue. Clear skies Wolfgang Like

7.75

kuechlew

#...

LP plays a role the issue

I have found this to be the case. I live in a bortle 7/8 city with my observatory on my balcony with plenty of local light pollution. When shooting narrowband the issue is almost non existent, some ringing in Oiii but that is the closest to broadband of the narrowband filters. So yes, Light pollution definitely plays a roll. Was hoping to do some test tonight but the weather had different plans. Will report back as soon as I have results from my spider teak up.

I'm not using Newts - and with any thread like this my Newt-phobia increases day by day - but in my bortle 5 zone I have very good experiences with an Astronomik UHC-E light pollution filter. Far from cheap but worth every penny. No idea how well it will work in bortle 7+ but if you're desperate about the issue it may be worth a try. I keep my fingers crossed for you that tightening up the spider will fix the issue.

Clear skies
Wolfgang

1.51 R8RO #... · view views
I do have an old Baader Neodymium filter that I could try for Lum, not sure how much difference it would make. Like

0.00 Tim Ellison #... · view views
Tim Ellison: Hi, Sorry to read that you haven't yet got to the root of the problem. It looks like it is a tough one to crack! I thought I would chip in again if I may, as a follow up to my comments a few days ago. I think I might be able to add a bit to your thinking. Firstly an apology. I hadn't previously read through the conversation sufficiently thoroughly and hadn't spotted the earlier mentions of the possibility of things tilting or shifting. I had thought I was the first to mention it, but I wasn't. Apologies for missing that! My telescope is similar to yours - Sky-Watcher 200P rather than your 200PDS. As I mentioned in my earlier post I had a similar problem, which took me a long time to figure out. In fact I haven't really figured it out I don't think, but seem to have fixed it "by accident". What I reckon happens with my kit is as follows. The flat frames contain a very subtle circular structure which is at or near the centre of the frame. This isn't visible when looking at the image on screen, but it is possible to devise processing techniques to amplify it and make it visible. Having done this I know the ring structure is there. And having thought about it quite a bit, I don't believe that this ring structure is a problem as such, but is something which SHOULD be present. I think it is probably related to the presence of the secondary mirror structure in the centre of the telescope tube. I believe - but haven't been able to prove - that a similar ring structure is present in the light frames. Because it is very subtle, and there is always a lot of other stuff going on in the light frames, I haven't been able to find a way to make it visible, but I think it must be there. When things are working correctly, I believe that the ring structures in all the frames will be precisely aligned, and during calibration those in the lights and those in the flats will cancel each other out and as a result the effect will disappear. However if anything moves between the taking of the lights and the flats, the rings will appear in slightly different positions in the two sets and will no longer align, and so will no longer cancel out during calibration. In this case with my images they gave rise to an effect which looked quite similar to what you are getting. Based on the above, I believe that "something shifting" is a strong candidate for being the cause of your problem. In my case the camera was able to rock slightly in the focuser and it appears it was this which was causing the problem. The rocking was only very slight - but it was enough. When I beefed up the camera support arrangements - which I did with the aim of getting rid of a differential flexure issue - the problem went away completely. Others have suggested that movement of either the primary or secondary mirror are possible alternative causes. I haven't seen either of those causing a problem with my system, but if either mirror is able to shift, then I imagine that might produce a similar effect, for similar reasons. You say you have made sure that your primary isn't moving, but that perhaps there might be slight movement of the secondary. Based on my own experience I think that rocking of the camera as you rotate to the flat-shooting position is the most likely cause, and ought to be a primary candidate for investigation. I would recommend looking quite hard at that. I hope the above is helpful. Good luck! Tim Thanks for your input Tim! I have also come to the conclusion that this might just be something to live with and can be contributed to the nature of Newtonians. What worries me though is the fact that the circle is so visible in my lights. You can definitely tell that there is a ring in the lights and this doesn't show up on other peoples images that I have compared with who have the same scope. One thought to this would be the nature of the TSGPU which pushes focus out by 20mm that may affect the dimensions of the ring (But then again the ring is also visible using my Baader MPCC MkIII). Another thing that threw me off is that the night before my latest successful attempt I did everything the same (took flats right before the sequence at the same direction of the target) and the ring was still there. So apparently it is not consistent. I feel like there is definitely slack or room for movement in the focuser tube, especially now that my camera sits pretty far out. Do you have any resources for stiffening this up? Getting real tired of this issue and I feel like I am running my head against a wall. At least I have narrowed it down to the tube as I get the same results regardless of what camera/CC combination I use. Any and all input is always welcome! P.S Don't worry about not reading through, I probably wouldn't either with all the responses that I've gotten. Hi R8RO and all of the gang, As Wolfgang has said, this is all turning into a good script for a movie. Although I think the plot has become rather too complex for me - I'm losing track of it rather! Lorenzo's input has been most interesting to read. Fixing what looks like a similar problem by replacing the spider is good news! In my case the symptoms were similar, but the cause, as per my previous posts, appears to have been slightly different - i.e. rocking of the camera in the focuser. I imagine both can probably cause similar effects. I know that shifting of the primary mirror has been ruled out here at a very early stage in the plot, but I think that is a possible third thing which might produce a similar effect, and is worth keeping in mind for anybody else with similar issues. Regarding my work to beef up my camera supports, I'm happy to share my thinking and my experience. But at this stage the whole thing looks extremely ugly, so I'm not going to share any pictures. Sorry! I constructed a quite heavy-duty bracing arrangement which locks my imaging camera solidly with the guide scope and guide camera. I used various components of the "Arca Swiss" variety (as commonly used by "non-astro" photographers for mounting their kit), together with some home made parts cut and drilled from pieces of aluminium. This has definitely fixed my issues with differential flexure - which was its primary aim - and has also removed the "glowing ring" effect from my images. However it has introduced new problems of its own - it is so big and heavy that it messes with the overall weight and balance of the system. So I need to come up with something better. I've taken a deep breath and started doing something I was always too afraid to do before - drilling holes in my telescope. For the moment I have drilled and tapped a hole in the side of the focuser so that I can fit a second locking screw. That is awaiting testing to see how stable the camera is with a second screw in place. I'm not sure it is going to be good enough. Drilling the hole turned out to be not too scary though, so I might do more! Assuming the second screw isn't enough, my next plan is to make another bracing structure, but this time a smaller and neater one than before. I am envisaging something like a metal cage which fits around the focuser and is bolted solidly to the telescope tube via some new holes which I will drill. The imaging camera will be mounted solidly on the frame - with a quick-release plate so it can be temporarily loosened and moved in and out for focusing. It sounds mad doesn't it!!?? I feel that I am way out of line with everybody else in what I am doing! Just a quick thought on your experiment with the laser. You report seeing the laser dot move as the system is rotated to different positions. That could be an indication of the secondary mirror moving as the telescope moves. But I wonder if a similar effect might be caused if, instead of the secondary moving, there is rocking in the focuser, which is allowing the body of the laser to move. So I'm not sure if that laser test can really tell you where the problem is. Unless I have somehow misunderstood something myself (which is very possible and does happen frequently!). That's all I've got for now. Good luck with your experiments and tests. I hope you manage to get to the solution without too much more pain! All the best, Tim Like

0.00

Tim Ellison

#...

Tim Ellison:
Hi,

Sorry to read that you haven't yet got to the root of the problem. It looks like it is a tough one to crack!

I thought I would chip in again if I may, as a follow up to my comments a few days ago. I think I might be able to add a bit to your thinking.

Firstly an apology. I hadn't previously read through the conversation sufficiently thoroughly and hadn't spotted the earlier mentions of the possibility of things tilting or shifting. I had thought I was the first to mention it, but I wasn't. Apologies for missing that!

My telescope is similar to yours - Sky-Watcher 200P rather than your 200PDS. As I mentioned in my earlier post I had a similar problem, which took me a long time to figure out. In fact I haven't really figured it out I don't think, but seem to have fixed it "by accident".

What I reckon happens with my kit is as follows. The flat frames contain a very subtle circular structure which is at or near the centre of the frame. This isn't visible when looking at the image on screen, but it is possible to devise processing techniques to amplify it and make it visible. Having done this I know the ring structure is there. And having thought about it quite a bit, I don't believe that this ring structure is a problem as such, but is something which SHOULD be present. I think it is probably related to the presence of the secondary mirror structure in the centre of the telescope tube. I believe - but haven't been able to prove - that a similar ring structure is present in the light frames. Because it is very subtle, and there is always a lot of other stuff going on in the light frames, I haven't been able to find a way to make it visible, but I think it must be there. When things are working correctly, I believe that the ring structures in all the frames will be precisely aligned, and during calibration those in the lights and those in the flats will cancel each other out and as a result the effect will disappear. However if anything moves between the taking of the lights and the flats, the rings will appear in slightly different positions in the two sets and will no longer align, and so will no longer cancel out during calibration. In this case with my images they gave rise to an effect which looked quite similar to what you are getting.

Based on the above, I believe that "something shifting" is a strong candidate for being the cause of your problem. In my case the camera was able to rock slightly in the focuser and it appears it was this which was causing the problem. The rocking was only very slight - but it was enough. When I beefed up the camera support arrangements - which I did with the aim of getting rid of a differential flexure issue - the problem went away completely. Others have suggested that movement of either the primary or secondary mirror are possible alternative causes. I haven't seen either of those causing a problem with my system, but if either mirror is able to shift, then I imagine that might produce a similar effect, for similar reasons. You say you have made sure that your primary isn't moving, but that perhaps there might be slight movement of the secondary. Based on my own experience I think that rocking of the camera as you rotate to the flat-shooting position is the most likely cause, and ought to be a primary candidate for investigation. I would recommend looking quite hard at that.

I hope the above is helpful.

Good luck!

Tim

Thanks for your input Tim!

I have also come to the conclusion that this might just be something to live with and can be contributed to the nature of Newtonians. What worries me though is the fact that the circle is so visible in my lights. You can definitely tell that there is a ring in the lights and this doesn't show up on other peoples images that I have compared with who have the same scope.

One thought to this would be the nature of the TSGPU which pushes focus out by 20mm that may affect the dimensions of the ring (But then again the ring is also visible using my Baader MPCC MkIII).

Another thing that threw me off is that the night before my latest successful attempt I did everything the same (took flats right before the sequence at the same direction of the target) and the ring was still there. So apparently it is not consistent.

I feel like there is definitely slack or room for movement in the focuser tube, especially now that my camera sits pretty far out. Do you have any resources for stiffening this up?

Getting real tired of this issue and I feel like I am running my head against a wall. At least I have narrowed it down to the tube as I get the same results regardless of what camera/CC combination I use.

Any and all input is always welcome!

P.S Don't worry about not reading through, I probably wouldn't either with all the responses that I've gotten.

Hi R8RO and all of the gang,

As Wolfgang has said, this is all turning into a good script for a movie. Although I think the plot has become rather too complex for me - I'm losing track of it rather!

Lorenzo's input has been most interesting to read. Fixing what looks like a similar problem by replacing the spider is good news! In my case the symptoms were similar, but the cause, as per my previous posts, appears to have been slightly different - i.e. rocking of the camera in the focuser. I imagine both can probably cause similar effects. I know that shifting of the primary mirror has been ruled out here at a very early stage in the plot, but I think that is a possible third thing which might produce a similar effect, and is worth keeping in mind for anybody else with similar issues.

Regarding my work to beef up my camera supports, I'm happy to share my thinking and my experience. But at this stage the whole thing looks extremely ugly, so I'm not going to share any pictures. Sorry! I constructed a quite heavy-duty bracing arrangement which locks my imaging camera solidly with the guide scope and guide camera. I used various components of the "Arca Swiss" variety (as commonly used by "non-astro" photographers for mounting their kit), together with some home made parts cut and drilled from pieces of aluminium. This has definitely fixed my issues with differential flexure - which was its primary aim - and has also removed the "glowing ring" effect from my images. However it has introduced new problems of its own - it is so big and heavy that it messes with the overall weight and balance of the system. So I need to come up with something better. I've taken a deep breath and started doing something I was always too afraid to do before - drilling holes in my telescope. For the moment I have drilled and tapped a hole in the side of the focuser so that I can fit a second locking screw. That is awaiting testing to see how stable the camera is with a second screw in place. I'm not sure it is going to be good enough. Drilling the hole turned out to be not too scary though, so I might do more! Assuming the second screw isn't enough, my next plan is to make another bracing structure, but this time a smaller and neater one than before. I am envisaging something like a metal cage which fits around the focuser and is bolted solidly to the telescope tube via some new holes which I will drill. The imaging camera will be mounted solidly on the frame - with a quick-release plate so it can be temporarily loosened and moved in and out for focusing. It sounds mad doesn't it!!?? I feel that I am way out of line with everybody else in what I am doing!

Just a quick thought on your experiment with the laser. You report seeing the laser dot move as the system is rotated to different positions. That could be an indication of the secondary mirror moving as the telescope moves. But I wonder if a similar effect might be caused if, instead of the secondary moving, there is rocking in the focuser, which is allowing the body of the laser to move. So I'm not sure if that laser test can really tell you where the problem is. Unless I have somehow misunderstood something myself (which is very possible and does happen frequently!).

That's all I've got for now. Good luck with your experiments and tests. I hope you manage to get to the solution without too much more pain!

All the best,

Tim

1.51 R8RO #... · view views
Tim Ellison: As Wolfgang has said, this is all turning into a good script for a movie. Although I think the plot has become rather too complex for me - I'm losing track of it rather! Lorenzo's input has been most interesting to read. Fixing what looks like a similar problem by replacing the spider is good news! In my case the symptoms were similar, but the cause, as per my previous posts, appears to have been slightly different - i.e. rocking of the camera in the focuser. I imagine both can probably cause similar effects. I know that shifting of the primary mirror has been ruled out here at a very early stage in the plot, but I think that is a possible third thing which might produce a similar effect, and is worth keeping in mind for anybody else with similar issues. Regarding my work to beef up my camera supports, I'm happy to share my thinking and my experience. But at this stage the whole thing looks extremely ugly, so I'm not going to share any pictures. Sorry! I constructed a quite heavy-duty bracing arrangement which locks my imaging camera solidly with the guide scope and guide camera. I used various components of the "Arca Swiss" variety (as commonly used by "non-astro" photographers for mounting their kit), together with some home made parts cut and drilled from pieces of aluminium. This has definitely fixed my issues with differential flexure - which was its primary aim - and has also removed the "glowing ring" effect from my images. However it has introduced new problems of its own - it is so big and heavy that it messes with the overall weight and balance of the system. So I need to come up with something better. I've taken a deep breath and started doing something I was always too afraid to do before - drilling holes in my telescope. For the moment I have drilled and tapped a hole in the side of the focuser so that I can fit a second locking screw. That is awaiting testing to see how stable the camera is with a second screw in place. I'm not sure it is going to be good enough. Drilling the hole turned out to be not too scary though, so I might do more! Assuming the second screw isn't enough, my next plan is to make another bracing structure, but this time a smaller and neater one than before. I am envisaging something like a metal cage which fits around the focuser and is bolted solidly to the telescope tube via some new holes which I will drill. The imaging camera will be mounted solidly on the frame - with a quick-release plate so it can be temporarily loosened and moved in and out for focusing. It sounds mad doesn't it!!?? I feel that I am way out of line with everybody else in what I am doing! Just a quick thought on your experiment with the laser. You report seeing the laser dot move as the system is rotated to different positions. That could be an indication of the secondary mirror moving as the telescope moves. But I wonder if a similar effect might be caused if, instead of the secondary moving, there is rocking in the focuser, which is allowing the body of the laser to move. So I'm not sure if that laser test can really tell you where the problem is. Unless I have somehow misunderstood something myself (which is very possible and does happen frequently!). That's all I've got for now. Good luck with your experiments and tests. I hope you manage to get to the solution without too much more pain! All the best, Tim Hi Tim, The cage does sound like something I could envision a mad scientist concoct in a laboratory, but I like the originality and creativity! I agree that the laser method won't give me the exact cause of the movement just by looking at it but when I tested the parts individually it required way more pressure on the focuser assembly than on the secondary to produce the same amount of sway. So I can with some confidence say that the secondary attributes more to this issue as supposed to the focuser. The search continues! Looks like I'll be having some clear skies in the coming week (even though the sky is as bright as it is every going to get) Like

1.51

R8RO

#...

Tim Ellison:
As Wolfgang has said, this is all turning into a good script for a movie. Although I think the plot has become rather too complex for me - I'm losing track of it rather!

Lorenzo's input has been most interesting to read. Fixing what looks like a similar problem by replacing the spider is good news! In my case the symptoms were similar, but the cause, as per my previous posts, appears to have been slightly different - i.e. rocking of the camera in the focuser. I imagine both can probably cause similar effects. I know that shifting of the primary mirror has been ruled out here at a very early stage in the plot, but I think that is a possible third thing which might produce a similar effect, and is worth keeping in mind for anybody else with similar issues.

Regarding my work to beef up my camera supports, I'm happy to share my thinking and my experience. But at this stage the whole thing looks extremely ugly, so I'm not going to share any pictures. Sorry! I constructed a quite heavy-duty bracing arrangement which locks my imaging camera solidly with the guide scope and guide camera. I used various components of the "Arca Swiss" variety (as commonly used by "non-astro" photographers for mounting their kit), together with some home made parts cut and drilled from pieces of aluminium. This has definitely fixed my issues with differential flexure - which was its primary aim - and has also removed the "glowing ring" effect from my images. However it has introduced new problems of its own - it is so big and heavy that it messes with the overall weight and balance of the system. So I need to come up with something better. I've taken a deep breath and started doing something I was always too afraid to do before - drilling holes in my telescope. For the moment I have drilled and tapped a hole in the side of the focuser so that I can fit a second locking screw. That is awaiting testing to see how stable the camera is with a second screw in place. I'm not sure it is going to be good enough. Drilling the hole turned out to be not too scary though, so I might do more! Assuming the second screw isn't enough, my next plan is to make another bracing structure, but this time a smaller and neater one than before. I am envisaging something like a metal cage which fits around the focuser and is bolted solidly to the telescope tube via some new holes which I will drill. The imaging camera will be mounted solidly on the frame - with a quick-release plate so it can be temporarily loosened and moved in and out for focusing. It sounds mad doesn't it!!?? I feel that I am way out of line with everybody else in what I am doing!

Just a quick thought on your experiment with the laser. You report seeing the laser dot move as the system is rotated to different positions. That could be an indication of the secondary mirror moving as the telescope moves. But I wonder if a similar effect might be caused if, instead of the secondary moving, there is rocking in the focuser, which is allowing the body of the laser to move. So I'm not sure if that laser test can really tell you where the problem is. Unless I have somehow misunderstood something myself (which is very possible and does happen frequently!).

That's all I've got for now. Good luck with your experiments and tests. I hope you manage to get to the solution without too much more pain!

All the best,

Tim

Hi Tim,

The cage does sound like something I could envision a mad scientist concoct in a laboratory, but I like the originality and creativity!

I agree that the laser method won't give me the exact cause of the movement just by looking at it but when I tested the parts individually it required way more pressure on the focuser assembly than on the secondary to produce the same amount of sway. So I can with some confidence say that the secondary attributes more to this issue as supposed to the focuser. The search continues! Looks like I'll be having some clear skies in the coming week (even though the sky is as bright as it is every going to get)

0.00 Tim Ellison #... · view views · 1 like
Good luck with the experimenting. It does sound like you are approaching it in a good and logical way! Not like most of my experiments! Anyway I hope you have some success and get it figured out. Fingers crossed! Tim Like

3.61 Nikkolai Davenport #... · view views
Something came up in another thread that I thought might be applicable here: by inserting a guide camera with a small lens attachment into the focuser, it can help identify light leaks in the OTA. The one I found most interesting is the light leak around the focuser tube, which might be relevant in this case. If there is movement in the focuser tube, that could be the source of the ring in your light/flat frames. The shift in the position of the secondary may also influence how this appears when the OTA is positioned in different directions. Like

3.61

Nikkolai Davenport

#...

Something came up in another thread that I thought might be applicable here: by inserting a guide camera with a small lens attachment into the focuser, it can help identify light leaks in the OTA. The one I found most interesting is the light leak around the focuser tube, which might be relevant in this case. If there is movement in the focuser tube, that could be the source of the ring in your light/flat frames. The shift in the position of the secondary may also influence how this appears when the OTA is positioned in different directions.

1.51 R8RO #... · view views · 1 like
Something came up in another thread that I thought might be applicable here: by inserting a guide camera with a small lens attachment into the focuser, it can help identify light leaks in the OTA. The one I found most interesting is the light leak around the focuser tube, which might be relevant in this case. If there is movement in the focuser tube, that could be the source of the ring in your light/flat frames. The shift in the position of the secondary may also influence how this appears when the OTA is positioned in different directions. This is actually something I've been wanting to try for a while. I don't have a lens for my guidecam but this might be something to get in the future. Thank Nikko, I'll have a look! Might look into creating a round piece of flocked cardboard to place on the secondary that overhangs a bit to see if this could be an issue. Like

1.51

R8RO

#...

· 1 like

Something came up in another thread that I thought might be applicable here: by inserting a guide camera with a small lens attachment into the focuser, it can help identify light leaks in the OTA. The one I found most interesting is the light leak around the focuser tube, which might be relevant in this case. If there is movement in the focuser tube, that could be the source of the ring in your light/flat frames. The shift in the position of the secondary may also influence how this appears when the OTA is positioned in different directions.

This is actually something I've been wanting to try for a while. I don't have a lens for my guidecam but this might be something to get in the future. Thank Nikko, I'll have a look! Might look into creating a round piece of flocked cardboard to place on the secondary that overhangs a bit to see if this could be an issue.

3.61 Nikkolai Davenport #... · view views
Might look into creating a round piece of flocked cardboard to place on the secondary that overhangs a bit to see if this could be an issue. If you want to try blocking potential light leak from the focuser, an aperture mask around the end of the focuser draw-tube with a lip that extends outward so it's larger than the diameter of the focus tube might do the trick. The idea is to try and block any direct light that might be leaking between the focus draw-tube/focuser, as well as between coma-corrector/draw-tube. Thinking the source is more likely between draw-tube/focuser, though. Edited ... Like

3.61

Nikkolai Davenport

#...

Might look into creating a round piece of flocked cardboard to place on the secondary that overhangs a bit to see if this could be an issue.

If you want to try blocking potential light leak from the focuser, an aperture mask around the end of the focuser draw-tube with a lip that extends outward so it's larger than the diameter of the focus tube might do the trick. The idea is to try and block any direct light that might be leaking between the focus draw-tube/focuser, as well as between coma-corrector/draw-tube. Thinking the source is more likely between draw-tube/focuser, though.

Edited ...

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