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Good afternoon everyone. This is my first forum post and I sincerely hope that this inquiry is being placed in the correct section and will end up on open ears. Before I explain my problem, here's my rig in detail: 12" Newtonian Reflector from Orion Optics UK Celestron CGX-L mount ASI Air plus ASI2600MC Pro camera Starizona Nexus (Reducer & Coma Corrector) ZWO EAF electronic focuser 60mm SVBony Guidescope ASI120MM Guidecamera The problem I have is, that after replacing the secondary mirror and equipping it with a dew heater, my focus of the stars and their sizes have worsended. Before, I had a starsize of 3 to 4 (I know, bad already). But now I have to be lucky to get under 6. Additionally, the stars now look like they have tails. It's not coma, since they look like this everywhere on the pictures, no matter what corner. And looking at the out of focus star (Sirius), I can see that there is an issue with a "missing piece" of the disc. I tried to adjust everything multiple time and over three days used every single collimation tool, from a simple cap with a hole, 2x different laser collimators and a concenter (my preference). Every single time, it looked good to me. And I have done this often enough, to be confident enough to say, that my collimation was good. However - I cannot seem to get rid of this issue... Any ideas or suggestions would be greatly appreciated!!! (And be kind to my writing - my native language is German) Pictures to support my claim, are below.    |
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| Thermal plume. You must wait until the system (mirror) equalizes with the outside sky temperature. Or switch off the secondary heater, it doesn't do any good and in countless hours of imaging I have never see the need of it. Just get an insulating long dew shield. |
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Thank you for the quick response. I thought of this too and that was the first thing I "eliminated" - no heat on the second and third night. However - the problem remained. Below is one capture of NGC4565 at midnight - even the heat from the day should have dissapated by then. Plus I have 3 fans behind my primary mirror, pulling cold air in from the front, which helps keeping everything inside at ambient temperature.  |
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Strange, looks exactly like thermal issues but given that you run mirror fans it is a bit puzzling. I would suggest trying without the secondary heater, it could well be the cause of your problems. Have never used one, nor needed one, in my typical conditions which are usually above 80% humidity and often closer to 95%. |
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Hi Buyman, the issue with the first image you posted seems kind of familiar to me. I've seen similar issues before, when there is an obstruction in the optical train, that's causing diffraction. Could be the head of a screw, or something similar. But the second and third image puzzle me a lot. To me, it seems like the main mirror and the focuser are not aligned, causing 'shading'. On a big reflector like yours, I'd recommend using a tilt adapter between scope and focuser. Leaving the 'smear' in the bottom left corner of the stars aside, your stars are all elongated across the Y-axis of the image. This could also be due to tilt between focuser and scope. Without a tilt adapter, collimating the scope with a laser will cause you to adjust the telescope to a tilted laser beam. Next time you are outside, also try plate solving the image, and check if the calculated focal length is not too far off the actual focal length. If it is, you should probably check with customer support. Clear Skies, Tim |
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The only other similar issue I can think of is that one of your mirror clips is somehow too tight. This can easily be seen at high magnification with an EP and it would definitely clear what the source is (I still think that the first picture is an heat plume). By any chance where is the scope stored when not in use? See an example of too tight mirror clip(s) and I'm sure you notice the similarities:  |
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Thank you all for your answers. To respond to some of your questions:Oskari Nikkinen: I did that on the second night already, but on the next clear night, I will keep ALL the dew heaters off - including the camera one. Just to exclude "plume". And they're not just for condensation - I had the mirrors "ice over" before. Takes a while to defrost with a hair blower and leaves streaks - keeping the dew heater on, helps. Tim Richter: The obstruction was/is a good idea, but wouldn't that also show on the defocused star? And I just checked - I don't see anything looking down the tube. I believe the tilt adapter might be the solution I need. But I want to exclude everything else first, to avoid spending extra money.  Good point on the elongation - I put that under the "not focusing" thought, but think you may have a point. But then again - wouldn't that show on the defocused star as well? I mean it actually is not perfectly centered, so maybe you are right. I always plate solve from "0". Every time I setup my rig, or uncover it, I zero the focal length of the camera. I had to learn it the hard way, when switching to a NB filter, and couldn't figure out why I can't plate solve... *duh* Uebrigens - klasse YouTube Kanal. Hab' viel von Dir gelernt! Mach bitte weiter so - bin ein grosser Fan!!! Gruss aus Pennsylvania... andrea tasselli: Thanks again for your fast feedback - much obliged. You don't live here, do you?  The Orion Optics tube doesn't have any clips to hold the mirror in place - none that I can see. And you can see that on the defocused star pictures. But I'd have to lie, if you'd ask me how it's being held in place. I might have to take a look in the rear of the tube, to see how they managed to fix its position. As for the storage: All parts are being stored outside in a poolshed with its doors open year around. Therefore there is not much time needed to adjust the scope to the ambient temperature. |
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| I doubt my experience is relevant, but I get misshapen stars like this on some night and not others. The only correlation seems to be when the jet stream is overhead or overlapping fronts causing high altitude turbulence. I tried so many “solutions,” only to find the stars perfectly round 2 days later. |
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Dave Rust: Interesting take. I have to keep an eye on this. The nights I was working on this issue did have some cloud coverage coming and going - maybe there were some unseen turbulences involved as well. Interesting idea - thx for the feedback! |
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I know it can be frustrating. It might save you some grief to see if it’s something out of your control. You mention clouds. I experience “double image” stars when invisible haze or thin clouds pass overhead, apparently refracting the image differently for that duration. I’ve learned to be a little more relaxed with the performance from my strong scope (2350mm), realizing that minor anomalies are magnified into major ones and perfect nights are to be relished. |
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Buyman3375: So thermal plume can be excluded unless it was unseasonally warm out there in Germany. Remains the clipping. You might not see them if they are shielded by an aperture ring. By the way, what OOUK scope do you have; VX, CT or the new one I forget the name of? |
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| Just to be on the safe side you can check whether the extra-focal and intra-focal images of the exit pupil are identical. If one is squashed (like the one you posted at the beginning of this thread) and the other is pronged then you have an issue with thermal plumes. Just to clear the waters; there is no way that clouds or jet-stream shenanigans can produce such an effect and, also, it isn't collimation. |
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andrea tasselli:Buyman3375: Thanks for sticking with me and my issue(s). To answer your questions, the OOUK scope is a CT, but the secondary mirror - even though bought from them - is not from this line. It possibly could be, but I bought it as a replacement from OOUK, because the first one got damaged (still worked), but needed to be replaced. Maybe the mirror is the issue? Sorry I forgot to mention the replacement before! Btw - I don't live in Germany, I moved to the US in 2011 and live in PA. It was unusually warm out here for March, but not enough to consider heat plumes from that. As for the extra-focal and intra-focal imgages - I am too close to my minimum limit to create the intra-focal image. The image wouldn't be to good to compare - unless I go the same maximum distance on the other side. I can try that... |
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| Coming from OOUK anything is possible, even an overstrained or astigmatic secondary (if you can replace back the orginal one that is something I would do). But I'll try first to identify whether a thermal plume could be the answer. And, yes a CT scope still has mirror clips as far as I am aware. |
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If the CT mirror cell is the same as the VX one, which looks like the same in images i have seen, then the mirror is glued to the cell with silicone sealant. At least my VX cell is, manufactured in 2020. I will say that my OOUK secondary mirror coatings have started to deteriorate only after 3 years of use, so i have no trouble imagining there are some quality issues with their secondaries. Might be a decent idea to buy an affordable GSO secondary mirror to test. |
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Oskari Nikkinen: Thanks for the feedback. Yeah, I think both mirrors are actually glued. I will submit a picture of the primary mirror, once I have a chance. But it won't be from "behind", just down the tube. Not sure if you saw my other response, but I forgot to mention on my initial post, that I replaced the secondary mirror because of the coating issue. That's what triggered the whole "adding a heater" thing. However - the coating on the first (secondery) mirror, came off after cleaning the cell, so I purchased another one - from OOUK. If something like this happens again, I will defenitely consider GSO next time. |
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Hello again. So I am still fighting tooth and nail to get this resolved. So far I had no luck. The most recent images haven't changed much, but I noticed an inconsistency with the spider vanes and was wondering if that could be the cause of my issues? The screenshot below is from a short exposure of Polaris. And when I noticed the weird spider vanes, I checked with the earlier pictures in this thread and noticed the same thing. It appears that's the only variable that doesn't change and is consistent with my unfocusable images Any advice on how to resolve this? I measured the center of the secondary, then tightened/screwed the vanes. I have done this several times, but it doesn't change the "look". Is there a proper or safe way to set the vanes right and tighten the screws in a certain order?  |
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It ain't the vanes for sure, that is a double diffraction from one set of vanes which means they aren't co-linear, but otherwise not a major issue. To cut the chase can you make a cardboard offset aperture mask such as shown below: The important bit is to mask both spider and secondary. Test again with intra-focal or extra-focal images if possible. In-focus images are not much help to diagnose the matter at hand. To my knowledge the CTs are closed behind the rear of the mirror, that is another area to investigate. I'm still convinced something is wrong with either mirror. |
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andrea tasselli: Hi Andrea. Thanks for the response and not giving up on me. (yet) Maybe it's my lack of vocabulary or I'm simply to dumb to understand what you're asking of me, in regards to the "cardboard offset aperture mask". Would you please be so kind to explain? I attached two pictures of the intra- and extra focus pictures. Sorry for the quality, but I'm too close to zero, to create a nice intra focus picture. And to keep them equal, I could one go "1800" steps in each direction. Both are terribly copped and resized - I hope these will be of any help...! I personally believe it has to do with the secondary mirror, since I never changed anything on the primary. The secondary was glued to the stalk by myself and offset, like the previous was. I'd like to say I was as precise as I could, but I'm not sure if I clued it exacly on the same position as before - in regrads to nm, or even mm. I would like to think i did... Intra Focus  Extra Focus  |
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Thanks for posting those images. Which are a text-book example of thermal plume. See page 143 (2nd row from top) of Suiter's "Start Testing Astronomical Telescopes". I can send you a copy of the page, if interested. Or one side pinching (which would be novel for me but you never know...). As for the cardboard mask well, it is just a cardboard (any other suitable material easy to work with) panel cut to the same diameter of the telescope tube (outer). Inside you should cut another circle in between the outer diameter of the secondary and the inner diameter of the telescope. This inner circle is as large as it can be to fit in one of the quadrants which the spider delineates in the telescope aperture. The picture below hopefully illustrates the arrangement (in gray are the spider and secondary as seen from the telescope front and hidden by the mask):  Any question just ask. |
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| Incidentally, you said you "glued" the secondary on the support. That's a bad idea which can lead to astigmatic images unless the "glue" is more like sealant rubber and spacing provided by rubber grommets. |
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andrea tasselli: Thanks again! So, if I cut out the hole within the template, what am I supposed to do with it afterwards? Am I supposed to put it into the opening of the scope and then take intra- and extra focus pictures again? And what would that show/achieve? As for the "thermal plume" - unless there could be another reason for a thermal plume to come to exist, it is not done by my heating equipment - anymore. Since I discovered this issue, every night I disconnected all my dew heaters - just to make sure. |
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andrea tasselli: Well... This is where I fear I am going to lose you. Because of my incompentence. The first mirror was only glued to the stalk with a silicone from the inside of the of the stalk, not underneath where it touches the mirror, nor on the outside. When I wanted to fix it in place, I played with a glue gun on an example first and realized that the dew heater would soften the glue and I feared that it might loosen during the heating outside and fall into the tube. Out of fear, I had the "great" idea to glue it with 3 different silicones. I used one for the direct contact to the mirror, one for the inside of the stalk to the mirror and one on the outside of the stalk. It doesn't look terrible and I did a good job at keeping it at a minimum. But I am slowly having this creeping feeling, that this might have caused my dilemma... Please tell me it can't be... |
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Buyman3375: Sticky-tape it in front of the scope and take pictures of Polaris or anything bright and convenient (remember you are now shooting at f/10 or more now so it would need longer integration times). All the diffraction effects due to mis-aligned spider are removed and all effects due to thermal plume and/or deformed *primary* mirror are lessened or removed entirely. If the issue is with the secondary mirror then it would show up verbatim. Note that also your depth of focus will be massively increased so you might be able to take just extra-focal images. Not too far away, just the same apparent size of the last ones you posted. As for the cause of the "thermal plume": any thermal difference between mirror and outside temperature will results in such a thing, even with just 1 degree difference. The real question is the magnitude and how fast you can dissipate the excess heat. With my OOUK 12" I have just a bare rear scope and it is always kept outside with proper reflective cover (say similar to a telegizmo). Question: what is the day-time recorded temperature in wherever you keep the scope and the night-time temperature? |
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andrea tasselli:Buyman3375: I will try that tonight. I'm located in PA and my temperature ranges nowadays from 25F - 50F degrees. During the day, we recently had it all the way up to 72F. And when I'm not storing it in the open shed, due to rain, wind or snow, I have it outside at the sme spot - under Telegizmo's! |
