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Hello guys, I've already read a lot of forum post about this topic. Usually the problem apears to be reflections inside the tube, light leaks, sometimes the corrector sticking inside of the tube or a GPU causing wierd stuff. Some people are doing flats wrong or using fauly bias /darkflats. All these errors can I exclude in my case. My equipment: Skywatcher 300 Quattro Touptek 2600 mono Starizona Nexus Reducer Corrector Astronomic LRGB Filter. Before you ask, I have tried every single coma corrector I am aware of existing and I had 3 different filter Sets. ZWO, Astronomik Deep Sky Filter and the 2c Astronomik Filter Set. Even mirror shifting is now fixed and not a problem too anymore. Nothing worked. My telescope is darker as the nightsky inside  I have this problem for 3 years know. Supprisingly on my 150 Quattro this problem seems not to be existing. I have tried everything and I came to one conclution. When imaging an object low on the horizon right in the light pollution, the flats will never work in LRGB and O3. Ha is normaly not a problem. When imaging outside of the city at B4 or better the flats will allways work, even with questionable flat taking methods. It seems that the flats needs to be way more accurate when taking images in bad bortle class, because the vignetting is getting stronger there. Ive also tried several different exposure times/ gains/ gain mode and flat methos like skyflats, extremly expensive flatfield and now even a IDY flatfield. I am very clueless anymore and I think I will just swap to a smaller sensor becaue I can almost never use my APS-C Sensor. Anyways, here is the error.  This is the Masterflat  I can upload the filts and some light with darks and bias when someone want to have look at it. On the left is a non stretched masterflat, on the right a streched single frame.  I hope anyone have an idea! Thanks for reading. Marc |
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| Is the backside of the Quattro covered with black-as-ink light-proof cap? I never had issue shooting in my B7 skies and correcting for flats and your image clearly shows light leak. And as for flats: is not with what you are doing them is how you do them... |
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Have you put tape or something around the focuser tube? I had some light leaks around my newt focuser. As for the flats, what adu value are you reaching, time? It almost looks like they're too bright, mine look similar on my hyperstar rig when the flat panel is too bright. I've switched to 10% adu at 3 seconds and haven't had a problem since. |
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andrea tasselli: Yes it is. In fact I've used 2 layers of dewcap and I have put the mirror on velours. So no light is hitting the glas from behind. I know it looks like a light leak, but i can take dark flats and daytime.... So no way light is comming from outside in my tube. |
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Have you put tape or something around the focuser tube? I had some light leaks around my newt focuser. *Yes it is completly covered. I can take dark flats during day, so much lighprove is my scope... I ve tried the normal 32.000 ADU with 1-5 sec (Ive tried longer and short ones) Also I did 10 - 90 % in 10% steps. nothing worked. I can try the 10% with 3 sec. I think my main goal is getting rid of the vignetting. Only solution is getting a larger secondary mirror, but it is already pretty large (110mm small axis) |
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Do you also a have a velour-lined long dewcap (say 40 cm above the edge of tube)? Otherwise, let's see what the darks/lights/biases/flats have to say about the issue... |
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| But, let's make it plain here: vignetting will always exist and if the light drop is large (larger than 30%) then even the best flat in the world won't be able to correct that. Shooting at f/3 isn't conductive to less vignetting but MORE vignetting and MORE issues with flats. Just to be in the clear... |
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This is usually not a flats exposure or other software issue, but a mechanical one with newtonians. You get that ring artifact in flats when your scope is not mechanically stable throughout the night. Something in your scope moves between taking the lights and taking the flats, which means the 2 no longer match and flat calibration is impossible. Now what that moving thing is, is harder to say but some key areas should be inspected: The mirror cell, the secondary spider, the tube itself and especially the focuser if you're using a stock Skywatcher one (not very good). If any of these produce a few microns worth of movement of the center of the optical axis flats will become impossible. I think the reason you have not noticed this in nicer skies is that gradients are simply not that strong, so poorly working flats are less of an issue. You can test the stability of your system by the way, and here is how: Take a flat at every tube orientation, for example one to the zenith, one with the tube horizontal to the west, one to the East, one to North, one to South. Then in either Pixinsight or Siril Pixel math do a division between any of the flats for example flat_south/flat_zenith. What you should get is a complete noisy, but even mess with no pattern or ring structure to be seen. If you see any kind of pattern your scope will never be able to produce good flats and you need to figure out which part is causing the instability. |
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Oskari Nikkinen: In general good advise but does this have to be accurate to microns? Thermal expansion itself will call that amount of movement. also, what is the repeatability of the filter wheel position? The last was likely the issue I had. |
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andrea tasselli: Untrue, flats will correct a 30% drop in brightness very easily. In fact i had more like 35% drop in brightness with my previous coma corrector and it was fine. If my scope was out of collimation it could have been as bad as 40% less light, and flats will still work. |
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Arun H:Oskari Nikkinen: I mean microns of movement on the sensor, so it doesn't have to be microns of movement of the mirror cell for example. Another way to put it is anything that makes the center of the light cone move by even a few microns across the sensor will make flats impossible. |
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hi the last time ,i saw that pattern it was caused by the problem with the back focus ( 10mm more ) clear skies |
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Since you've been through the ringer here, of which I truly sympathize with you, let me suggest something to try of which I theorize is a cause for incorrect flats more often than folks think. Here is my theory: When I first began my learning curve, I use to focus quite aggressively throughout the night, always trying to keep my scope at perfect focus. In doing so, I keep myself very satisfied that my focus was spot on. However, this is also the time period in which I had continual flat correction issue. The correlation, at least for me, has become quite evident......especially with sensitive optics. It's my belief that, focusing after the start of a session, increases your risk of miniscule optic movement such that the flat profile (which are taken at a single point in time and position) are slightly different than the average of your focus positions. Given that I had these similar issues (especially with fast optics or with reduced optics/severe vignetting), when did these issues suddenly stop for me? They suddenly stopped for me when I quit focusing after I started the session....and yes, I have electronic focusers installed on all of my imaging systems. My suggestion........ do a test session. 1) Go to a target in the sky where you are currently having issues. 2) Obtain initial focus 3) Absolutely do not touch your system after initial focus is obtained and you've started your test session. 3) Once complete, remotely slew using the slowest speed (again do not touch your system) to your flat-taking position. 4) If using a flat panel (Note: Ensure your flat panel has been outside to achieve ambient stability before using), and very gently place it over your scope allowing it to seat over it and try not to move anything whatsoever. 5) Take your flats appropriately. I'm very curious of your results if you decide to try this suggestion. And BTW - that 'ring' pattern is a product caused by your nexus reducer. I have the Starizona apex and it's almost identical. It CAN be corrected but It's also VERY sensitive to the issue I describe above. |
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Arun H: Pretty much nailed the issue here, Arun. Obviously isn't an issue of "few microns" and in fact even "many microns". I suspect is an issue with internal (to the image train) source of diffuse light. But let's not put the cart before the horses until a sample of the images produced is made available. |
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| I started getting this issue when I moved to the IMX571. Was still using RC back then. After weeks of troubleshooting I discovered that it only happened when my flats were too bright. Started taking all my flats between 500-800 ADU and it has never happened again. When I switched to Newt I thought perhaps the issue would be gone with the new scope. Took some flats at about 18k ADU, artifact is back. Started taking a series of flats lowering the ADU with each new set until the artifact went away. Found again that it stopped happening below 1000. I still don't have an explanation as to why all I know is since I started taking lower ADU flats the issue has never resurfaced. If I take flats much above 1000 ADU it returns like clockwork. Try taking some sub 1000 ADU flats and see if that makes a difference. |
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Oskari Nikkinen: I wrote greater than 30% but I didn't quote a number did I?. |
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Oskari Nikkinen: Balderdash. I routinely have them and flats correct just fine. Same as many SCTs out there I suppose. |
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I have had issues with light creeping in on my newton through the focuser tube as well as the primary mirror holder. So applying - a hair band around the focuser tube and - a black shower cap around the primary did the magic :-) Arny |
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Arny: *Thank you for your reply. As I said, my tube/imagetrain is so dark, that I can take darks during sunshine... So no light leak is possible. |
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Reg Pratt: Hey Reg, I allready did, but I guess I am giving it another shot... |
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Marc-Antonio Fischer:Reg Pratt: How low in ADU did you go? |
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Reg Pratt:Marc-Antonio Fischer:Reg Pratt: Ive tried 400 /1000/ 1200/3000 and so on. |
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| @Oskari Nikkinen That was very helpfull! Thank you very much. I will try this out! |
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Focus drift is not a key issue with flats if everything else is stable and the focuser has no lateral movement. The flat panel is maybe a kilometer or two out of focus, so refocusing of any amount due to thermals will have little to no effect. The light cone stays practically the same shape and in the same spot over the sensor. If there is lateral movement, such as because of the focuser having too much tilt in relation to the secondary (or poor collimation) then this could be an issue but i doubt it. The least on one's worries with a newtonian i would say. I have done a synthetic shift test with my flats where i shifted the center (by cropping lights and flats from opposite sides) by 8 pixels, which is 30,08 microns with my camera, and flats failed. So i am confident in claiming that 30 microns of lateral shift of the light cone (for any reason) will make flats fail. |
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So guys, an update.. i was very shocked when I saw this result. The following is a substraction of two lights, one before and one after the flip.  as you can see after the substraction, the patters in clearly visable! Suggesting that after the flip, something moves or changes the vignetting. I have also proved that by using two light directly imaged after another. The result was a flat noisy image, like it should be. Also ive testet it with flat, doing it northside and southside. Also you can very notice the pattern here!  the question is, what causes this vignetting changing after the flip ? I mean technicly i am able to take two pair of flats one for and one after ghe flip and try if they work with the light... but this is very tedious to do and the result is also nicht clean eighter Ive installed my ocal pro and cant see any movement of the primary mirror mark when slewing the socpe .. |
