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In looking at my image I noticed some stretching of the stars at the edges of the image. I’m looking at CN forums I came to the conclusion this may be sagittal astigmatism caused by improper back focus? Can someone with more experience confirm this might be the case before I end up messing with back-focus? https://www.astrobin.com/y6rf7j/B/ WO GT71 w/FF |
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| Assuming that the FF would work as advertised (some do and some don't), yes you're are too close. Add spacers until the issue disappears or it flips the other way (and then you'd know that the FF can't cover the full frame). Maybe a tiny bit of tilt too but it is secondary to the spacing error. |
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| Thanks, Andrea. I am thinking it’s the spacing in the flattener itself. It is adjustable. Forgive me for my ignorance, but what do you mean by off tilt? Guiding? |
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| It means that the sensor is very slightly tilted with respect to the optical axis. First address the issue with the backspacing and then we'll see if the tilt still exists. |
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| Andrea is correct. You need more spacingDAD88ED3-556C-4FFF-B952-46724BF991C5.jpeg |
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| Thanks, John. I’ll give it a shot and see how it turns out. |
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| Assuming you have a WO FF, which is adjustable as you said, just add more space using it. Have the same issue with my WO81ZS and fixed it this way. |
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| Thank you, Mauricio. I will give it a try. Just waiting for another relatively clear night… |
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Mauricio Moreno: Mauricio, which spec did you adjust yours to? Looks like on the WO website mine says 9.3, but I’ve seen different specs. |
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Since you appear to be running NINA, you might consider installing the HocusFocus plugin if you haven't already done so. In addition to better focusing than the stock focusing, it also gives you an aberration inspector, which will show your back focus error (in focuser steps) and sensor tilt. Patriot Astro has a very nice tutorial on Hocus Focus, including information on using the aberration inspector. You can also use the FWHMEccentricity script in PixInsight (under the Script->Image Analysis menu). Extract a grayscale image from your image, then run the script and click the "Measure" button. When that completes, click the (very unintuitive) "Support" button and you'll get both FWHM and Eccentricity contour maps. Both are quite useful in diagnosing back focus and sensor tile issues. - Ken |
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You may also experience color fringing of the stars (different colors inside /outside of the stars) , especially when imaging low hanging object and it is a very common problem with One shot color cameras, but the issue can be corrected in WBPP in PI, and luckily, Adam Block just put out a video on Youtube on the subject. Optolong filters usually have a glass thickness of 1,85 mm, so you should to add 0,6mm to you backfocus distance. Roy |
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I am in the process of solving a similar issue and someone sent me this guide which has been really useful... |
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Jeremy Phillips FRAS: Thank you! |
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I made adjustments… I think this is looking better on the edges. Looks like I was adjusting my flattener spacing incorrectly. https://www.astrobin.com/s0jpy0/B/ |
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I'm having similar issues with a new GT71 & 6AIII. See below images for PI and ASTAP eval. Not uniform across the field. Wierd cross stars upper left. Upper right looks like sensor too far. Bottom left too close?    |
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Jeff Horn: Jeff, what camera are you using? |
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I have similar issues in my APO, first I worked on the back focus using spacing rings like these https://www.telescopi-artesky.it/en/artesky/t2-adapters/1487-spessore-acciaio-m42-artesky-8055517130704.html (sorry, Italian only, but you'll get the idea), the image that Jeremy posted above helped me understand how to get to the best distance. Once it was fixed, I noticed that the stars were elongated radially on one side only, and with this test https://astronomy-imaging-camera.com/tutorials/reason-of-field-tilt-problems-and-how-to-solve-it.html I understood that the problem is in the optics (but since star profiles were acceptable with after-stack cropping and the scope has been collimated recently, I decided to deal with it) |
