Unable to attain full frame flatness

jiman

Image train is WO FLT91 - FLAT68iii - sensor tilt adapter - M68 OAG - 2"EFW - ASI6200MC
If anyone may have similar setup, I am unable to attain full frame flatness. From center to about 70 percent out stars are solid. Rechecked configuration multiple times, measurements are all in advertised requests. The FLAT68iii is adjustable and I've tried from 0 to about 10cm out.
Jim

Tech@ZWO

jiman Here is an article for your reference.
https://astronomy-imaging-camera.com/tutorials/reason-of-field-tilt-problems-and-how-to-solve-it.html

enaiman

You should try CCD Inspector. They have a fully functional trial version which works for 30 days and that can help you.
This is not an issue related to any ZWO components and shouldn't be posted in this forum but taking it as a cry for help I will try to help.
You need to check the image to see if it is happening the same in all corners (edges) or only on some of them.
If you see elongated stars all around the edge with the center having round stars then your backfocus is wrong.
If you see those elongated stars only on two, three corners then you have a tillt problem (and possibly backlash)

Anyway, it is not very easy to fix it (I have the same problem and working through it step by step) but you can do it with research and patience.

jiman

Thank you for your responses but I've obviously not explained my issue well enough. My stars at all four corners are elongated to the outside which from my knowledge indicates more distance needs to be given from FLAT68iii to image train. I don't have a tilt issue.

My apologies for posting to the incorrect forum.

enaiman

jiman there you go - you have correctly diagnosed your issue.
Find what backfocus distance the manufacturer recommends and make sure you get that really close (keep in mind that the backfocus distance is to the camera sensor) because 1-2 mm can be the difference between elongated stars and perfect image.

ckoos

Also, filters can change the required backfocus. As I recall, you need to add 1/3 of the thickness of the filter. Backfocus of the 6200 is 55mm, so add about .3mm to that for the typical filter. According to this you don't have enough backfocus: https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus#:~:text=No%20matter%20which%20camera%20brand%20you%20use%2C%20T-rings,be%20ready%20to%20go%20with%20only%20a%20T-ring.

-chuck-

w7ay

ckoos add about .3mm to that for the typical filter

FIlters are normally much thicker than 0.9mm.

The glass thickness from good brands like the Astrodon and Chroma are 3mm thick (even the Radian Quad is 3mm thick). Cheaper 2" filters seem to range from 1.8mm thick to 3mm thick. Both of my 36mm and 50mm Chroma sets are 3mm thick. My IDAS filters are either 2.5mm or 3mm thick.

If the flattener/reducer asks for 55mm, the sensor really needs to be more like 56mm away once there is a glass filter in between the flattener/reducer and the camera' sensor.

For reference, see page 8 in Katie Schwertz' MS thesis, that includes cases for glass that have refraction index which is not 1.5 (the "1/3" rule is for a refraction index of precisely 1.5):

https://www.researchgate.net/publication/260399858_Useful_Estimations_and_Rules_of_Thumb_for_Optomechanics

Chen