Tilt/Curvature??

[Saturn488]

I think I have a combination of tilt and curvature in my setup. It has been driving me nuts.

Running a WO GT81 + 1600MM + Field Flat and 36mm Filter Wheel.

I have it at 55.5mm back focus (closest I can get to 55.6mm as I need to add 1/3 the thickness of my 2mm filters).

I am using both ASTAP and CCD Inspector (mentioned in my other thread) to check for tilt and both always have different results (CCD says I have way less tilt). I have also tried 10% and 20% focus windows in SGP (the higher the value the slightly better result).

I tried using a Gerd Neuman tilt adapter which is so easy to use but didnt really improve my results.

See attached for the most two common results I get.

My curvature is anywhere between 16% and 24%.

What could be at play here?
- Titled camera sensor?
- Flexure or weight issue?
- Faulty field flat?
- Focus issue?

Any advice or help or something I can try? If I can get the curvature at a constant 15% and my tilt less than 10% then ill be happy.

[Joshua Bunn]

Hey mate,


In these situations, I prefer to go back to the basics.
As you say, this could be several things, but you can do some tests to isolate things.


To remove the possibility of flexure or random shifting, ensure your focuser and everything else is firm. secondly, point the scope at the zenith to do your tests. Gravity will not sag your focuser and you should have better seeing looking straight up.


Are you collimated?? if you're not sure if its collimation or tilt causing wonky stars, you can rotate your camera then take another shot and compare. A tilt issue (from the rotation point to the camera) will follow the camera - stay in the same chip position. Collimation will not, it will move to a different place.

The other thing to try, do you own focus runs in the corners of the chip, and one in the middle. Use the best focus position at each off the 5 locations to determine where your chip sits relative to the focal plane. From there you will know what part of the chip need to move in what direction.


Regards
Joshua :)

[Saturn488]

Thank you.

I will give that a go next clear night. I'll take everything apart and reattach.

I've read online that the field flat 6aiii has to be set at 7.1. Some other sources state otherwise.

So I'll have:
- 1600MM - 6.5mm
- EFW - 20mm

Required back focus is 55mm but need to add 1/3 filter thickness which us 0.67mm.
Total of 55.67mm of back focus so need to make up 29.17mm.

I hope im doing that correctly.

Linking back to my other post. CCD inspector usually has better results than ASTAP then I use Pixinsight to check all corners. What should I be using to check my results?

Thanks!

[Joshua Bunn]

based on this page

https://williamoptics.com/new-adjustable-flat6a-t-mount-not-included
You need to set the ring at 7.9mm which will give you 62.3mm of backfocus to the image plane, plus 0.66 for your filter -> 63mm close enough.
Currently you have 26.5mm so you need to make up 36.5mm with an extension tube or part thereof and the remaining with some other accessory if you want..


Where does it say you need 55mm of backfocus for this reducer?


Personally I only use CCDIn at the moment, but I confirm the results with focus runs in the corners and a visual inspection at 2-400%


Josh

[Saturn488]

I have the FLAT 6AIII 0.8X.

The 1600MM requires 55mm back focus.

[Joshua Bunn]

Okay, so you need 62.7mm of backfocus including your filter, which leaves 36.2mm to make up.

https://agenaastro.com/william-optics-all-new-0-8x-reducer-adjustable-field-flattener-6aiii-for-gt71-gt81-z103-telescopes-p-flat6aiii.html


Your camera doesn't require 55mm of backfocus, it uses up 6.5mm of backfocus.

[Saturn488]

OK I think I finally managed to solve it.

I had elongated starts on the top left, bottom left, hardly any on the bottom and top right.

So I dialed in the backfocus and my field flat went from 7.1 to around 11.1 and now I have around 2% tilt and 16% curvature. Stars on the top left and top right are much much better. Ever so slight elongation but would never notice it.

So total backfocus I have now is 66.7mm

Time to tighten it all down and start imaging!

Thank you :)

[Joshua Bunn]

No problems, I'm glad you have it sorted.