9 replies to this topic

[sulamar]

Hello,

I have the William Optics gt 71 scope and I'm using it with the WO adjustable field flattner. My camera is ZWO asi 294mm pro.

 I've been battling with these starshapes for a while. My backfocus should be correct and I have even tried adjusting it to different lenghts without any improvements.

This is an example of the lower right corner and the other corners follow the same pattern but are a bit different.

Does anyone have any ideas what could be causing these shapes or is this just normal for refractors?

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[TheSheriff]

I'm no expert diagnosing optics issues and it's hard to make conclusions based on a single corner crop.  But that said, I did own a GT71 and researched it quite thoroughly.  So based on that as a starting point for you...

 

If your stars look good in the center and your extreme corner stars look uniformly bad, It may still may be a back focus issue.  The GT71 can be very sensitive to back focus errors to the tune of .5 mm or less.  It may also be your specific scope may not be up to a 4/3 sensor and have well corrected stars all the way to the corners.   Acceptable star shape is subjective, but to summarize what I have read on the GT71 (FWIW)...  It is best suited to a 1" sensor and most people have no issues and are very happy with it.  Many, but not all have had good luck with a 4/3 sensor, though in some cases you may need to crop the extreme corners.  A lucky, much smaller group have acceptable results with a APS-C sensor.  I was not one of those.  In my case, by the time I cropped, I was down to the smaller sensor size FOV which defeated the wide field I wanted.

 

If you have good (or better) corners mixed with bad once your BF is dialed in, you may have tilt in your image train.

 

And of course, a mixture of all the above is possible.

[CharLakeAstro]

I have the same GT71 with a Flat6Aiii adjustable flattener, and I use the same camera.

These star shapes are not normal.

Q1- what model adjustable flattener are you using?

Q2- is the flaring direction the same in all 4 corners?

Q3- can you post each corner sample?

[sulamar]

I have been thinking that it could have something to do with tilt.

Im using the William Optics Adjustable Flattener Reducer Flat6AIII.

Here is a four images one from each corner.

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[CharLakeAstro]

Those images point to the reducer spacing being incorrect

[900SL]

Looks like backfocus.

Run a fits file through software that can analyse tilt, eg Siril

One issue with the WO adjustable flattener is that the 'adjustable' part is a screw thread and ring, and this can actually introduce tilt in some cases. This has been covered in previous threads.

Some workarounds if this is tilt..

Tilt adapter

Wind the adjuster fully in and use conventional spacer rings to get backfocus

If the corner is only an issue when you pixel peep, ignore it and start imaging, they tend to average out in a stack

Edited by 900SL, 26 September 2023 - 02:00 AM.

[osbourne one-nil]

I had this issue with the ZS61 and it was radically improved when, instead of using the adjustment ring of the reducer, I just used my own spacers. There's a lot of slop in the threads I think so best to have everything clamped together as much as you can...in my experience!

[osbourne one-nil]

I should have read the post before more thoroughly....or at all!

 

Complete agreement. 

[sulamar]

Thanks for the answers! I will look into these things and try to fix the problem. Now I can try all these things if one doesn't work.

 

Appreciate the help, thanks!

[Av8ForFun]

I’m just starting to use my GT71 and 6AIII. I have the same thing in my data. I just haven’t enough nights to work it out. When I get a rare night out these days, I just use my tried and true setup. If you find a solution, please come back and share it.