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WO GT71 star issues

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18 replies to this topic

#1 RPF

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Posted 09 December 2023 - 11:19 PM

I have a relatively new William Optics GT71 scope with the associated .8x flattener and noticed some seriously odd artifacts on the stars. Especially the bright ones. They look like little diffraction flares.  It seems like the star shapes are OK in general so I don't believe this is a pinched optics situation.  Could this possibly be some internal obstruction creating diffraction or maybe some protruding screw somewhere?  Is anyone familiar with this scope and issue?

 

Is there a fix for this particular GT71 scope?

 

Thanks,

Bruce

 

M45_2023-12-09_issue.jpg



#2 Koop

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Posted 09 December 2023 - 11:30 PM

Wow, I don't know what to think here. I just had fisrt light on my GT71 with the WO 0.8x reducer flattener and didn't see anything like that.

M45_Pleiades-RGB-session_1-3-St-1GMP.jpg



#3 mehdymo

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Posted 09 December 2023 - 11:39 PM

Should be pinching. Did you use heater strap? That may cause this if it is too hot. Try it without the heater to see how it looks like.



#4 RPF

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Posted 09 December 2023 - 11:43 PM

Hi Mike,

 

Thanks for the reply.  Actually, I think I am seeing the same issue in your image albeit not as obvious possibly due to the slightly stronger contrast. I zoomed in on my stars for M45 quite a bit.

 

Screenshot 2023-12-09 223907.jpg



#5 RPF

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Posted 09 December 2023 - 11:47 PM

Reading elsewhere, others claim pinching has a distortion effect on the star shapes themselves.  The stars are mostly round here as opposed to triangle shapes others say are representative of optical pinching.  I am inclined to think this is some obstruction.

 

My observing conditions were ideal for this image. The temps were about 65F and the humidity was very low.

 

@mehdymo, is there some specific thing you see in the image that leads to your pinching conclusion?

 

Thanks for the reply.


Edited by RPF, 10 December 2023 - 08:53 AM.


#6 RPF

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Posted 10 December 2023 - 11:12 AM

So, looking online further, I get more confused as to what pinched optics looks like. My image above looks like it is generated from some obstruction but I just got through looking down the tube toward the sensor and I don't see anything. So, going back to the pinched optics idea.  Is it possible to have this asymmetrical diffraction pattern with pinched optics? Would it be possible that pinched optics produce a diffraction spike like the one I show?  I don't have much experience with this issue.

 

Some say there are screws (in my case 4) that you can loosen at the front of the scope  to alleviate the problem. Anyone with a WO scope have any experience with this?

 

Thanks



#7 Rotciv.greene

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Posted 10 December 2023 - 10:00 PM

Are these images processed at all? What do the stars look like in a linear state?

#8 RPF

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Posted 10 December 2023 - 10:22 PM

Above was the processed image but the subs all have the same issue. Here is a single raw prestretched luminace frame of the same image.

 

 

Thanks

 

Screenshot Luminance 2023-12-10.jpg


Edited by RPF, 11 December 2023 - 01:06 AM.


#9 mehdymo

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Posted 11 December 2023 - 03:46 AM

Don't touch the collimation screws until you ask WO first and you are 100% sure about what you are doing. 



#10 900SL

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Posted 11 December 2023 - 05:44 AM

What is the rest of your image train? Any filters? 

It looks like a diffraction pattern caused by something protruding into the light cone. Anything at all visible in the lens cell?

 

Older vixen scopes show a six pointed pattern caused by small spacer tabs 



#11 900SL

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Posted 11 December 2023 - 05:48 AM

This seems a similar issue https://stargazerslo...he-final-straw/



#12 RPF

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Posted 11 December 2023 - 09:35 PM

I had contacted AgenaAstro Products, where I bought the scope and they were very responsive and helpful. They actually contacted William Optics and John from AgenaAstro told them about my issue. (Thanks to John). They confirmed that I should do the procedure according to this video: https://www.youtube....nnel=Astronotna

 

I had no visible obstructions down the scope.  The filters were the same as I have used on my ZS61 and the issue existed with all filters. The GT81 issue mentioned above can probably be fixed the same way.

 

I did that tonight. Here is the result. The left is guided and the right is not guided as I needed access to the adjustment screws and my guide scope was in the way. Everything else was the same.  Comments?

Attached Thumbnails

  • Lens pinch GT71-75.jpg

Edited by RPF, 11 December 2023 - 10:25 PM.


#13 Koop

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Posted 12 December 2023 - 01:43 PM

The YouTube video of the adjustment looks simple enough, but it's the ZenithStar - a doublet. The GT71 is a triplet and has three sets of screws. Which ones get the tweak?



#14 RPF

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Posted 12 December 2023 - 06:26 PM

I adjusted the 4 screws that are located around the front end of the scope. I show one of them in the picture (Yellow arrow). I turned them counterclockwise about 1/8 or slightly more of a turn.

 

WOGT71_75-s.jpg


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#15 RPF

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Posted 13 December 2023 - 12:40 PM

I was contacted by a WO service tech who sent me the following information. That was very kind of them. My steps above were similar but I never tightened the screws back as suggested here. My diffraction issues are mostly gone now. As pointed out, the picture  is for the doublet but the triplet you do the same thing for WO GT series scopes- pick the screws closest to the scope end. But I would be careful to not do things in the extreme like over tighten. I only turned things to loosen about 1/8 of a turn.

 

WO pinced optics info.jpg


Edited by RPF, 13 December 2023 - 12:41 PM.


#16 StevenBellavia

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Posted 23 February 2024 - 10:19 AM

FYI:  This is a similar thread, I started about 6 months prior to the OP's, which was actually a response to a much older thread.

 

https://www.cloudyni...ar-flare-tests/

 

And there are many others on this very topic, for the GT-71 and other scopes.

 

I am not convinced it is entirely the lens cell screws.  Loosening the screws helped a little on my GT-71, but I think there is something going on with the baffles, or something else internal to the OTA.

 

Overall the William Optics GT-71 is a magnificent telescope.  The resolution of details, flat field (using the flattener) and complete absence of chromatic aberration, is amazing, and worth the little bit of star-flaring, which I think I (and you) have tamed to an acceptable level in my (and your) scope.

 

But if I have free time, I might open it up and see if something in the OTA is causing it, like a baffle or something. Baffles should ideally be knife-edge. A "soft" edge can cause scattering. A larger imperfection might show up as what we are seeing (?)

 

Steve


Edited by StevenBellavia, 23 February 2024 - 02:11 PM.


#17 Kevin_A

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Posted 23 February 2024 - 12:44 PM

Most of the time you can just loosen the appropriate single screw that is affecting it by referencing the screw that is in the same location as the abberation. WO and many others seem to be shipping lots of scopes that are overly tight for winter imaging. In the summer months most tight lens cell issues do not show up.


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#18 StevenBellavia

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Posted 23 February 2024 - 01:54 PM

Most of the time you can just loosen the appropriate single screw that is affecting it by referencing the screw that is in the same location as the abberation. WO and many others seem to be shipping lots of scopes that are overly tight for winter imaging. In the summer months most tight lens cell issues do not show up.

Maybe they overtighten a little to help it get here safely from overseas?


Edited by StevenBellavia, 23 February 2024 - 02:11 PM.


#19 StevenBellavia

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Posted 23 February 2024 - 01:59 PM

I am 99% sure this wasn't causing the flaring issue, but it always bothered me that there was a shiny screw sticking up, visible in the scope.

Too far down the tube (actually part of the focuser) to catch the light cone, but I had to fix it.

 

I removed it, added a tiny plastic washer (that I had to open up just a little), and added more black (the tip was already black).

 

As I said, I don't expect any change (and I am perfectly happy with the telescope as it is now), but being OCD, I had to do this.

Now it is essentially flush with the surface, still serving its purpose, but out of my view. when I look down the scope from either direction.

 

(:

 

Steve

Attached Thumbnails

  • GT-71_screw-01.jpg
  • GT-71_screw-washer-01.jpg
  • GT-71_screw-washer-ink-01.jpg
  • GT-71_screw-washer-ink-installed-03.jpg

Edited by StevenBellavia, 23 February 2024 - 03:48 PM.



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