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Asymmetric artifacts on stars

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#1 petercoxphoto

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Posted 11 August 2020 - 03:30 AM

Hi guys -

I'm using an ASI1600MM Pro on a Skywatcher Quattro-8S Newtonian. I'm seeing this sort of asymmetric burst pattern on bright stars (in this image between the top and right diffraction spikes), and wondering if anyone can point me in the right direction to find the source of it? The pattern is always on the same side of the stars, no matter where they are in the frame. 

 

assymetric_star.jpg

 

Thanks!

 

Cheers,
Peter


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#2 zenon

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Posted 11 August 2020 - 04:23 AM

Hello,

 

it seems that a protruding focuser into the OTA could be your problem.

 

Here's a video explaining what could be done about it:

 

https://www.youtube....h?v=Afi5B0B3dwA

 

Good luck

 

EDIT: You seem to have checked already whether your coma corrector is protruding into the OTA or not (here). Sorry I can't be of more assistance.


Edited by zenon, 11 August 2020 - 05:03 AM.

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#3 petercoxphoto

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Posted 11 August 2020 - 10:22 AM

Thanks for that, indeed that's not the problem. I replaced the coma corrector with the MPCC III as it was shorter and wouldn't protrude into the OTA.

 

Cheers,
Peter



#4 petercoxphoto

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Posted 11 August 2020 - 10:59 AM

Actually having looked at the video there are some things to check. Thanks!



#5 TelescopeGreg

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Posted 11 August 2020 - 01:13 PM

I was seeing the same sort of "blob" on my images, though it's off to the upper left instead of right.  Per the video, it's not the focuser protruding into the optical path - the Paracorr-2 still has about an inch to go before intruding.  I didn't try hiding the mirror clips, since the blob isn't symmetrical or triangular. 

 

I was thinking that it's just the way the main mirror was manufactured, and inherent in the scope, so for that and other reasons I ended up getting a new scope as my main OTA.  I absolutely love the new scope, but still have the Newt.  I might try revisiting things, if I get some time to play with it.  Good to know I'm not the only one with the issue.  An 8" f/5 should be able to do things that a 5" f/7 can't do, if it can be made optically clean.



#6 petercoxphoto

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Posted 14 August 2020 - 05:10 AM

So coming back to this. I thought that perhaps the problem was the focuser mounting plate protruding into the tube, as seen in this image. However, it's lower than the baffles by a fair margin, so that seems like it would rule it out - or is that a false assumption?

 

IMG_20200603_112313 (2).jpg

 

If it's not that protrusion, how do I go about troubleshooting the cause? I'm pretty sure it's not the mirror clips as the problem is not symmetric. How do I know if my mirror is pinched? Could it be the secondary heater cable, seen here zip-tied to the spider vane? I would imagine this would lead to bloating the normal diffraction spikes and not creating a new one between them.

 

Thanks!

Peter

 

 



#7 PhilHoyle

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Posted 27 August 2020 - 05:57 PM

Mirror clips.
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#8 petercoxphoto

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Posted 28 August 2020 - 06:08 PM

So I solved the problem. As suggested here, it was indeed the mirror clips holding the primary in place. Initially I thought it couldn't be as the flares were asymmetric and didn't seem to match the clip pattern. However, the problem was disguised because my secondary wasn't even close to sitting centered under the focuser tube. It was only after I got a sight tube that I realized this - so a laser collimeter alone is not good enough!

 

I could collimate the scope with the laser and get decent images, but because the secondary wasn't correctly centered some of the clips were not part of the image reaching the camera, hence the asymmetric flaring.

 

After centering it was immediately obvious that the clips were the source of the problem. I made a crude circular mask out of cardboard to confirm and placed it over the primary such that it covered the clips and the results speak for themselves.

 

This image is pre-secondary centering showing the original problem (focus is poor here, this is while troubleshooting):

frame_and_focus_5-sm.jpg

 

This one is after centering the secondary and fitting my cardboard 'donut' mask. 

frame_and_focus_70-sm.jpg

 

Thanks for the suggestions, glad to have gotten this sorted!

 

Cheers,
Peter


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#9 elmiko

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Posted 28 August 2020 - 06:15 PM

Glad you figured it out! I did the same thing with my Orion Imaging Newtonian. Beautiful stars now!

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#10 petercoxphoto

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Posted 29 August 2020 - 04:48 AM

For the benefit of others, here's a picture of the mask I made in place. It was super easy to make and done largely by estimation. I'm going to see if I can get a nicer one 3D printed, which might clean up my stars a bit more.

 

IMG_20200829_020517.jpg

 

You can see it's very rough, but it covers the mirror clips completely. I had to remove the primary cell and place it in the back of the OTA, then reinserted the primary. A quick recollimation of the primary was all that was needed to get back on track.

 

Cheers,
Peter




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