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Old 26-10-2019, 04:30 PM
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Ant0nio (Tony)
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Any ideas on this artifact?

Hi all, I shot a sequence of 12 frames of M78 last night & this gradient showed up after pre processing. It's in every frame. I've deliberately over stretched the image to make it obvious.
The night before I shot the same sequence, same setup, same target & no artifacts were present. Details are:

12 x 600sec frames, bias & darks subtracted
Scope - William Optics FLT98
Focal reducer/flattener - William optics AFR IV
Camera - SBIG 8300C
Filter - Baader IR UV

Any ideas what has caused it?
Thanks in advance,
Tony
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Old 26-10-2019, 06:53 PM
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Merlin66 (Ken)
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Top left of frame?
There's a spectrum and a secondary reversed spectrum usually caused by extraneous light and a glass edge.....
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Old 26-10-2019, 09:56 PM
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Ant0nio (Tony)
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Quote:
Originally Posted by Merlin66 View Post
Top left of frame?
There's a spectrum and a secondary reversed spectrum usually caused by extraneous light and a glass edge.....
Hi Ken, thank you for your expert analysis, I spent hours trying to find some clues to this on the internet to no avail. Your description fits though,
I thought it might have been a finger print on the filter so I took it off to clean it & noticed the glass rattled a bit. The retaining ring had loosened,
so maybe my local light pollution (neighbors floodlights) caught an edge of the filter glass.Sounds feasible.
I think this issue is resolved thanks to your wisdom.
Many thanks,
Tony
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Old 27-10-2019, 02:36 PM
ericwbenson (Eric)
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Hi Tony,

What you have there is diffraction of an off-axis star by the anodized barrel of a tube inside of your OTA/focuser/imaging train. Starlight impinging at grazing angle on the somewhat smooth wall of a tube (with microgrooves from machining) produces that effect. You can possibly see it with your naked eye by observing the daytime sky (or a bright screen in a dark room) without the camera/filter wheel in place. (don't look anywhere near the sun obviously).

I had to solve this problem with my telescope. See the bottom of this thread here:
http://www.iceinspace.com.au/forum/s...d.php?t=105990


Good luck,
EB
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Old 27-10-2019, 08:34 PM
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Ant0nio (Tony)
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Quote:
Originally Posted by ericwbenson View Post
Hi Tony,

What you have there is diffraction of an off-axis star by the anodized barrel of a tube inside of your OTA/focuser/imaging train. Starlight impinging at grazing angle on the somewhat smooth wall of a tube (with microgrooves from machining) produces that effect. You can possibly see it with your naked eye by observing the daytime sky (or a bright screen in a dark room) without the camera/filter wheel in place. (don't look anywhere near the sun obviously).

I had to solve this problem with my telescope. See the bottom of this thread here:
http://www.iceinspace.com.au/forum/s...d.php?t=105990



Good luck,
EB
Thank you Eric, that is rather conclusive. The artifacts in the two images are virtually identical. I'm using a set of the cheap Chinese spacers for back focus & the inside surfaces are really quite glossy,
I'll have a go at coating them with flat black & failing that replace them with some of good quality.
Many thanks Eric,
Tony
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Old 27-10-2019, 08:46 PM
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Ant0nio (Tony)
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My sincere thanks to both Ken & Eric for taking the time to help me resolve this. Truly a testimony to the value of the collective wisdom available via the IIS forums.
Even though the insight offered by Erics' experience seems to have identified the cause, Kens' contribution has undoubtedly helped me avoid another lurking issue.
Thanks again to both of you & clear skies.
Best,
Tony
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