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Help understanding source of red vignetting

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#1 waykreid

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Posted 31 March 2020 - 12:24 PM

Hello from a new member.  You've all been very helpful to my journey already.  

I've been dabbling in AP for while but started to get serious about the hobby in the last few months.   I practice every clear night the skies give me (few and far between this winter)

I'm trying to understand the source of the vingetting and imaging halo I've been getting.   Here's me setup:

ST80 (Meade Adventurer 80mm, stock)
SVBony UHC filter
1.25" to EOS adapter
APC-C Canon DSLR

The attached image is a single sub-exposure, 90s  ISO3200.  I have heavily processed and color-adjusted it in photoshop to highlight the red/purple halo around the edges.   The fall-off beyond the halo is typical to-be-expected vingetting (from the 1.25 focuser, yes?).  I am in SW Ohio under bortle 4/5 skies.

Is this a typical issue?  Will flats alone resolve or mitigate this problem?  Is it possible these are from stray reflections inside the cheap adapter, filter, or the OTA itself?

Thanks for any assistance,
Wayland

Attached Thumbnails

  • st80_vingetting.jpg

Edited by waykreid, 31 March 2020 - 12:25 PM.


#2 scadvice

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Posted 31 March 2020 - 12:32 PM

I've gotten something similar from not keeping my red headlamp away from the telescope lens far enough.



#3 descott12

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Posted 31 March 2020 - 01:03 PM

I have very similar results when doing EAA with my IDAS LP filter. I think some of these filters can really affect the color  balance, especially when making flats. I would try it again without the filter and see what happens.


Edited by descott12, 31 March 2020 - 01:05 PM.


#4 Chucke

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Posted 31 March 2020 - 01:20 PM

I had that problem once.  Turned out it was light leaking in through the camera viewfinder.  Canon had thoughtfully already supplied a cap for the viewfinder which solved the problem.



#5 klaussius

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Posted 31 March 2020 - 01:43 PM

To add another possibility to the laundry list, I get similar vignetting just due to the fact that red is significantly noisier in my Canon 650D, and after applying flats, the outer portions noisiness gets accentuated.

 

Check if the shape of that red vignetting matches the shape of the flats (inverted of course).

 

Edit: upon rereading your post, it seems you haven't applied flats in that image. If so... the above can't be the issue. shrug.gif


Edited by klaussius, 31 March 2020 - 01:45 PM.


#6 waykreid

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Posted 31 March 2020 - 02:01 PM

I've only had the telescope and filter for a few weeks - so I haven't had too many opportunities to really investigate.  But these tips are very helpful.

I have a few unfiltered test shots laying around.  I can't seem to reproduce the distinct "weird" vignette in them when adjusting with PS or Camera RAW.  This points to the UHC filter as the main source, but the light-through-viewfinder may be a secondary culprit.   Thanks for the leads.



#7 Der_Pit

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Posted 01 April 2020 - 08:59 AM

APS-C through 1.25" is for sure vignetting substantially.  If I read your post correct you are (so far) not doing any flat correction?  You definitely should.....

For the existing data, how are you trying to correct?  For sure it would have to be done channel by channel separately....


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#8 waykreid

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Posted 01 April 2020 - 12:29 PM

So far in my AP journey, I have not been doing flats. Mostly due to my limited time and just trying to learn/improve the very basics. My results have either been cropped to deal with the vignette, manually manipulated in PS, or I've just called it "ok" and left them in, as a testament to my beginner status.  The time to make good flats has arrived, though. 

I did some experimenting and the red fringing is definitely coming from the filter somehow.   Just eyeballing through it at a bright light, the "purple" hue is more pronounced near the edges of the filter (due to incidence angle, I think).   Then I also notice extra light entering the filter reflected off the somewhat shiny rim of the filter itself.  I took some test shots to locate weaknesses in my light train. 

In this shot, I held a bright light into the scope through the filter.   Then adjusted the result to amplify the rings I saw.  I see a "thread" pattern in the outer ring which makes me thing stray reflctions or light from the barrel of the telescope adapter have something to do with it.   But maybe not.  This is not a scientific experiment.  

filter_rings.jpg

Then, curious about light leakage through the eyepiece, I capped the scope.  Then took a 90sec exposure (3200 ISO) shining a bright light into the eyepiece.  This is the result straight from the camera.   It was eye-opening seeing how not light-proof my little camera is.   Time to cover that eyepiece.   I use the flip out LCD for astro anyway.   

eyepiece.jpg

 



#9 klaussius

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Posted 01 April 2020 - 12:43 PM

You may have the filter backwards if possible, or its anti-reflection coating may have flaws near the edges. It's been reported to happen in low quality filters.



#10 nimitz69

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Posted 01 April 2020 - 05:19 PM

regardless of the issues with your filter taking cal frames EVERY session is a basic fundamental AP skill ... and if properly prepared they take all of about 10 mins.  Your darks & bias should be taken at other times.

Not having the time to take them is quite frankly a poor excuse, you simple need to budget the time to do so.  They make a huge difference in your final image and taking shortcuts now as you are learning and developing basic habits means you’re likely to not cnly continue to do so but will also take other shortcuts as well ...

there are no shortcuts designed to make you a better imager ... dalek12.gif




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