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Purple frame/rectangle in images like burn-in or perhaps shadowr/reflection cast?

DSLR DSO Astrophotography
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#1 Elessar-lad

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Posted 20 October 2021 - 10:40 PM

Hello everyone!

I hope you guys are doing great, I wanted to say thank you for this awesome community. I've learned a lot here through the years.

 

Recently I stumbled into a problem I can't figure out. I seem to have some sort of burn-in/reflection/shadow on the long exposures pictures I've been taking.

 

I haven't changed anything on the camera side, same telescope and filter. I thought it was some sort of reflection from the full moon or something, but not the case. I also thought it was the viewfinder that has a light leak of sorts, or even a gap between the camera mount and sensor. I wrapped out everything with a cloth, nothing changed. Another thing I tried was to put a delay between the mirror flip and the actual opening of the shutter, 2, 4, 6 seconds, and no change. It's really driving me crazy!

 

This ghost rectangle has proven to be hard to remove on the final stack, I suppose that due to the color tint the stacking software sees it as a nebula.

 

Any help is greatly appreciated.

 

My gear:

 

Canon T4i with the IR cut filter removed
Sharpstar 61EDPHII
Optolong L enhance
Orion 1x field flattener.

Attached Thumbnails

  • issue 4.jpg


#2 sharkmelley

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Posted 20 October 2021 - 11:10 PM

It's caused by shadows from the mirror box.  It can be mitigated to some extent by using flats (you may need to experiment with different ways of taking flats) but flats will never completely deal with the issue because there are additional problems such as light being scattered off the sides of the mirror box - especially the parked mirror.

 

Mark


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#3 galacticinsomnia

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Posted 20 October 2021 - 11:17 PM

Hello everyone!

I hope you guys are doing great, I wanted to say thank you for this awesome community. I've learned a lot here through the years.

 

Recently I stumbled into a problem I can't figure out. I seem to have some sort of burn-in/reflection/shadow on the long exposures pictures I've been taking.

 

I haven't changed anything on the camera side, same telescope and filter. I thought it was some sort of reflection from the full moon or something, but not the case. I also thought it was the viewfinder that has a light leak of sorts, or even a gap between the camera mount and sensor. I wrapped out everything with a cloth, nothing changed. Another thing I tried was to put a delay between the mirror flip and the actual opening of the shutter, 2, 4, 6 seconds, and no change. It's really driving me crazy!

 

This ghost rectangle has proven to be hard to remove on the final stack, I suppose that due to the color tint the stacking software sees it as a nebula.

 

Any help is greatly appreciated.

 

My gear:

 

Canon T4i with the IR cut filter removed
Sharpstar 61EDPHII
Optolong L enhance
Orion 1x field flattener.

I have the exact same rectangle in my modified camera, canon M6II. 
Whoever modded your cam should have a good idea of why this is there.  It sort of bothered me at first but found that as mentioned, you can mitigate most of it with a good set of flats or false flats in post.  For me having a lot of integration time 5hr + also helps substantially.

Clear Skies !
 


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#4 Elessar-lad

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Posted 21 October 2021 - 08:24 AM

It's caused by shadows from the mirror box. It can be mitigated to some extent by using flats (you may need to experiment with different ways of taking flats) but flats will never completely deal with the issue because there are additional problems such as light being scattered off the sides of the mirror box - especially the parked mirror.

Mark


Thanks for confirming. I heard of people completely removing the mirror, this is my dedicated astro camera or paint the mirror box with matte black paint?

Thanks!

#5 michael8554

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Posted 21 October 2021 - 08:40 AM

Also blank off the eyelevel finder window, extraneous light gets in there.


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#6 Elessar-lad

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Posted 21 October 2021 - 03:50 PM

I have the exact same rectangle in my modified camera, canon M6II. 
Whoever modded your cam should have a good idea of why this is there.  It sort of bothered me at first but found that as mentioned, you can mitigate most of it with a good set of flats or false flats in post.  For me having a lot of integration time 5hr + also helps substantially.

Clear Skies !
 

Did you ended up finding what caused it? I tried blacking out the inside of the mirror box with fine strips of black electrician tape and taped out the viewfinder, same issue. I have a feeling the sensor is too far (deep) from where it's supposed to be and the frame around the sensor where the shutter is, creates some sort of vignetting. I even took a 120 FPS video to see if the secondary mirror was blocking for a split second the sensor when it finishes taking the picture but in slo-mo you can see the shutter closes first, and then the main and secondary mirror follow. That leaves me with not many options. 

I'm taking the camera apart to see what could be the problem. I should add I didn't have this issue before, just in the last 3 days which makes it more strange. 

 

Regards, 



#7 galacticinsomnia

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Posted 21 October 2021 - 08:59 PM

Did you ended up finding what caused it? I tried blacking out the inside of the mirror box with fine strips of black electrician tape and taped out the viewfinder, same issue. I have a feeling the sensor is too far (deep) from where it's supposed to be and the frame around the sensor where the shutter is, creates some sort of vignetting. I even took a 120 FPS video to see if the secondary mirror was blocking for a split second the sensor when it finishes taking the picture but in slo-mo you can see the shutter closes first, and then the main and secondary mirror follow. That leaves me with not many options. 

I'm taking the camera apart to see what could be the problem. I should add I didn't have this issue before, just in the last 3 days which makes it more strange. 

 

Regards, 

My T6s doesn't show this phenomenon and its full spectrum, but has a mirror, Canon m6ii is mirrorless, full spectrum and no mirror, but has the problem.  My unmodded M6ii, is factory defaulted and does not have this issue.

I don't really have any problem with it because most of it gets removed in post, and not sure what causes it.  I theorized it was some kind of sensativity with the outer edges of sensor since it seems like these areas have a higher exposure than the center areas.  No answers but it doesn't bother me much.  Would like to understand why also though...

Clear Skies !!



#8 KLWalsh

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Posted 23 October 2021 - 10:43 AM

As others have said, this may be mirror box vignetting.
But I’m surprised it’s so asymmetric. I’m wondering if your optics are misaligned somehow.
You should shoot some ‘sky flats’ - point the scope straight up at a clear blue sky near dusk, then take some photos with the histogram of the flat sky illumination at around 50%, adjusting the ISO as needed. The result should be a flat uniform image with fairly even vignetting at the edges and corners.
If it’s noticeably asymmetric, then check your imaging train to see what causing the asymmetry. Something maybe be out of collimation.

#9 galacticinsomnia

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Posted 23 October 2021 - 06:38 PM

I think this is a normal size effect from being modded.. 
Can't be a mirror box on my account, not to try an Hijack the thread, but am posting in order to show it happens with mirrorless and I don't have a mirror box.
Possibly canons color science ;)  I don't let it bother me, it isn't evident in final output.

Here is a recent dark frame, edited to more easily reveal the similar pattern.




2021-10-23_163157.jpg

 

Clear Skies !!

 



#10 Elessar-lad

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Posted 26 October 2021 - 09:23 AM

Hi everyone,

Thank you all for your input. I investigated more about this and the whole sensor assembly was tilted to one side, likely it was me when I dropped the camera a few days ago.... and I completely forgot about that event. I noticed the flash mount is completely pushed in, likely the one that took the blunt of the fall.

Since then, I've taken the camera apart and readjusted the sensor, I don't know if this is the right method, I installed my field flattener on the camera and let the camera rest on the table with the flattener facing down, then leveled my table, finally, using a bubble level on top of the back of the sensor I adjusted the screws so the sensor can be parallel to the table. The big problem was that I didn't know how close or far away was the sensor from the front to determine the focal point. A lot of trial and error, and I can focus fine with my Canon and Nikon lenses. I also ran into no power issues, I thought I fried the motherboard, thankfully I didn't, it looks to me the issue was the ribbon-like cables are super finicky, this no power issue delayed everything for maybe 3 days. I took the camera for a spin this weekend and I can focus and take pictures just fine now.

Weather has been crap so haven't been able to test for real, using the telescope in my basement doesn't show that purple frame anymore, but who knows for real.

I'll keep you posted.
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#11 Elessar-lad

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Posted 02 November 2021 - 09:58 AM

Quick update. 
 
After I made the changes the purple frame is still there, however, not as noticeable and distracting. The flat frames are taking care of it. See the link below. 
 
https://imgur.com/a/VlqXRKi
 
Thanks everyone for your help!




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