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Samyang 135, Canon 6d, problems with flat background.

optics equipment dslr astrophotography
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#1 qendis

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Posted 13 January 2021 - 04:45 PM

Hello!

For quite some time, I have been looking for a solution to the problem of a strip, which appears at the bottom of the picture. The strip is visible when the histogram is streched.
I use the following setup: Canon 6d, Samyang 135mm (both new). However, I encountered the exact same problem with another similar setup, namely a used Samyang 135 and a Canon 6d.
I tried to use various types of flats (proper exposition, overexposition, long exposure time). Nothing seems to help.
I stacked the photos in Siril, DSS, APP and PIX and the result is still the same. I can't find a solution anywhere.
Do you think the problem is with the camera or the lens?

I'd appreciate any advice or suggestions you could give me.

Best regards,

result.jpg

 

resul2t.jpg



#2 Jim Waters

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Posted 13 January 2021 - 05:01 PM

Does it happen on other lenses?  You should give it a try.  What about different ISO's?  Have you tried PI 'CanonBandingReduction'?

 

Is the banding in the RAW images?  How do you take your Flats?


Edited by Jim Waters, 13 January 2021 - 05:13 PM.


#3 photoracer18

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Posted 13 January 2021 - 05:23 PM

Personally I would take another series with the camera at a different orientation because that line does not look straight.



#4 Cfreerksen

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Posted 13 January 2021 - 05:32 PM

Are you talking about the horizontal line at the bottom of the image? Are you plate solving after a meridian flip? Or maybe you are seeing a problem that has been documented in Canon cameras. Search "Canon Banding" I don't use DSLRs often so you might want to take a look into the posts.

 

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#5 t-ara-fan

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Posted 13 January 2021 - 07:48 PM

That line is the shadow of the mirror on the sensor.  It is most pronounced with fast lenses = fatter light cone coming into the sensor.

 

https://www.cloudyni...shadow-or-what/

 

Solutions: stop down the lens, crop the image, remove the mirror from the camera.

 

I just crop. My f/1.4 lens isn't that great off axis when wide open.


Edited by t-ara-fan, 13 January 2021 - 07:48 PM.

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#6 Jim Waters

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Posted 13 January 2021 - 07:54 PM

Here's an image with the same setup.  I had no banding in the RAW images and no issues with processing.  BIAS, Flats and Darks were taken.

 

https://flic.kr/p/2grP49d

 

EDIT - Shot at f/2.8


Edited by Jim Waters, 13 January 2021 - 07:55 PM.

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#7 michael8554

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Posted 14 January 2021 - 05:18 AM

It's not clear to me whether the bright-up is on the Raws or just on the stacked image ?

 

I would carefully examine a stretched Light and a stretched Flat.

 

If those are clear then possibly the bright-up is due to the stacked result of images that are drifting slightly in Dec over the length of the session.


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#8 qendis

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Posted 14 January 2021 - 10:49 AM

Does it happen on other lenses?  You should give it a try.  What about different ISO's?  Have you tried PI 'CanonBandingReduction'?

 

Is the banding in the RAW images?  How do you take your Flats?

There is no banding in the light frames.
I make flats from 1200 / s to 1.6 s. I am just trying different variants.



#9 qendis

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Posted 14 January 2021 - 11:11 AM

Are you talking about the horizontal line at the bottom of the image? Are you plate solving after a meridian flip? Or maybe you are seeing a problem that has been documented in Canon cameras. Search "Canon Banding" I don't use DSLRs often so you might want to take a look into the posts.

 

Chris

Thank you for your help. I did as you say. I have some new ideas.

Chris



#10 fishonkevin

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Posted 15 January 2021 - 08:57 AM

Do you block the cameras viewfinder?  It may be a light leak.



#11 qendis

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Posted Today, 09:02 AM

Do you block the cameras viewfinder?  It may be a light leak.

Yes, I block the camera viewfinder. I am using black insulating tape.



#12 qendis

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Posted Today, 09:09 AM

Welcome! I removed the strip from the photo.
I used a flat 0.8 s / ISO 100. Thus, the flat fixed the problem.
Thank you for all your help.
Greetings from Poland!

 

ostateczna_szkola_macka.jpg

 




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