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Dark bar at the edge of my stacked image; where could it come from?

astrophotography ccd imaging
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#1 marcel.noordman

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Posted 20 September 2020 - 05:51 AM

Hi everybody,

 

Maybe someone can offer a possible explanation for the dark bar that I get at the bottom edge of my calibrated, stacked image of the flying bat?

 

Some background info on the picture: WO Star 71 with Atik 383L, Atik OAG and Atik EFW, Baader 36 mm Ha 8nm filter, 35x 300 sec subframes, properly calibrated with bias, darks and flats. Processed using PI with drizzle integration. I am imaging in a Bortle 9 area ;-(

 

I tried to process without flats, and got the same result.

I tried to move up the pick up prism of the OAG a bit, same result.

 

The attached picture is a boosted stretch of the integrated image.

I find it very difficult to remove the sharp gradient at the bottom in PI with ABE or DBE.

 

Any suggestions on where this might come from and/or how to remedy in processing?

 

Thanks in advance for your reply.

 

Flying Bat with bar.jpg



#2 Zebenelgenubi

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Posted 20 September 2020 - 08:14 AM

Check the subframe drift in the stack.  The bottom of the frames may be moving out of the FOV after star alignment so the bottom of the stacked image has fewer integrated frames than the rest of the image.  Try a different master frame for the stacking and work on polar alignment. 


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#3 marcel.noordman

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Posted 20 September 2020 - 10:00 AM

Desktop.jpg Hi Zebenelgenubi,

 

Thanks, good suggestion.

 

However, when I look at an individual frame, I can still see the bar running at the bottom (see attached single frame).

In case it is relevant; I noticed that there was a meridian flip halfway through the series as well.



#4 michael8554

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Posted 20 September 2020 - 12:39 PM

I don't believe the prism shadow would stretch the full width - is that the edge the prism is on ?

 

Is it present on a Flat ? I can see a hint of a shadow of my prism on my flats, but this removes the shadow on the Lights when I stack.

 

Try an exposure with the camera rotated 90 wrt the EFR etc.


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#5 KTAZ

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Posted 20 September 2020 - 12:47 PM

I'm guessing prism shadow.


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#6 tloebl

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Posted 20 September 2020 - 01:52 PM

Have you removed the camera assembly from the scope and see what things look like looking toward the chip?


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#7 marcel.noordman

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Posted 01 October 2020 - 07:50 AM

I think I have found a clue to why I see this band at the edge of my pictures. I found it in a document published by Baader Planetariums on their website: https://www.baader-p...gest_causes.pdf

 

Horizontal lines and stripes

Some customers told us about bright horizontal lines or stripes which were seen in some combinations wit CCD-cameras (mostly with Atik). They did not exist on the dark frames with lens-cap on the telescope, so that a problem with the camera seemed unlikely. Rotating the filter did not change the image much, the lines were perfectly aligned with the edges of the sensor. This way, the problem could be identified in the end:
Some CCD-chips have a highly reflective edge – that is on the place where the wires are connected. In combination with the Atik-filter-
wheel, the Backfocus was so that there was a forward-backward-reflection, when a filter was used. The result: bright lines on the sensor.
So, the filter made the reflections inside of the camera visible, but it did not cause them – an optical system is not designed to suppress scattered light from the camera. After the cause was found, the solution was easy: The filter was mounted with an additional 5mm spacer ring, so that the maxima of the reflection were suppressed, and the system worked as intended."




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