Author Topic: Annoying background brightening  (Read 1146 times)

Offline bulrichl

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Annoying background brightening
« on: 2017 July 05 08:35:34 »
Hello,

I have an issue with the processing of images of the Helix nebula (NGC 7293). These have been shot with a Takahashi FSQ 106 N and a Canon 600D (modified). The camera is connected via a CA-35 (TS 102) adapter and a Wide T-Mount adapter (with Canon bayonet joint) to the refractor.

At first I guessed that a bad flatfield correction (with sky flats) was the cause, but when the flatfield correction with a LED flatfield box produced the same result, I rule that out. This is the integration of 104 light frames (360 s each) after calibration (MasterBias, MasterDark, MasterFlat; the flat frames have been calibrated with a flat-dark before integration), debayering, aligning and cropping (because of dithering). It's the not further processed, linear image, auto-stretched by Pixinsight:

http://ge.tt/4CeMwfl2
(Image 1: NGC 7293)

In the outer region of the image you see a brightening of the background, mainly in the red channel (about oval ring-shaped) but also in the green channel (at top and bottom, pulvinated). The blue channel is flat.

I suspected that the cause of the brightening is a reflection and illuminated into the refractor with a small LED lamp:

http://ge.tt/4CeMwfl2
(Image 2: Reflex)

Clearly visible is a ring-shaped reflection which is caused by the Wide T-Mount adapter.

http://ge.tt/4CeMwfl2
(Image 3: Wide T-Mount adapter)

I can imagine that the relative bright object NGC 7293 near the image center also produces a reflection that in turn is reflected by one of the lenses in the direction of the sensor, but maybe there are other possibilities.

My questions:
1) What is the cause for the brightening in the red and green channel?
2) How can I eliminate this cause?
3) How can I correct the present images?

By now this result puts me off astrophotography thoroughly. I'd really appreciate your ideas.

Regards, Bernd

Offline Greg Schwimer

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Re: Annoying background brightening
« Reply #1 on: 2017 July 05 09:54:13 »
At quick glance what I see in your image is sky gradient that is heavier on the right and tapers down toward the left. This is typical and should be easily remedied. I would start with a good application of the DynamicBackgroundExtration (DBE) tool. DBE should be able to fix that with no problem. I've attached a DBE process icon that should get you started. The points were based on your image.

If you are concerned about reflection of the adapter causing this then consider using a flat, black paint to mitigate that.

- Greg
Scottsdale, Arizona, USA

Offline bulrichl

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Re: Annoying background brightening
« Reply #2 on: 2017 July 06 09:48:58 »
Hi Greg,

thank you very much for your quick response.

Actually I could not imagine that this artifact was correctable and so I was somewhat frustrated. Your process icon already did the main work although the integrated linear image that I dropped was only in JPG format, 886 x 584 - I am really perplexed that your process icon worked so well anyway.

I then fiddled with the DBE parameters (had not to do much starting with your settings). After DBE there always was still some uneven background left at the margins. This could be removed by a subsequent application of AutomaticBackgroundExtractor (ABE).

Then by chance I found out that - with this image - the DBE processing could also be omitted: the application of only one ABE step achieved complete removal of the background brightening! The important parameter was 'Function degree' in the section 'Interpolation and Output'. It had to be set quite high, and when I "dared" to try 8 or 10, the brightening was gone completely! It was like magic.

Again, thank you for pointing me to the right direction. I am very happy with this result now (still linear image, auto-stretched in Pixinsight):

http://ge.tt/2Sm2wgl2


As to the supposed reflection: the ring-shaped surface of the Wide T-mount adapter is black, but not flat. I would not like to paint this surface because if the paint doesn't adhere optimally, the lenses of the field corrector could be contaminated. To tell the truth this doesn't seem important to me any longer, now that the artifact can be removed that easy by image processing.

Regards, Bernd

Offline Greg Schwimer

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Re: Annoying background brightening
« Reply #3 on: 2017 July 06 12:56:18 »
Bernd,

Your result looks good!

I understand you do not wish to paint the part. An alternative would be to sand it lightly with fine grit sandpaper. This should result in a matte finish.
- Greg
Scottsdale, Arizona, USA