Author Topic: Markarian's Chain with DSLR  (Read 7060 times)

Offline Ignacio

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Markarian's Chain with DSLR
« on: 2013 April 19 08:41:25 »
Hi everyone,

I wanted to share this result with you, which was the subject of another discussion/thread related to subexposure weighting in ImageIntergation. The thing is that, when your subexposures are subject to large variations in gradients (ie, when the session starts and/or ends at low elevations, and more so in light polluted skies), then straight SNR weighting can be off. As it turns out, SNR evaluation weighting uses dispersion as a proxy for signal, which picks up gradients as "useful" signal. If the gradient is pretty much the same thru the session, then, it doesn't matter. But if there are significant variations, then poor frames with more intense gradients get overweighted in the final stack.

The Markarian's Chain is a low elevation and dim object from my location. Hence, I had to extend the session way into both ends of the horizon haze. When processing the series, I noticed the problem, which has surfaced before when doing comet images approaching the sun. To solve this problem, I had to flatten the subexposures, by using a linear model in ABE, before registration. This removes most of the "bad signal", and can be done quite easily (as suggested by Carlos),  using an image container.

With this approach I got about 5% lower noise, and lower fwhm, in the stacked image. Also a smoother, less lumpy background in the final processed image.

Anyway, here it is in full resolution, 4.5 hours under pretty poor seeing conditions. Taken with an AP130GT at f/5 and a cooled canon 1000D.

http://www.pampaskies.com/gallery3/var/fulres/MarkarianChain_hires.JPG

cheers
Ignacio

Offline Jan

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Re: Markarian's Chain with DSLR
« Reply #1 on: 2013 April 19 11:18:38 »
It is lovely result, nice details! Otherwise - your image has a green cast (SNCR recommended:-).

Actually you may try to load your image into Aladin and you will see that you may have even some very distant objects on your picture! Last year, on my shot of Markarian I have found one QSO at the distance of 11,5 billion LY (short focal lenght and problems with collimation, but just to illustrate see link http://astrofotky.cz/~Oko1/1334592584.jpg)



Offline cs_pixinsight

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Re: Markarian's Chain with DSLR
« Reply #2 on: 2013 April 19 11:37:50 »
Ignacio, this is another wonderful result.  If you hadn't mentioned all the issues you worked through nobody would have known.  I followed your previous thread and will be attempting your workaround to see if it improves my images taking from my light polluted back yard.  I'm guessing it will.

Craig

Offline Ignacio

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Re: Markarian's Chain with DSLR
« Reply #3 on: 2013 April 19 11:57:04 »
Thank you, Jan and Craig, for your feedback. Glad you like it.

Jan: I don't see the the green cast in my monitor (the image background has an SCNR pass). What I do see is some minor residual color lumpiness, but with no particular cast. Monitor calibration is always an issue, but judging by the histogram, it should be fine (on average). I will look into Aladin, it sounds very interesting. Thanks!

best
Ignacio


Offline Alejandro Tombolini

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Re: Markarian's Chain with DSLR
« Reply #4 on: 2013 April 19 19:46:24 »
Hi Ignacio, excellent image and very good framing.

I also see what Jan mentioned, and is not possible to remove with SCNR. However it can be removed working in chrominance in the layers where that noise is located.

In the next screen shot is exagerated the stretching to see the colored spots in the background.



First I use Gerald formula to correct dark spots



Then generate a range mask to protect high signal areas that has not to be afected by the noise reduction. 



Finally apply ATWT deleting layers 5,6 and 7 in chrominance protecting the galaxies with range mask




Saludos, Alejandro.

Offline Ignacio

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Re: Markarian's Chain with DSLR
« Reply #5 on: 2013 April 20 09:40:22 »
Hola Alejandro,

Yes, that is the color lumpiness I was referring to. Thank you for sharing these techniques, for which I was totally unfamiliar. I will definitely try it on the 32-bit fits file, and repost.

So, Gerald's formula replace pixels below certain value (0.12 in this case), with an average of the median and the original pixel value, right?

best
Ignacio
« Last Edit: 2013 April 20 09:51:29 by Ignacio »

Offline Alejandro Tombolini

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Re: Markarian's Chain with DSLR
« Reply #6 on: 2013 April 20 10:22:43 »
Hola Ignacio,

Yes, the formula evaluate if the value of the pixel is bellow of a detailed value, in that case replace such value by the median of the corresponding channel, if not lets the value as is.
But I recomend you see Gerald's explanation in its video that is very very clear.

Saludos. Alejandro.

Offline marekc

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Re: Markarian's Chain with DSLR
« Reply #7 on: 2013 April 20 22:18:08 »
Hi Ignacio,

That's a really nice Markarian's Chain. It made a good `first impression' on me when I opened it, and it was fun to look at for an extended time! Very pleasant image to examine! Thanks for posting it.

- Marek

Offline Ignacio

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Re: Markarian's Chain with DSLR
« Reply #8 on: 2013 April 21 07:37:54 »
Thanks a lot, Marek. Glad you enjoyed it.

Ignacio

Offline boris sputnik

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Re: Markarian's Chain with DSLR
« Reply #9 on: 2013 May 21 17:16:43 »
Very nice !
Nexstar 8SE ,stellarvue SV80ST ,HEQ5 pro ,Canon T3 (1100D) ,0.63 focal reducer/corrector ,CLS clip-in filter , Magnif.mini SSAG Guiding kit ,BacyardEOS ,PHD guiding

Offline lucchett

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Re: Markarian's Chain with DSLR
« Reply #10 on: 2013 May 25 13:55:23 »
That's an astro(photographer)'s picture.

Your framing is superb.

Andrea