Author Topic: Question about Canon DSLR milky background removal - limited success  (Read 4017 times)

astropixel

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What is the best way to deal with the milky background associated with some DSLR cameras in PI. I notice that with a significant non-linear stretch my images reveal uneven whiteness across the image. To a large extent I have minimized the effect with Windorized or Percentile clipping, but the residual makes it impossible to stretch and obtain brightness and colour. Background extraction tools have not helped, and I have tried various different approaches during non-linear processing with little success.

Basically, this artefact prevents getting the best out of significant data.

Online pfile

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are you sure this is a camera artifact and not due to light pollution? i'm not sure i've ever seen uneven whiteness but i definitely get some strange gradients sometimes...

Offline georg.viehoever

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Could you post an example image. I think I dont see this problem with my Canon EOS 40D.
Georg
Georg (6 inch Newton, unmodified Canon EOS40D+80D, unguided EQ5 mount)

astropixel

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Thanks for taking an interest in this.

I extracted the L* channel to highlight the issue. I will resist the temptation to think out loud, except to say that this translates to a very muddy background.

Offline Cleon_Wells

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Have you tried not calibrating your subs to eliminate the possibility that the Master Calibration files are not causing this problem?
Cleon
Cleon - GSO 10"RC/Canon T1i-Hap Mod, 100mmF6/2Ucam/MG, EQG/EQmod

astropixel

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I'm hesitant to say that it's a calibration problem. I don't want it to be >:( The initial stretch produces a very flat image - evident in the RGB and extracted L*. But this is only achievable with Windsorized clipping on the large set - 247 lights, and Percentile clipping on smaller experimental sets - 3 lights.

Integration with Linear clipping, the problem was evident from the first stretch - I assumed to have selected the wrong clipping algorithm.

I have no reservations concerning the bias and dark frames - they are easy to acquire and process. But, I dread the thought that it may come back again to the flats that I am producing???

Offline georg.viehoever

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I wonder if this is the general background "dust" in this region of Orion. See http://de.wikipedia.org/wiki/Datei:Orion_Belt.jpg for example.

Georg
Georg (6 inch Newton, unmodified Canon EOS40D+80D, unguided EQ5 mount)

Offline Cleon_Wells

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Georg may have the answer. Just what is the the total time in the 247 lights?
Do you examine the three channel list created with II after integration?
Cleon
Cleon - GSO 10"RC/Canon T1i-Hap Mod, 100mmF6/2Ucam/MG, EQG/EQmod

astropixel

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I did think of dust, but discounted it on the basis of the total exposure time - just 67 mins, made up of 10, 15, 20 and 30 second exposures - f/3.2, iso800.

The other problem was differential flexure. Exposures are unguided, but alignment was unusually good and I should have dithered the frames periodically - could this contribute to the problem? Originally the image was streaky, but I managed to process a lot of this out. Maybe it shows up with non-linear processing and more extensive stretching.

If it is dust, then this is all too funny - I've spent hours attempting to eliminate good data. :'(

astropixel

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Here is a masked version just to illustrate the extent and nature of the problem. Is this recoverable?

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Re: Question about Canon DSLR milky background removal - limited success
« Reply #10 on: 2011 February 09 08:56:14 »
check this widefield - the entire area is full of dust...

http://www.flickr.com/photos/31986095@N05/5281522077/

it does show up even in shorter exposures. and it looks... kind of milky when the signal is low...


astropixel

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Re: Question about Canon DSLR milky background removal - limited success
« Reply #11 on: 2011 February 09 18:20:42 »
Comparing images, I see faint evidence of dust areas, but overall I think this is an artefact - possibly the result of trying to squeeze to much out of the image. I've just about given up on this set.

Offline David Raphael

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Re: Question about Canon DSLR milky background removal - limited success
« Reply #12 on: 2011 February 09 20:35:05 »
Here is the same field - I don't think you have milkyness...I see dust ;-)

http://www.flickr.com/photos/21315110@N00/5343866972/

Don't look too closely at my picture...it is not a very good one...
David Raphael

astropixel

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Re: Question about Canon DSLR milky background removal - limited success
« Reply #13 on: 2011 February 10 01:57:04 »
Very nice David. Thanks everyone, I concede - dust! Stunning area - just needs more data.

This time I started out with ACDNR following the first non-linear stretch.

Given my skills, this is as much as I can get out of it - a long way to go, I understand.

Comments/feedback most appreciated.

Offline Redshift

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Re: Question about Canon DSLR milky background removal - limited success
« Reply #14 on: 2011 February 10 02:56:23 »
I think you've got the start of a good image there. I like the colour in the Flame and the Orion nebula. Stick with it and get more subs.

Regards
Dave