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Old 20-12-2010, 08:23 PM
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dugnsuz (Doug)
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The argument for using darks and flats

Hi all,
Bored due to weather - thought I might try comparing data processed with and without darks and flats applied/subtracted.
Processed in PS4 - all the same processing steps performed on both images.
Which do you prefer? Please forgive the compression artefacts.
Doug

EDIT: 3rd image is Flats Only
Attached Thumbnails
Click for full-size image (No-Darks copy.jpg)
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Click for full-size image (Darks+Flats copy.jpg)
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Click for full-size image (Flats-Only.jpg)
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Last edited by dugnsuz; 23-12-2010 at 11:40 AM.
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  #2  
Old 20-12-2010, 08:30 PM
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jjjnettie (Jeanette)
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I prefer the second one Doug. Far more pleasing to my eye.
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Old 20-12-2010, 08:31 PM
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What are you on about Doug?.Are you implying they are comparable?, you are bored.
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Old 20-12-2010, 08:33 PM
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Octane (Humayun)
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The non-calibrated image clearly shows vignetting.

Are you using a dark library (smack!) or darks taken on the night?

H
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Old 20-12-2010, 08:37 PM
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Quote:
Originally Posted by Octane View Post
The non-calibrated image clearly shows vignetting.

Are you using a dark library (smack!) or darks taken on the night?

H
Dark library - temp as close to that of imaging session, ie Dark library@ -12.5ºC, imaging session subs average temp@ -13ºC
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Old 20-12-2010, 08:38 PM
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Narrowfield rules!

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Of course, ignor H (smack).
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Old 20-12-2010, 08:43 PM
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dugnsuz (Doug)
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Quote:
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What are you on about Doug?.Are you implying they are comparable?, you are bored.
Yep - b-o-r-e-d!!
Data is from new cooled DSLR - images all taken at around -13ºC, just wondered if using darks from a temp matched library would yield much better results.

The 'Darks+Flats' image was much easier to stretch and control. And the flats certainly clean up the vignetting big style.
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Old 20-12-2010, 08:48 PM
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Hi Doug,
I prefer the Darks and Flats applied. Without it looks a bit sort of washed out to me.
Great to see the comparison.
Duncan
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Old 20-12-2010, 10:04 PM
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Octane (Humayun)
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OK, OK, if you can guarantee that the darks match that evening's light exposure temperature, then, that's cool.

Otherwise, it's a mortal sin. : )

I'll ignore you in a minute, Fred!

H
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Old 20-12-2010, 10:45 PM
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Quote:
Originally Posted by Octane View Post
OK, OK, if you can guarantee that the darks match that evening's light exposure temperature, then, that's cool.

Otherwise, it's a mortal sin. : )

H
As close as I can to within a degree or so H
Doug
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Old 20-12-2010, 11:39 PM
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All is forgiven. : P

H
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  #12  
Old 21-12-2010, 08:47 AM
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Of course the 'Darks + Flats' image is better as far as contrast etc goes, but the stars are more pinpoint in the first image, at least on my laptop.

Cheers,
Jason.
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  #13  
Old 21-12-2010, 12:46 PM
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I prefer the second but Jason might have a point. Will have a later look on a bigger monitor. Is there a higher resolution image you can post Doug?
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  #14  
Old 21-12-2010, 01:03 PM
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Doug bored - H commenting and Fred reprimanding……….. there is balance in the world once again – of course it looks better with darks and flats
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  #15  
Old 21-12-2010, 01:21 PM
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I always take darks, darkflatts, flats and biases each ttime I image. Ive found that theyy ccan vary from night to night sso that somtimes they dont work well enough.
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Old 21-12-2010, 02:47 PM
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For pretty pictures you do whatever it takes to get the prettiest picture.

For Scientific imaging you better be using Darks and Flats or you are wasting your time.

Cheers
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Old 21-12-2010, 08:02 PM
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Fred should be ashamed of himself for abusing anybody prepared to experiment and learn!

I think there are several common assumptions that you could prove wrong with this experiment (I've done it myself).

Clearly the flats win in terms of reducing vignetting. Absolute must have with just about any imaging system (you might get away without it for awhile with some tele lenses but not when you start pushing it).

Unless you had a camera with serious amp glow or other large scale artefact, dark subtraction will not show at all at the resolution of the images you've presented.

Can you give us a 100% crop from one part of the image with and without darks. I believe that there will be less difference than most people expect. The darks can only reduce hot pixels (things that are constant from one image to next). They can't reduce the random noise which is all that will be affecting 99.9% of the pixels in the image (from a high quality CMOS sensor).

Depending on what software you are using (eg ImagesPlus), you can tell it to use the dark frames to only subtract the hot pixels from the light frames, which prevents it adding further noise to other parts of the image (by subtracting one lot of random noise from another). It also means that a slight temperature mismatch between lights and darks becomes less of an issue.

I applaud all experiments!

Phil
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  #18  
Old 22-12-2010, 07:30 AM
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dugnsuz (Doug)
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Thanks all for commenting - my photobucket pro account lapsed and I didn't renew so high res images may be a wee while coming!
Might look elsewhere for a hosting site - any suggestions?

Phil I'll investigate the dark subtraction options in IP.
Next experiment looks like trying the flats alone - will post soon.
Doug
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  #19  
Old 22-12-2010, 12:54 PM
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Second looks the best for sure. As for darks and flats subtration with a DSLR camera cooled; I used a darks and flats library. Using IP I just used the standard darks subtraction and this produced good results on many occasions. I don't agree with H that you need to take them on the night. I found variations of up to 5 degrees or so did not really matter much. Especially in wide field mode.
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Old 22-12-2010, 04:14 PM
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dugnsuz (Doug)
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Quote:
Originally Posted by Paul Haese View Post
Second looks the best for sure. As for darks and flats subtration with a DSLR camera cooled; I used a darks and flats library. Using IP I just used the standard darks subtraction and this produced good results on many occasions. I don't agree with H that you need to take them on the night. I found variations of up to 5 degrees or so did not really matter much. Especially in wide field mode.
Cheers Paul - you're past experience with the Astro 40D is helpful.
I'm trying to match darks temperatures as closely as I can.
Thanks
Doug
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