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Old 18-04-2010, 08:08 PM
lookus
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dark subtraction vs in camera noise reduction

have spoken to several people regarding dark subtraction using DSS (or similar) as opposed to using the in camera noise reduction function(canon camera) and am getting mixed responses.

fo example- am i better off taking 10 x 10 min subs with ICNR turned on ( so 1x10 min sub really takes 20min) and then stacking these 10 frames in DSS?

or

am i better off taking 20 x 10 minute frames( spending the time taken doing ICNR and taking more light frames) and then stacking these 20 frames in DSS.? my idea here is to build a library of dark frames for various exposures and temperatures. so i would initially have to invest some time in building the library but these dark frames could be reused for a while.

i am very interested to hear peoples opinions on this and what they think is better.
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Old 18-04-2010, 08:18 PM
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Mathematical analysis suggests it is better to take separate dark frames.. final S/N ratio is slightly higher.
http://www.samfahmie.com/node/5

Last edited by bojan; 19-04-2010 at 02:10 PM.
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Old 18-04-2010, 09:47 PM
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dugnsuz (Doug)
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Quote:
Originally Posted by lookus View Post

am i better off taking 20 x 10 minute frames( spending the time taken doing ICNR and taking more light frames) and then stacking these 20 frames in DSS.? my idea here is to build a library of dark frames for various exposures and temperatures. so i would initially have to invest some time in building the library but these dark frames could be reused for a while.
This would be the optimum imaging workflow I think - maximize your number of light frames and dark subtract from a known library of temperature matched darks.

Unfortunately I could never be bothered with the building up of the library or critically recording the temps of each light throughout the imaging run so I stick to ICNR which subtracts a dark of the same temp but as you state doubles the time needed.

Much easier to build up a library of exact temp matched darks with a CCD I think too.
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Old 19-04-2010, 07:28 AM
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Each imaging session I am taking darks in the middle of it or at the end (while packing), usually 10-15 frames. This way I do not double the imaging time.. yes, this way I have some extra actuations (and this is considerable part of ~50,000 actuations life span of the shutter) but also slightly better S/N at the end of the day.
Building a library up front is not the same thing as taking care of this during the imaging session, when all conditions are really matched (which is the idea behind taking dark frames anyway).
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Old 19-04-2010, 08:27 AM
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Take more lights and make a library of dark frames is the best option. ICNR just takes away from imaging time. Do your darks on a cloudy night. Do 16 darks and as many lights as you can on any given night. Some objects take more than one night to get good results even with a DSLR.
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Old 19-04-2010, 08:43 AM
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Originally Posted by Paul Haese View Post
Do 16 darks and as many lights as you can on any given night. Some objects take more than one night to get good results even with a DSLR.
Paul,
What if on cloudy night the temperature is not the same?
You will end up with badly calibrated images.
Flats and bias frames can be done on cloudy night (or day) though, and the library of them does make sense.
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Old 19-04-2010, 09:01 AM
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Well I found that generally with CMOS imaging that use of a dark library had little negative impact on the final image. The only time this would be problematic is if I did not have enough lights to work with. If you are concerned about it do darks on a moon lit night with clear skies. You can check my website for images that were taken with this method. Non if any noise present in all but a few of the images taken with the 40D.

Anyway don't take it from me, use your own method. I have the images to back up my statement. The choice is really an individual thing.
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Old 19-04-2010, 09:43 AM
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OK, fair enough..
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Old 19-04-2010, 11:08 AM
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Previously, I used ICNR. However now I take darks. I haven't quite got it all worked out yet (correct temps, etc), but Im getting close. What I do is record the image run ambient temp, as it falls, I will stop capturing lights and take some darks frames. Then continue with the rest of the lights, followed by a bunch of darks at the end of the run. I was finding that ICNR was stealing some of the faint data from the image, obviously this is not good. Taking actual darks seemed to solve that issue
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Old 19-04-2010, 02:02 PM
astrospotter (Mark)
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Canon in-camera darks match chip temp well

The most recent 3rd Edition of the Backyard Astronomers Guide by Terence Dickinson and Alan Dyer has greatly added to their DSLR astrophotography section of past issues. In this book the claim is that they have done it both ways for quite a few runs and feel the in-camera method is best (for quality, not for productivity) because the darks are taken when the chip temp is as close as possible to actual chip temp.

The book claims that darks for a fully cooled CCD camera have no problems with setting up a little fridge at home even and take darks at home which is the ultimate time saver for the field. Warning: Wife alert if you try this in the normal fridge and forget to put temp back!

As far as taking darks I have found that I must take darks for 4 minute or so exposures as they are the only thing that will take out tiny single pixel blue or green or red dots for my camera which starts to show these dots at such long exposures. These dots are even more of an issue when stacking or bringing out detail with certain post processing steps.

My experience in Astrophotography is very very minimal (mostly have been a visual guy) so I cannot offer my own opinion beyond the pixel issue. That is why I mention Terence Dickinson book as he is one of the key contributors to astronomy and I respect his authority due to reputation.

The Backyard Astronomers Guide is fantastic book/reference for many other reasons as well. ISBN -13:978-1-55407-344-3. It is a bit expensive but if one is into astrophotography, one is already numb to the cost of many things.
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Old 19-04-2010, 02:37 PM
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I choose between the two for purely practical reasons.
On a pristine cloud free night I'll have time on my side so I'll use ICNR.
But if there's a little cloud about I'll be wanting to capture as many subs as possible in the time I have, then take darks later.
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Old 19-04-2010, 02:54 PM
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Using ICNR provides a better match of temperatures between the light and dark frames but it uses only one dark frame to calibrate each light frame. Taking a bunch of darks and creating a median combine master dark will reduce any random noise introduced by the dark frames.
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Old 19-04-2010, 04:41 PM
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Good answer on use both as time permits

The post from Nettie sums it up nicely. It's a tradoff of time which is precious in the field (or anywhere else). When pressed for time with a DSLR use darks you collect later. If you have the time in the field, do them in camera.

Off topic: I have a t-shirt with the saying 'One by one...the penguins steal my sanity!' because we have a joke at work where managers we started calling penguins due to reasons I will not get into. But is there a reference in some movie to this very odd and non-sequiter line? Please answer as a message which I think Ice In Space supprots? so this does not clutter this thread with off topic chatter. Thanks.
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Old 19-04-2010, 05:16 PM
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I cant find this feature in MaximDL, but CCD stack has an "adaptive dark" feature I have used with a CCD, which scales different dark temp/exposure times to lights, works well, and may in itsself make a dark library with an aproximate temp to the lights worth while. I dont know how you would tell CCD stack what the light temp was though, perhaps by editing the FITS header
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Old 19-04-2010, 06:44 PM
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thanks very much for all the responses. i am glad to hear that a dark library can work well if well temp. matched. i hoped this would be the case as i find it much more enjoyable to take more light frames. i find it quite frustrating sitting waithing for the ICNR frame to complete. i think i will proceed in this direction for now.

thanks very much all.
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Old 19-04-2010, 06:48 PM
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Quote:
Originally Posted by lookus View Post
thanks very much for all the responses. i am glad to hear that a dark library can work well if well temp. matched. i hoped this would be the case as i find it much more enjoyable to take more light frames. i find it quite frustrating sitting waithing for the ICNR frame to complete. i think i will proceed in this direction for now.

thanks very much all.
Could always get your DSLR cooled!
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Old 19-04-2010, 06:53 PM
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iceman (Mike)
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I've tried both, and have always had better results from ICNR.

I never have the time or inclination to take darks at the same temperature, and as others have said, when the temp drops, if you don't have darks to back it up then you may end up with no ability to do any dark frame subtraction.

It's up to you though - if you can get dark frames working well for you, it's definitely much better for productivity.

My experience hasn't been as good with them, so I stick to ICNR simply to ensure that a good, clear night of imaging (all so rare for me these days) isn't ruined by not having any dark frames.
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Old 19-04-2010, 07:49 PM
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i have built up a bit of a library at various temps by controlling the temp in my office. it is good down to about 15 degrees which is about the temps i am getting when imaging at the moment. this however will get lower and then i will have to build a library in the field i guess.
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Old 19-04-2010, 10:31 PM
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I am also figuring out a few things with darks. Did a few a few days back.

First - If I alter the lights level do the flats get adjusted as well? It seems logical.
Second - the EOS450D has what the manual noise reduction which is supposed to take a dark and subtract it automatically. Is this proceedure any good?

Haven't had enough time to test it yet.
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Old 20-04-2010, 03:29 AM
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Quote:
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Second - the EOS450D has what the manual noise reduction which is supposed to take a dark and subtract it automatically. Is this proceedure any good?
Malcolm, that's what everyone is referring to in this thread as ICNR - In Camera Noise Reduction.
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