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Old 01-10-2009, 08:21 PM
TrevorW
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A question of darks - by C Stark

If anyone hasn't read these they might be of interest

A qoute from part 1.

"if we want a very good estimate of the average dark current (so we can subtract this expecetd valu of the dark current from our lights) we need even more dark frames to average togethor"

futhermore

"How many dark frames should I use ? depends on your dark current. If you have a lot of it and you image gets considerably brighter and noisier as you take longer dark frames you are going to need more darks. If you don't collect enough darks you are going to inject noise into light frames when you pre process the images"

refer this link for articles

http://www.stark-labs.com/craig/articles/articles.html

under 2009
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Old 01-10-2009, 08:34 PM
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[1ponders] (Paul)
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This is correct, under a number of assumptions.

1. that you ambient temperature is stable. If temp goes up or down then you are stuck with only a short number of darks. Do not combine darks from different temperatures.

2. Dark frames contain noise. Dark Current is not noise. It is unwanted signal. It is not noise because it is repeatable, it is not random, therefore by definition it is not noise. But there is noise in a dark current. This is why you need multiple dark frames. The more dark frames you have to combine the greater the signal to noise ratio and the less effect the dark 'noise' will have when calibrating your lights.

3. In a light polluted situation as in a city dark current noise is basically swamped by unwanted light "noise" (again it really isn't noise because it's not random). The lower the signal (ie background of a dark sky) the more noise will predominate. In a light polluted environment the light noise predominates not the random elements of the dark current so you can get away with far fewer dark frames for combining.

Bury and Brunell have a great chapter on this topic in the AIP4WIN book which I think should be on every astrophotographers bookshelf.
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Old 01-10-2009, 08:53 PM
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Quote:
Originally Posted by [1ponders] View Post

Bury and Brunell have a great chapter on this topic in the AIP4WIN book which I think should be on every astrophotographers bookshelf.

I agree, this is a great book but might be a little too mathematical for some folks especially those seeking simple answers. It reminds me of my old physics texts.

Mark
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Old 01-10-2009, 08:56 PM
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One of the things I like so much about the first half of the book (the second half basically left me gasping ) was that I could look at the maths, leave it if I didn't understand it but still get the important part by reading the text. Funny thing is my mathematics has improved considerably over the past couple of years after reading it a couple of times.
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