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Old 15-04-2006, 06:16 PM
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Dark Frame Subtraction

Hi there to all,

I have not been doing digital imaging for long, however i have read up on it and followed the advice from the books, with reasonable success.
My question is this, is there any need for dark frame subtraction if the camera that i use has a noise reduction function in the custom settings, is this basically the same as subtracting dark frames afterwards.
I use a Pentax IstD, my exposures normally are from 30 sec's to 3 or 4 minutes, tracked.
During my imaging periods i have taken dark frams as suggested, and hopefully subtracted them successfully, i say this as API4Win can be a challenge to the novice.
I decided to check how noisy the dark frames were, on my computer and enlarged the image by 1600 percent, and then scrolled through the image and hardly found any noise at all , just the odd signal white pixel.
I would be interested to hear in what more experienced imagers may think, and maybe give me some advice.

Thanks Leon
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Old 15-04-2006, 07:55 PM
Dennis
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Hi Leon

I have the Pentax *istDS and I have used it in a limited manner for astrophotography.

When I use the Pentax Remote software and expose for the (maximum) of 30 seconds using the interval function in the software, then I leave the auto-dark frame to the camera as it chugs along exposing the light frame followed by the obligatory 30 second dark. That is, I do not take any separate dark frames.

However, if I were to use the bulb mode and expose for say, 60 secs or 2 to 3 minutes, then I would take an equivalent dark frame with the lens cap on and manually subtract it from the light frame.

The important things are:

The dark frame should be exposed for the same duration as the light frame.
The dark frame should be acquired at the same temperature as the light frame.
Every 6 deg C drop in temperature halves the dark current.

Hope that helps.

Cheers

Dennis
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Old 15-04-2006, 08:45 PM
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Thanks Dennis, I understand what you are saying and will follow your suggestions and see how we go. However just out of interest, is your imaging wide field, and i so what lens do you use with the IstDs

Thanks, Leon
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Old 15-04-2006, 08:49 PM
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JohnG (John)
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You also need to take a Bias frame as well, that is, a dark with the lens cap on and the shortest exposure on the camera.

JohnG
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Old 16-04-2006, 07:18 AM
Dennis
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Quote:
Originally Posted by leon
Thanks Dennis, I understand what you are saying and will follow your suggestions and see how we go. However just out of interest, is your imaging wide field, and i so what lens do you use with the IstDs

Thanks, Leon
Hi Leon

Here is a link to an image of Omega Centauri taken with the *istDS through my Vixen 102mm f9 refractor:

http://www.iceinspace.com.au/forum/s...ighlight=Omega

I have the two stock Sigma zoom lenses, but I haven't been to a dark sky site for ages to use these for deep sky, so have no results to post or comments to make.

Cheers

Dennis
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Old 16-04-2006, 04:01 PM
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Hi Dennis,
Had a look at the site you suggested, and wow, what a shot, great stuff dennis, hopfully i will reach that stage some day.
thanks for your help, i'll just keep plodding along and i'm sure i'll learn heaps along the way, especially from you guy's.

thanks

leon
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Old 16-04-2006, 05:53 PM
Dennis
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Hi Leon

Glad the stuff was helpful. There is a steep learning curve but honestly, once you have grasped what is going on and seen the results from your own eforts, you will be hooked, so please persevere, it really is worthwhile.

Cheers

Dennis
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