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Old 07-04-2013, 03:35 PM
E_ri_k (Erik)
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Dark frame exposure length

Hey everyone, quick question. If I have a set of images of 10 minute, 15 minute, and 20 minutes, would it be feasible to do a full set of 20 minute darks to cover the other exposure lengths?

Erik
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Old 07-04-2013, 03:45 PM
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rogerco (Roger)
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Limited experience but I understand the idea of a dark is to reproduce the hot pixels that appear after a certain exposure time. If you use a 20 sec dark for a 15 sec exposure you may end up with a hole where the hot pixel is being subtracted from a light frame that doesn't show the hot pixel.
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Old 07-04-2013, 04:22 PM
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The simple answer is yes. Dark frames can be scaled. You have to remove the offset first though. ie a 0 sec exposure. This is done automagically by lots of preprocessing software.
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Old 07-04-2013, 08:07 PM
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You are best to have your dark's at the same length and hopefully the same temp as your lights.

Leon
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Old 07-04-2013, 09:06 PM
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Yes, dark frames can be successfully scaled, but you have to take bias frames. This is because the bias is constant, no matter what the length of the dark frame exposure, so to start with a level playing field you need to remove the bias. My usual exposure is 600sec, which I use for galaxies and nebulae, but if I image a bright star cluster I shorten the exposure to prevent stars saturating. I scale my 600 sec dark and things work fine. That said, scaling may not work quite as well as darks of the same length as your exposure. Scaling assumes darks scale linearly with time (eg twice the exposure gives twice the dark current). This works pretty well for reasonable changes-- scaling a 600s to 300s, but I would hesitate to scale a 1000s dark to 50s. It is important to do all darks and lights at the same temperature, but this is a non- issue with current ccd cameras as the temperature control is usually good.
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Old 07-04-2013, 11:44 PM
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I don't know anything about CCD cameras, but if you are using a DSLR,
why don't you double the length of your imaging session, and let your
DSLR do the darks for you, and then you won't have to worry about
matching exposure lengths.
raymo
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Old 08-04-2013, 05:54 AM
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Yes Raymo, that is exactly what I used to do, it took a bit longer but there were no problems at the end of the session.
I used a Canon 5D and had the ICNR set and it did its job well

Leon
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Old 08-04-2013, 04:26 PM
E_ri_k (Erik)
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Thanks guys. I'm not going to keep my 600 second subs, just going to keep and continue shooting the 900 second ones.

I have done 100+ Bias frames, so that's covered. How do you "scale" dark frame?

Erik
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Old 08-04-2013, 06:26 PM
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Octane (Humayun)
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The software scales it automatically.

H
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Old 08-04-2013, 07:41 PM
E_ri_k (Erik)
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Quote:
Originally Posted by Octane View Post
The software scales it automatically.

H
Oh yeah, that's easy. Does the software gets all the info from the file header? And work the rest out.

Erik
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Old 08-04-2013, 08:25 PM
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Yep, exposure length is a field in the FITS header.

H
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