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Old 26-05-2013, 11:28 AM
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DavidTrap (David)
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Straw Poll about Dark Frames

Who uses a stack of long dark frames (eg. 30mins) and lets the software scale it back for image processing and who uses appropriately matched dark frames (eg. 30sec, 5min, 15min)?

Reason for asking - time for me to redo my dark library, and wondering what is the consensus??

ta

DT
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Old 26-05-2013, 11:32 AM
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multiweb (Marc)
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Quote:
Originally Posted by DavidTrap View Post
Who uses a stack of long dark frames (eg. 30mins) and lets the software scale it back for image processing and who uses appropriately matched dark frames (eg. 30sec, 5min, 15min)?

Reason for asking - time for me to redo my dark library, and wondering what is the consensus??

ta

DT
I use darks of 20min because I figured it's the longest I'll ever practically expose for. I scale them all the time when calibrating in CCD Stack. Lately I've been doing 5min or 10min subs and always use the 20min master.
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Old 26-05-2013, 11:46 AM
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I've found that for my STF-8300M, matched dark frames works better than scaled dark frames (judged visually with extreme stretching).

There was something funny going on with the bias frames - the SBIG engineers said they fixed a bug in the firmware when I reported it wasn't working properly. Might be something to do with the overscan pixels that RickS found/discussed a while back.

I'd assume that scaling for reasonably close exposures (i.e. scaling a 30 min dark down to a 25 min exposure) would work well, but extreme scaling (e.g. 30 min darks for a 1 min exposure) wouldn't be as effective since the ADU counts might saturate or go into the non-linear zones for some pixels.
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Old 26-05-2013, 11:50 AM
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Thanks Marc and Dave.

Dave - what you say does make sense. I have had appropriately matched darks until now, but had started to experiment with longer subs (15-30min).

Might take some short and long ones, but not necessarily at every exposure length I'd conceivably use.

DT
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Old 26-05-2013, 11:55 AM
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David,

I've generally had good results with dark scaling in PI.

I haven't got around to it yet but I took darks at 5, 10 and 30 mins when the U16M came back from Apogee and I was going to compare scaled darks to non-scaled (and also compare bias calibration with and without overscan). I'm waiting until I have some data that is good enough to justify the extra work - something from a dark site.

Cheers,
Rick.
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Old 27-05-2013, 06:59 PM
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Same as Rick. If you're using PI, it's dark scaling is pretty good. Dark library about 2-3 times longer exposures than the longest you normally take I think is the rule of thumb. I used 20x1800s for 1x1 bin and 20x900s for 2x2 bin for dark library and 100x biases each binning.

I'm about to redo my library as well and think this time I'll do more than 20 for the darks. The benefit of 100x biases is surprisingly good.
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Old 27-05-2013, 07:07 PM
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My usual subs are 10 min, so that's what I use for dark frames. If I do a glob or bright cluster I may use a shorter exposure and then I just scale my 10 min darks.
Geoff
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Old 27-05-2013, 07:41 PM
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Quote:
Originally Posted by troypiggo View Post
The benefit of 100x biases is surprisingly good.
I did 500 last time but the average value varied so much over time (in a non-random way) that I started to investigate the use of the overscan region for bias calibration.

Cheers,
Rick.
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Old 27-05-2013, 08:22 PM
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Go hard why don't you.
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Old 27-05-2013, 08:38 PM
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It's no harder to do 500 than 100
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Old 28-05-2013, 10:43 AM
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Scaling darks is a bit lazy. If you standardise your subexposure lengths and temperature to just a few like 10mins, 15 or 20 or even just 10 and 20minutes its only 2 sets of darks.
My camera can hit -30C all year round so I changed my standard temp from -35C to -30C (it would not hit -35C on some summer nights).

If you get a cloudy night its a perfect time to do darks so its not a lost night.

But start using scaled darks on a scope that uses correctors etc then you are going to get poor results from flats.

My Proline doesn't even really need darks. Just a few hot pixels I could PS out. A few times I didn't use darks when flats weren't needed either (nicely illuminating scope).

Greg.
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Old 28-05-2013, 03:45 PM
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I am quite anal about darks and flats. If you want to get a signal that is very faint any erroneous 'calibration' will insert artefacts.

I always use up to date darks at the set temperature and duration. Thirty at least.

I do not bother with bias frames as the darks already have them embedded. Bias frames are only needed to fudge some sort of temperature or exposure calibration.


With flats I am even more anal as I take darks for the flats and calibrate them accordingly. I also expose at least thirty flats to get a statistically relevant average.


Bert
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