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Old 08-04-2013, 08:09 PM
DannyV (Danny)
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EOS ICNR vs Dark Frames

Hi Folks,
Reading on from another thread prompted me to ask, (with respect to my Canon EOS 60Da):

Does using Canon's Noise Reduction Processing per shot perform as well as taking dark frames?

Should I take dark frames in addition (belts and braces)?

Or am I better turning the ICNR off and taking dark frames at the end?

My brief testing has failed to show any difference.

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Old 08-04-2013, 08:25 PM
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Octane (Humayun)
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Leaving ICNR on halves your effective imaging time.

With so few precious nights available to us, I know which way I'd be going (and, have been going since I started imaging).

If you're doing nightscape-type work, then, it's a no brainer -- use ICNR.

H
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Old 08-04-2013, 11:02 PM
raymo
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Hi DannyV, Whether you or the camera take the darks, the end result
will be the same. If available imaging time is not an issue, it is good to let the camera do it because it is desirable that the darks be of the same duration as the imaging exposures, which obviously happens when the camera does it. Adding extra darks will gain you nothing, indeed it can be detrimental. If imaging time is precious, then Octane is spot on, do them
later.
raymo
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Old 09-04-2013, 08:42 AM
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Multiple dark frames averaged should give you a better result that ICNR. This is because there is a statistical variation from one dark frame to the next, so we average many dark frames to get close to the "true" dark current. If you do ICNR, you are using only one dark frame per light frame, so once you have stacked your lights the net result will be a little noisier than if you had used multiple averaged dark frames. It's like using an autodark with a ccd camera. However (as usual) there are caveats. A DSLR does not have temperature control, so if you take lots of darks with a view to averaging them, then temperature changes during the acquisition could null out the theoretical advantage. Also, it depends on your sky.If you have a bright sky, you need fewer darks (because sky noise overwhelms residual noise in the dark current), and so ICNR will be good enough. However, you would be better off taking 3 or 4 darks at your subframe exposure time and using them--much quicker than using ICNR for 20 or 30 light frames.
Geoff
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Old 09-04-2013, 11:40 AM
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avandonk
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It is even worse than Geoff says. If you are starting off with a 'cold' camera that is you have just switched it on it and started a long exposure the sensor will heat up during this long exposure. The subsequent ICNR dark frame will then be to hot and leave black spots all over your image.

I bet you will find your first frame has these dark spots and the rest do not when doing sequential ICNR corrected exposures.

I measured the temperature of my Canon 5DH sensor before and after a long exposure. Within ten minutes it had risen by 17C degrees!

A C type sensor will have a smaller temperature rise at least 10+C.


Bert
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Old 09-04-2013, 01:01 PM
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Octane (Humayun)
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Geoff and Bert nailed what I was going to say, but, left my answer succinct as I was short on time.

Thanks, gents.

When I imaged with a DSLR, I often did a bunch of dark frames before I began, during a meridian flip/in the middle of the run, then at the end (along with flat lights and flat darks).

H
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