Question about darks and DSLR camera long exposures

[5ash (Philip)]

I appreciate why darks are taken to remove thermal noise and hot pixells and that they must be taken at the same temperature as the lights. This is best achieved with short exposures where the lights and darks can be taken within a short time interval . However when taking long exposures over several hours the darks will be taken at often very different ambient temperatures whether taken before or after the lights. This does not take into account the heating of the sensor over that period. My question is " for long exposures using a standard uncooled dSLR would it not be better to use the noise reduction routine available in the camera "? I know this doubles the time for each exposure but it would overcome problem of temperature changes over a photographic session of many hours.

Regards philip
PS any recommendations on a 2" light pollution filter for dSLR astrophotography

[rmuhlack (Richard)]

You're correct that darks taken at the beginning or end of a long imaging session would invariably be at a different temperature to the light frames. For this very reason I take my darks on cloudy evenings that are not suitable for imaging, and then match up the relevant dark master frame to the approximate temperature (to +/- 2 or 3 degrees C) that the light frames were acquired at.

[jjjnettie (Jeanette)]

I take a few darks while setting up, a few more half way through the session, then once I've finished imaging, just keep shooting darks till morning. All the darks are thrown into one folder to become one Master Dark.
I keep adding more darks each session.
Before I moved onto my cooled DSLR I would have approx 80 darks in the folder.
I would also shoot a completely new set of darks as the season changed. Not only because of the temperature variation but because the camera, as it ages, will develop new hot pixels not covered by older darks.

[acropolite (Phil)]

Regarding ICNR, I don't believe long exposure noise reduction works with bulb mode, it's OK for in camera timing (up to 30 seconds) only.

I also try to allow a few minutes between exposures to allow the sensor to cool down.

[Jon (Jonathan)]

All of the above work. I have also taken darks inside during the day (ambient 20 degrees) temperature matched with lights taken outside at 2 am in Winter (ambient 2 degrees) by the expedient of putting the camera in a plastic bag in the fridge. With the USB cable snaking out to the laptop on the kitchen bench.

Wife thought I was mad.

Backyard EOS tracks your sensor temp and writes it into the filename for each sub. Invaluable.