Dark frame scaling
Stacking program, astrometric (plate) solver, fits file viewer
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Dear Han,
Before all, thank you so much for this great program!
My question is related to calibration process with ASTAP. Suppose you haven't included dark frames of the same exposition time than light frames, and you don't check the "Dark exposure time" check box near the stack button. When you calibrate your light frames, the dark frame correction is applied (I think the program uses the first checked darks frames in the dark frames tab but not sure) and a warning message is showed about this time difference between light and dark expositions. Under these conditions:
- What dark frame substraction is done, the very same than the dark frame applied or some scaling is done?
- If not, could it be possible to have another option to scale dark frames (I suppose bias images should by applied to create master darks in order to have pure dark frames and then scale them to reflect the proper exposition time of the light frame).
Perhaps I'm missing something. In that case, please, let me know.
This suggestion appears to try to avoid doing dark frames for every different exposition times I use every night. Depending on the target, asteroid, NEO, comet, etc, the exposition time can vary a lot because of the speed of the target so it's a big time wasting procedure ;)
Clear skies,
felipe gomez
No there is no correction applied. Normally the exposure length is not so important. Most dark value comes from the pedestial value so a fixed value to have a reading above zero. So like a bias value and is required for correct applying the flat.. The other thing what is fixed by a dark are the hot pixels but this correction will work less effective with the wrong exposure time.
I think your best option is just to use the dark you have. If it has the wrong exposure time is not desirable but still using it is much better then no dark. Even the temperature difference is acceptable. but better too have it within 5 Celsius correct.
I use only two exposure times,. 50 seconds for bright object like M31, M13 to avoid saturation. For all other 200 seconds. since the readout noise of modern cameras is very low compared with the noise from the sky. So exposure length doesn't matter so much as long the image is not saturated. The only advantage of my 200 seconds is that I loose less time between exposures but on the other hand I loose a 200 seconds if there is a guiding problem.
So I would avoid too many exposure times. Use standard exposures of something like 30 to 60 seconds and a longer one like 180 or 240 seconds and stick to it. Then you can reuse your darks a long time. I have dark for -25, -20, -15 Celsius but moistly use -15 Celsius. They can be a year old or longer. With a DSLR not cooled you can still classify them on ambient temperature
Clear skies, Han
Last edit: han.k 2020-09-21