Bad Pixel Map
No, this will have no significant effect on a Bad Pixel Map, assuming the Bad Pixel Map is created out of darks of considerably exposure length ( more than 60 seconds at least) and plenty of darks (at least 20).
The principle of the BPM is that it has information on the non-linear pixels of your sensor. With higher temperatures they (the hot pixels) are more apparent, so if you make darks for a bad pixel map you can just make them on room temperatures inside (with some coffee 😉 ). Once you have created a good BPM you can use it for months (or even years) to come.
The temperature of darks is important for the dark current/signal & noise calibration, but not important for the creation of the BPM. If the BPM works, it just works and will keep working 😉
Yes for the creation of flat frames, it's always recommended to shoot the flat frames with the exact same optical train as your light frames.
For the darks of the BPM however, this has zero influence. You can even make the darks for the BPM without the same telescope. For instance, with a DSLR, shoot them indoors with a regular objective. The only requirements are
- make sure no light enters the camera, so it's a real dark
- use an exposure time of at least 60 seconds per dark (more will be better, I would use 5 minutes per dark)
- shoot enough darks for good statistics on location of the pixels that behave non-linear. So shoot at least 20, in this case, more is always better, it will improve statistics.
If you are using a DSLR, you can use any ISO value, but I would recommend to use the same ISO value as your light frames 😉
Let me know if you have more questions.
Thanks a lot for your answer this helps me a lot further.
A very last question.
If you use also flats for your BPM, what about the orientation of your DSLR connected to your telescope.
You can never get this exactly the same.
In the case by using flats for cold pixels do you have to make a new BPM every time?
Regarding the flats for the BPM, in this case, the orientation of the optics isn't important. APP uses the flats to detect if there are cold and/or defect pixels. Any set of flats will do for this purpose 😉 And just like with darks fot the hot pixels, you only need to make the BPM once. If it works, it just works and keeps working.