Averaging darks: wi...
 
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Averaging darks: with or without rejection?  

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(@schullerfred)
White Dwarf Customer
Joined: 2 years ago
Posts: 19
June 6, 2019 13:09  

Hello,

I am trying to improve my calibration scheme, now producing master darks and flats for all gain settings that I've been using so far with my camera (ASI385MC).

When averaging the dark frames,  should I use 'no rejection' or can I use e.g. a MAD rejection? I tried both, and I understand that the number of hot pixels and the high-end distribution is different for both cases. But I don't understand why the median value is also different. For example, with gain=150 I get a median value of 4 with rejection, and 6 without rejection. For gain=300,  I get again 4 with rejection, but  11 without. Which one is correct, and more importantly, which method should I use to calibrate my flats?

Also, the bad pixel maps look quite different when I run with / without rejection. I have read in another thread that _long_ darks should be used to compute bad pixel maps, which is not my case here since I took the darks with same exposure time as the flats. Does this mean that I can't use these to derive a bad pixel map?

 

Cheers,

Fred


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3575
June 6, 2019 13:35  

First of all, darks with the same exposure time as the flats are more like bias frames with potentially some extra hot pixels. They are good to calibrate the flats with, but not for your regular images. So it's best to use a very long exposure dark for the bad pixel map, but if you don't have that, use the longest you have (which usually is 5-10 min depending on your light frames exposure time) + calibrated flats and bias. You can then use the bad pixel map almost indefinitely.

For extra info on how to create them, I'd advise a look at this video: https://www.astropixelprocessor.com/data-calibration-and-integration-the-pelican-nebula/

At about 2 min, things start with the data.


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