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Master Calibration frames with Dark frame scaling
Hi there, I have a basic question about calibration. What I have is this (all from the same setup):
- 120 Bias frames
- 120 dark frames taken at -20C, 900 seconds long
- Flat frames taken with various filters over various nights, all at -20C. Max exposure is 180seconds.
- Light frames taken with various filters over various nights, all at -20C. Exposures going from 600 seconds to 1800 seconds.
My understanding of the process is the following:
- Create a master bias with all 120 frames. Will use average and outlier rejection (MAD winsor clip, kappa 3, iteration 1)
- Create a master dark frame. Will use average and outlier rejection (MAD winsor clip, kappa 3, iteration 1). Here I have the first question, when generating this master dark, do I use master bias? My understanding is that yes, this way you are creating a master dark that is purely representing dark current without the the bias noise in it.
- Create various master flats (based on session and filter). Use both master bias and master dark. Dark scaling is enabled. Average combination, winsor sigma clip kappa 2.5 iterations 2
- Calibrate light frames with all 3 masters, dark scaling enabled. Flat Masters will be based on session and filter. Average combination, LNC 1st degree, MAD winsor sigma clip kappa 3 iterations 1 (light frames in any single session will be less than 10, but total is about 50 or 60)
Does that sound about right?
What I am confused about is the dark scaling. Will it work in this fashion? (both scaling up for Lights, and scaling down for flats?). And the requirement to use a master bias with Master dark when wanting to do scaling. Does this somehow mean that I should not be using master bias when creating the master dark? since I will be using it when using the master dark (on flats or lights) with the scaling turned on?
Another question is this: Will dark scaling work well enough that I don't have to redo darks at 1800 seconds for my longest exposures?
- No you don't have to calibrate the darks with the bias yourself, APP will take care of this automatically during the entire process.
The rest sounds about right, does it produce the result you're expecting? Regarding dark-scaling I found this little quote from Mabula:
On the other hand, dark-frame scaling is rather robust now in APP and I have used it myself on lots of data now and it works very well. With Dark frame scaling, you can simply create 1 very good masterdark of let's say 200 darks with a long exposure time and use that always besides a good Masterbias. A masterbias is required for dark frame scaling to work.
It also depends on the sensor if you can use dark frame scaling to good effect. For instance, the zwo asi 183 cmos camera has severe amp-glow, in that case, dark frame scaling will lead to worse results since the amp-glow can't be properly scaled. This is due to it's non-linear behaviour with respect to exposure time and temperature. So with such a camera, you will need to create proper darks of fixed temperature and exposure time.
I don't know for sure if it also scale up to be honest, I wouldn't think so, but that's one for @mabula-admin.
Thank you Vincent.
Results seem to be ok visually. Not able to quantify since I don't have a master dark that's covering the longest exposure. If scaling up does not work well, I will go ahead and create a master dark that goes upto and slightly beyond my max exposure.
Would be great to hear from Mabula.