After a nightly session the first thing to do is to analyse the lights. This can be done visually or mathematically. Or both 😉
First of all I pre-sort the lights visually. But since I use an AutoGuider I can use almost every light frame. Only if there are air planes or satellites I’ll sort them out.
Temperature readout – EXIF tool
The next step is to check the sensor temperature of each light, dark and bias with the command line application exif-tool. As we know all subs should have the same temperature as much as possible. So subs with a high deviation will not be used for stacking.
Command line example: exiftool(-k).exe -Filename -CameraTemperature -fileOrder CameraTemperature -T -q -ext CR2
Finally, I use the SubframeSelector in PixInsight to analyse my pre-sorted lights and reject specific frames, based on a formula. I’ve found this expression in PixInsight forums and it does a good job for me:
FWHMSigma < 2 && SNRWeightSigma > -2 && WeightSigma > -2
Let’s take a look at the SubframeSelector steps (Script – Batch Processing):
- Add (calibrated) light frames into SubframeSelector
- Add approval expression and weighting:
Approval: FWHMSigma < 2 && SNRWeightSigma > -2 && WeightSigma > -2
- Start selection process by clicking “Measure”
- After the measurement all rejected subs are deselected in the table
- Also open the plots to have a better overview
- By clicking on a point in the plot, you can reject/include single frames manually
- Finally select an output folder (Approved directory) for the approved lights and click on “Output Subframes”
Now we have a good base to start the stacking process.