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.

Bias | Darks | Lights

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):

  1. Add (calibrated) light frames into SubframeSelector
  2. Add approval expression and weighting:
    Approval: FWHMSigma < 2 && SNRWeightSigma > -2 && WeightSigma > -2
    Weighting: SNRWeight
  3. Start selection process by clicking “Measure”
  4. After the measurement all rejected subs are deselected in the table
  5. Also open the plots to have a better overview
  6. By clicking on a point in the plot, you can reject/include single frames manually
  7. 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.

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