Arnold,

The sky parameter is a byproduct of SEP star extraction. Take a look at the Background and Background Map sections for details:  sextractor.readthedocs.io/en/latest/Background.html It is the all-sky RMS background (whole image noise standard deviation).

I was thinking of converting it to an SQM measure, but I believe I'd need to know camera gain, and probably need darks and bias images. I decided to keep it simple, but am open to suggested measures.

As a relative matter (e.g. how the sky is improving or degrading throughout the night) the current measure is probably fine. A similar measure from that respect (relative sky conditions) is # stars detected in the main and guide cameras, and guide star SNR.

Hy

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