astronebulee could someone tell me what they mean.

They are approximately the Half Flux Diameter (HFD) in pixels. Notice that it also converts that to arc seconds (in parentheses) as long as you have given ASIAIR the correct telescope focal length and camera model.

That being said, it is not that accurate. I have found that when the sky background is lighter (e.g., in astronomical twilight), the ASIAIR star size is smaller than when the sky is darker. Longer exposures tend to also give higher HFD with whatever algorithm ASIAIR uses to calculate HFD.

Use a real program instead, if you wish to have more accurate HFR/HFD/FWHM measurements.

Chen

astronebulee I'm undersampling with my images if they are this value or higher?

HFD of 2 is pretty good sampling wise, Lee; if it were 6, then you would be oversampling (i.e, more than two pixels per star diameter) and not getting anything in return (i.e., resolution already limited by Dawes limit and telescope aperture).

Don't worry too much about under/oversampling with DSO. When you oversample, the star's photons is spread over more pixels; however, notice that the sky noise is also spead over (the same) more pixels.

I.e., the signal to noise ratio of your nebula, when compared to the sky background is pretty much unchanged.

What changes is the nebula versus the camera's dark current, read noise and quantization noise of the ADC. With typical suburban skies, those are small compared to the noise from the sky background (i.e., if your average ADU is over even just 5K).

As long as you are within a factor of 1.5 either way, I wouldn't lose sleep over it. The difference between a low end filter and a high end filter will make a bigger difference than minor under/oversampling for us suburban dwellers.

Chen