Hi All,
I am wanting to test my setup for proper sampling, Can someone help me devise a proper test, id like to test my sampling with and without a focal reducer, the guide says to take a random pic of star field near zenith, then examine stars with good snr and measure fwhm, so after i take the image,
1) first do i apply calibration frames?
2) if i use nina or sharpcap to capture and astroimagej to analyze, what is the best way to first measure the snr in capture software or aij? explain how its done?
3) then i believe to look at fwhm in aij, i use the apature tool and use the "seeing" tool which i believe gives me fwhm in pixels correct?
4) once i measure a few stars and average fwhm, i then multiply by image scale to get seeing in arcsecs?
5) is calculating seeing is only for informational purpose as what im really looking for is the fwhm to be around 2-3 pixels correct?
" first do i apply calibration frames? "
Yes, I would use calibrated images for this.
" then i believe to look at fwhm in aij, i use the apature tool and use the "seeing" tool which i believe gives me fwhm in pixels correct?"
I'm not a user of AIJ, but yes, I would use the tool which gives you the FWHM's of the stars in pixels. This is the sampling for that particular image. You don't need to know the SNR to measure sampling, but using stars with good SNR's should give more accurate results (as long as the star images are not saturated).
This sampling of the seeing disk will be affected by "seeing", focusing, optical aberrations, tracking accuracy, local conditions at the observing site, etc.. It can vary over time on the same night, and when pointing to different parts of the sky.
"once i measure a few stars and average fwhm, i then multiply by image scale to get seeing in arcsecs?"
Yes.
" is calculating seeing is only for informational purpose as what im really looking for is the fwhm to be around 2-3 pixels correct?"
Correct. But, keeping a record of your seeing is useful information. For example, it can help you make better choices when you want to upgrade your equipment, e.g. whether to use a focal reducer
Phil