Ahh, thanks, I understand better now. I haven't noticed if that circle can have its diameter changed in Astrometrica.
I assumed it would calculate the magnitude based on the brightness of other stars in the image and then using the single brightest pixel value from the estimate star, I wonder why having more surrounding the star being included in the estimate has n impact? I should RTFM on the subject.
Flats, stretching, etc - have that covered.
Thanks,
Roger.
Quote:
Originally Posted by Terry B
What I mean by bigger aperture is the size of the circle that the stars flux is measured. In your image there is clearly lots of the star flux outside the little circle. Your measured FWHM os 3.5arcsec so the circle needs to be 3 or 4 x this size. You will need to work out how many pixels this is as I don't know the resolution of your scope.
The UCAC3 databases are good but the accuracy depends on the filter being correct.
There is some advantage to averaging a few exposures to improve noise but you are probably better to measure individual exposures and average the results.
Any form of "stretching" of the exposure will make photometry measurements unreliable so just use plain dark and flat processed exposures. Flats are very important and must be used for accurate photometry.
cheers
Terry
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