Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 09/05/2019 - 03:33
I've wondered for some time why the default aperture for a time series is 1.5 x FWHM ? I always change this to 2, unless the seeing is very stable, and then select a fixed aperture of 2 - 2.25 FWHM.
Seems that should be 2 x FWHM for better sampling of the flux, no? I've wondered about this for nigh on 5 years now... Logic behind this setting?
Thanks,
Brad Vietje, VBPA
Newbury, VT
Brad,
VPhot expresses the size of the aperture as pixels across the RADIUS of the FWHM. In my experience most texts on this subject, including the AAVSO CCD Photometry Guide, express the size of the aperture as pixels across the diameter of the FWHM. This is a constant source of confusion.
I think there should be a Black Box warning about this in both the CCD Guide and VPhot Guide .
Phil