Photometry

yanked out of my intro talk. needs to be reformulated, fixed, integrated properly. varoujan has some stuff from GAVRT to share.

• Every astronomer does things a little differently. (It’s kind of amazing that any two astronomers working on the same object and the same wavelength ever get the same answer.) Everyone does what they think is right.
• The shape of the point-source pattern is called the “point spread function (PSF),” or sometimes the “point response function (PRF).” (Technically these two things are subtly different, but never mind that for now.) The PSF is HUGE, and there is a lot of flux surprisingly far from the star that needs to be included.
• One can measure fluxes from point sources (like stars) in two ways: aperture photometry or PSF fitting.
• Aperture photometry measures all of the flux within a (usually circular) aperture centered on the star, minus the flux in an annulus around the aperture. This is quick, but not necessarily accurate, and can lead to large errors (especially if the background is complicated), and is essentially impossible in crowded fields. One must take into account fractional pixels within the aperture (which matters particularly when the units are MJy/sr). Usually one needs to apply an aperture correction to correct for the ‘missing’ flux outside the aperture.
• PSF fitting takes the basic shape of the PSF and matches it to the point source, thereby taking into account the fluxes at large distances from the star, as well as ignoring complicated structure in the background and other nearby point sources.
• MOPEX does both aperture and PSF fitting, and understands the units of Spitzer images. In practice with MOPEX, one needs to use aperture photometry for the brightest stars and PSF fitting for the rest. And, as of July 2007, one ought to use MOPEX aperture photometry for IRAC, and either aperture or PRF fitting for MIPS - this has to do with how well-sampled the PRFs are in the various channels.