Photometry

yanked out of my intro talk. needs to be reformulated, fixed, integrated properly.

• 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. Perhaps we should add something to the effect that, That doesn't mean they're making it up; it's just the creative nature of the science process that you truly are starting from scratch.? --Don't want people to think ANYTHING is okay.--Weehler 05:58, 3 August 2007 (PDT)
• 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.

yanked out of Varoujan's GAVRT stuff. needs to be reformulated, fixed, integrated properly.

Photometry is the measurement of the brightness of an object. The brightness that is recorded on an electronic detector (or any kind of detector) is a combination of the brightness of the source plus the brightness of the background that the source is on.

In the case of the center of the galaxy in our project, a great deal of the "background" is in fact from the surrounding host galaxy. But what we are interested in is only the light from the center and not from the rest of the galaxy. To account for this, we will determine how much light is coming from where the center of the AGN is, and then compare it to how much light is coming from near the center of the AGN. We assume that the "background" near the AGN is the same as the background right on the AGN, so by subtracting the two, we are left with only the brightness of the center of the AGN.