Aperture Photometry

•Not too dependent on the particular psf shape

•Works well when in “clean” fields – not many nearby stars, and smooth sky background

•Breaks down when object has a close companion

PSF Fitting Photometry

•Semi-analytical approach: fit psfs with adjustable function

•Make a good model on isolated stars; then fit to position, intensity, background of other stars

•Works well when light from neighboring stars have overlapping psfs, interfering with background determination – fit models simultaneously to multiple stars, conserving flux

•May not work so well with very crowded fields (hard to detect stars), variable background, or if psf is not well sampled/not well behaved in the wings overestimates brightness by 5-25%

Original field |Nearby neighbors removed

Difference Image Photometry

•Allows for atypical psf shapes, as well as variation across an image

•Deals well with non-uniform background

•Does better at identifying variables in crowded fields

•ISIS (Alard 2000) is a popular publicly available program

Where are the variables?Here they are!

Removes background

Orion Nebula Cluster (Irwin et al.)

Good for finding SNe too…

When to use which method??

Aperturevs. PSF:

clusterfield

ISISvs. PSF:

clusterfield

Some Photometry Records…

•Gilliland et al. (1993) : 0.25 mmag photometry on 12 stars in M67

•Everett & Howell (2001): 0.19 mmag by binning multiple points

•Hartman et al. (2005): 0.36 mmag precision on stars in NGC 6791

•Lopez-Morales (2006): 1 mmag precision on V<9 stars

The “nitty-gritties”

Linearity

Guiding/Tracking

Flatfields: sky vs. dome screen

10% variation!

Fringing