Aperture Photometry Not too dependent on the particular

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. Here are theyvariables? are!

Removes background Orion Nebula Cluster (Irwin et al. )

Good for finding SNe too…

When to use which method? ?

Aperture vs. PSF: cluster field

ISIS vs. PSF: cluster field

Some Photometry Records… • Gilliland et al. (1993) : 0. 25 mmag photometry on 12 stars in M 67 • 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