2. 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

3. 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%

5. Original field | Nearby neighbors removed

6. 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

7. Where are the variables? Here they are!

8. Removes background Orion Nebula Cluster (Irwin et al.)

9. Good for finding SNe too…

10. When to use which method??

11. Aperture vs. PSF: cluster field

12. ISIS vs. PSF: cluster field

13. 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

14. The “nitty-gritties”

15. Linearity

16. Guiding/Tracking

17. Flatfields: sky vs. dome screen 10% variation!

18. Fringing