Re: Stopping down the I-band camera

[Michael Richmond <richmond@stupendous.cis.rit.edu>]

  Tom wrote:

> Michael, on a "good" night I get a fwhm number (near the end in
> make_list.out) of 3.5 for V and 4.0 for I +/- 0.1.  Given these numbers,
> where should I set the aperture?  General rule? Where should the aperture
> be set in relation to the observed fwhm?

  Advantage of using large aperture: you get a more equal fraction
of the stellar light from stars all over the field if the PSF changes
shape or size from corner to corner.  Advantage of using a small
aperture: you add less background noise to the stellar signal.

  A decent general rule for relative photometry on a single
frame is to use an aperture radius of about 1-3 FWHM.  So,
given the numbers Tom quotes above, I'd say that an aperture
radius of around 4-7 pixels would be fine.   

  If you have a good set of data from one nice night, you might
run the pipeline two or three times from the "stars" step onwards,
changing the aperture size each time and seeing which value gives
the best sigma-versus-mag plot.

                                     Michael