Re: HD 145913

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Arne Henden wrote:

>  I usualy also recommend apertures around 5x fwhm diameter
>  when trying to do accurate photometry of bright objects, so you
>  might experiment with aperture size and see what happens.

  Michael Koppelman replied:

> There is nothing going on (that I can see) after 4 or 5 pixels. You are 
> suggesting a radius (in this case) of 7 pixels or so. Seems like I 
> would get a lot more noise and a lot less signal in that case.

  There are two reasons Arne (and others) suggest using such large
apertures.

      a) if you are trying to calibrate your photometry of stars in 
         the field of a variable against, say, Landolt standards
         in a different set of images, then the PSF might change
         significantly from the target star images to the standard
         star images (especially at large airmass, as Michael K is
         observing).  In this case, a different fraction of light
         might fall within a "small" aperture, and so one would
         not make a fair comparison between target and standard
         frames.

      b) even if you are doing purely differential photometry of stars
         in a single field, there can be small differences in the PSF
         across the field.  Again, using large apertures protects you
         from this effect.  

  Note that Arne qualified his statement "photometry of bright objects"
with the word "bright".  For bright stars, the majority of the noise
in the measurement is due to variations in the photon count from the
star; noise from the sky is negligible.  In this case, using a large
aperture doesn't hurt.

  If you are working near the plate limit, trying to measure the 
faint stars, then, yes, using a small aperture may be necessary 
to avoid the extra noise from the sky.

                                Michael Richmond