comparison star probabilities

Comparison stars are used in differential photometry to correct the target star for transparency variations - the brighter the comparison star, the better the differential photometry. This form allows you to calculate the probability of finding a comparison star of a given R-band magnitude with, e.g. HiPERCAM/ULTRACAM/ULTRASPEC. The probability depends on the galactic latitude of the star, with the all-sky average given by a galactic latitude of approximately 30 degrees, and the search radius. The latter depends on the size of the CCD chip or window. For example:
  • HiPERCAM uses five 2048x1024 e2v CCD231-42 detectors, with a platescale of 0.3 arcseconds/pixel on the WHT and 0.081 arcseconds/pixel on the GTC, resulting in maximum search radii of approximately 687 and 185 arcseconds on these two telescopes (as the target star can be put in one corner of the detector and the comparison star in the diagonally opposite corner).
  • ULTRACAM uses three 1024x1024 e2v CCD47-20 detectors, with a platescale of 0.3 arcseconds/pixel on the WHT, 0.35 arcseconds/pixel on the NTT and 0.15 arcseconds/pixel on the VLT, resulting in maximum search radii of approximately 434, 507 and 217 arcseconds on these three telescopes.
  • ULTRASPEC uses a 1024x1024 e2v CCD201-20 detector, with a platescale of 0.45 arcseconds/pixel on the TNT, resulting in a maximum search radius of approximately 652 arcseconds on this telescope.
The star counts used in this probability calculation have been taken from the Gemini techical note written by Doug Simons in August 1995: Longitudinally averaged R-band field star counts across the entire sky.

                          galactic latitude (degrees)
                          search radius (arcseconds)
                          R-band magnitude