2012 programme

Sat 7th
16:00-16:30
TBA

A Method for Differential CCD or DSLR Photometry Requiring No Determination of Air Mass or Extinction Coefficients

Roy Axelsen

Astronomical Association of Queensland

Where comparison stars and the variable star differ in colour index, and particularly when observations are made at high air mass, accurate classical differential photometry requires the determination of air mass and extinction coefficients. This paper describes a method in which these determinations are not required. It is a modifcation of a method for DSLR photometry published on the AAVSO web site. The requirements are: (1) the field of view is sufficiently small that the extinction gradient across the field is negligible for the particular project, (2) several comparison stars, of differing colour index, are present in the same field as the variable and check stars, (3) the colour index of the variable does not differ markedly from the colour index of the comparison stars, and (4) the instrumental magnitudes of the comparison, check and variable stars can be determined in two colour bands (e.g., B and V). In principle, the method first uses the equation of the transform relationship between (B-V)-(b-v) and (B-V) for sevaral comparison stars, and then the transform relationship between (V-v) and (B-V) for the same comparison stars, to calculated (B-V) and then V for the variable and check stars. In the above, V, and B are catalogue magnitudes; v and b are instrumental magnitudes.

Roy Axelsen is the Director of the Variable Star Section of the Astronomical Association of Queensland, and is member of the AAVSO. He has an interest in methods for two-colour PEP and DSLR photometry. A current interest is the changing periods of Delta Scuti stars.