3 Basic reductions

The linearity of the Loral/Lesser CCD was tested by taking dome flat fields with an increasing number of openings of the shutter, i.e. thereby increasing the exposure time but eliminating the finite shutter opening time. Unfortunately, the quartz lamp that was used as a light source was variable at a 5% level, and hence a non-linearity below this level could not be measured. However, according to the telescope manual (Storm et al. 1996) the CCD is linear below 1% up to the 16 bit data saturation level of the CCD, which is then our assumed limit for this effect.

The raw science frames were bias subtracted and flat fielded. A weak dependency of the bias level on the ambient temperature was determined from several bias frames (0.9 ADU/K). We found that our sky flat fields were not as reliable as the dome flats (different gradients across the field were often seen in two consecutive frames). Hence we decided to use dome flats for the flat fielding procedure. A master flat field for each filter was constructed using the median of several dome flats. The significance of our choice of flat fields is diminished by the flat field correction that is applied later.

After bias subtraction and flat field division the frames of the open clusters were reduced by using the MOMF software package by Kjeldsen & Frandsen (1992). The MOMF programmes use a combination of PSF- and aperture photometry.

Figure 1: Contour plot of the flat field correction surface that has been applied to the flat field for the y filter. The abscissa and ordinate show the position (in pixels) on the CCD