Data reduction: description

Flat fields

The contributors to the flat field are the pixel-pixel sensitivity variation, differences in fibre transmission profiles with wavelength, and differences in the total fibre throughput. The pixel-pixel sensitivity variations can be removed by a flat field made by uniformly illuminating the detector not through the fibres, or by illuminating the detector by moving the fibre slit along the detector in the spatial direction. Here, however, we are assuming that differences in fibre transmission profiles are important. In this case the method described below to remove these differences also removes differences due to pixel-pixel sensitivity variations, and so the pixel-pixel flat is not required. If the fibre transmission profiles are different then a dome flat can be used to remove these differences.

The differences in total fibre throughput are found by taking sky flats. The illumination of the IFU by the sky is assumed to be flat. However the number of counts per pixel in the sky flat spectra will be too low to allow the fibre transmission variations to be calculated from these spectra. As when creating the dome flat, the thermal background and dark current have to be removed from the sky flats. This is achieved by taking two sky flat observations, one of the bright sky and one of the faint sky. Subtracting the faint sky observation from the bright sky observation then removes the thermal background and dark current and still leaves a sky signal.

Rachel Johnson <raj@ast.cam.ac.uk>

Last modified: Thu Oct 19 15:30:05 2000