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