Calibration for Spectroscopic Modes

Spectroscopic observations require the following calibration frames:

Contents

Flat fields

The procedures for obtaining flat fields are described in greater detail elsewhere (follow the links under the different spectroscopic modes).

The different types of flat fields used for spectroscopy are :

  • Dome flats        (pixel-to-pixel gain variation)
  • Internal flats    (pixel-to-pixel gain variation)
  • Sky flats            (slit illumination/transfer function)

In spectroscopy dome and sky flats serve different purposes unlike in imaging. Because of the presence of a large number of prominent absorption features in sunlight due to atmospheric absorption the sky is not a flat screen for spectroscopy. So dome flats or internal flats, taken using a featureless lamp, are essential for determining the pixel-to-pixel gain variation of the CCD.

Dome Flats   Dome flats are taken by pointing the telescope towards an illuminated flat field screen. This is usually done during the afternoon since pointing the telescope to the flat screen takes a while. In general this is sufficient for flat-fielding observations taken through the night.

Internal Flats   On a couple of occasions recently we have been unable to eliminate some residual fringes in some red grisms using the afternoon dome flats. We suspect it may be due to a combination of CCD distortion (with pointing) and strong fringes in the red part of the spectrum ..... leading to low level but discernable fringes after flat-fielding. This is more noticable since EFOSC2 was moved to the NTT, as the instrument flexures are (currently) larger than they were at the 3.6m. One solution which seems to work is to take flat frames with the telescope pointing towards the science target. This can be done using internal flats immediately following the science observations of the target.

Sky Flats   Several factors like uneven slit edges and vignetting (not a problem with EFOSC2) can introduce a variation of the light throughput along a slit.  Observing a spectrophotometric standard star does correct for the slit throughput but only at the location of standard star on the slit. However a spectroscopic sky flat allows one to determine this slit illumination/transfer function all along the slit. This can be used to derive the appropriate correction factors for  objects observed at other locations along the slit (large extended objects, multiple objects along a slit etc)

Flat Field Intensity Level   In general the red photons emitted by the flat field lamps greatly outnumber the blue photons - in some cases the disparity is as high as 40:1. It is recommended that the peak level on any single flat field frame should be about 40,000 counts. 5 frames of this level are usually more than adequate. However a peak of 40,000 counts may mean a count of only 1000-2000 in the blue. One can build up the blue signal by taking more exposures (say 10-15 frames).  Note : one needs flat fields for each combination of grism, slit, binning, and readout speed .

Additionally one can combine flat frames taken on several days - the instrument is general stable over many days. Important  The spectrum of a flat field lamp varies with its temperature and hence the time for which it is on. Since flat field lamps are only used to determine the pixel-to-pixel variations (and not the absolute calibration) it is good practice to make a separate normalised flat field frame for each day and then average them instead of combining them all before normalising them.

There is hardly any vignetting in EFOSC2 - a single sky flat is sufficient, if at all required.

Calibration for long slit spectroscopy

Calibrations for SpectroPolarimetry