CFHT Information Bulletin, number 38, First Semester 1998

  


Difficulties flux calibrating red spectra with MOS

Geneviève Soucail (soucail@obs-mip.fr,) Jean-Francois Le Borgne, Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées

 This short note reports some comments about the problems encountered when one attempts to flux calibrate the spectra observed over a large wavelength range. It concerns all the modes of the MOS/SIS/ARGUS instrument in the reddest part of the optical range (from 7500 Å to 9000 Å ), now available thanks to the high efficiency of the CCDs from 4000 Å to 9000 Å. Similar problems were mentionned by Crawford and Vanderriest (1997, MNRAS 285, 580) who observed quasar environments with MOS/ARGUS. In order to correctly reproduce the line ratios of the quasar already published, they had to include a "correction factor" increasing up than a factor of 2 at 9000 Å.

 Contamination by the second order is responsible for it as no order separation filter is included in the instrument setup of MOS. An illustration of the effect is shown in Figure 32: two spectra are displayed, the top one being obtained with an Helium-Argon lamp and the bottom one with the same lamp and a B filter. The presence of the contaminating lines from the second order is obvious. In addition, the most UV lines (the strong He line at 3888 Å for example) are stronger in the second order than in the first one! This is due to the blaze function optimised in the first order, which is distorted in the second order towards much bluer wavelengths.

 The consequences of this effect which was not taken into account in the MOS instrument are multiple and we present only a few of them:

 

 
  

Editor: Dr. T. M. C. Abbott, tmca@cfht.hawaii.edu

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