variability of measurements with specphot standards
Response curve and efficiency measurements using spectrophotometric standard stars in MOS and IFU modes can show significant variations. On the nights of 11 and 12 September 2008, repeated observations of the same standard star have been carried out during the night in stable atmospheric conditions for several MOS and IFU modes. Efficiency curves have been determined by using VIMOS pipeline recipes. Some example results are shown below.
The following figures show efficiency curves derived from observations of G 158-100 using the HR_red grism. Results for all four quadrants are plotted (back: quadrant 1, red: 2, green: 3, blue:4). In the second figure, the efficiency curve for quadrant 4 is significantly lower than in the first one. This illustrates a typical problem for MOS standard stars: observations in one setting must be carried out sequentially for quadrants 1 to 4. Acquisition is executed only once (before observing quadrant 1), the star is then centered on the other quadrants by blind offsets. Pointing errors may accumulate so that the star is slightly off the slit when quadrant 4 is observed. This results in an apparently lower efficiency for this quadrant.
IFU example results are shown for HR_blue (first two figures) and for
HR_red (last two figures). The standard was LTT 9293. Here, the situation
is more complex as different quadrants are subject to variations. It is
yet unclear whether the observed variations are instrument-intrinsic or
due to reduction problems (e.g. incorrect sky subtraction).