Spectrophotometric variability

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.

MOS results

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.

Observation of the standard G 158-100 using MOS and HR_red. First set from 11 September 2008.

Observation of the standard G 158-100 using MOS and HR_red. Second set from 11 September 2008.

IFU results

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).

Observation of the standard LTT 9293 using IFU and HR_blue. First set from 12 September 2008.

Observation of the standard standard LTT 9293 using IFU and HR_blue. Second set from 12 September 2008.

Observation of the standard LTT 9293 using IFU and HR_red. First set from 12 September 2008.

Observation of the standard LTT 9293 using IFU and HR_red. Second set from 12 September 2008.