MOSCA

MOSCA was commissioned at the start of the semester, just after the last STC meeting. Most of what I write below has already been included in my report to the Council in November 2002.

MOSCA has been commissioned on October 18, 2002. Although the weather was not very good, with part of the time cirrus clouds and relatively poor seeing, it was possible to do most of the tests. Analysis has shown that the camera is well aligned, and that the PSF is stable across the field within the limits set by the seeing. During the commissioning night also observations were obtained of the optical counterpart of GRB021004 which provided support for the existence of a break in the optical light curve.

Tests of the CCDs showed very similar results to those obtained in Copenhagen before shipping. The gain and read-out-noise is practically the same for all CCDs at 1.3e-/ADU and 8.5e-, respectively. The shutter was found to been extremely accurate, having an average error of $\sim$0.002 sec with a spread of $\pm\sim$0.001 sec across the field-of-view. A minor problem was found with the data of one of the CCDs which was (in retrospect) also found to exist in test data taken in Copenhagen, but which does not affect the data in any significant way. The main limitation is the relatively low full-well capacity of one of the CCDs, which varies across the CCD, being lowest in the corner closest to the center of the mosaic at a level approximately twice as low as for the other CCDs.

The zero-point (in electrons) for the different broad-band filters are given in Table 4. They compare very well with the zero-points of ALFOSC, with MOSCA especially being more efficient in the blue as expected.

Table 4: MOSCA zero-points

Filter	Zero-point
U	24.6
B	26.2
V	26.0
R	25.7
I	25.2

Under normal operations one would expected a data volume of various GB per night with MOSCA (which is a problem for archiving without the carousel). However, under many (if not most) circumstances 2x2 binning (providing 0.22$\times$0.22 arcsec pixels) appears more than adequate, which reduces the data volume significantly and has the added advantage of a reduced readout time for a full frame of 37 sec compared to 113 sec for a 1x1 binning.

Two visitor observing run have been made with MOSCA. Some minor problem have been encounter with the BIAS control software and with one of the FASU filter wheels, but the camera itself is performing without any problems. Up to now, the observers have selected to obtain all their data with 2$\times$2 binning.

A further observation of GRB021004 was made during the night of November 3, 2002, which helped to constrain the brightness of the underlying host galaxy. The measurements of GRB021004 made with MOSCA have been included in an article which has been published the Astronomical Journal.