4 OM performance

The first light observation for XMM-OM took place on 2000 January 11. Since then the various engineering and science data taking modes of the instrument have been commissioned including full-field image engineering mode (not generally available for science observations because of the very large telemetry overhead required to transmit the data to the ground), and the Image and Fast science modes. The telescope focus has been optimised using the heater-based fine focus control, and the gain of the image intensifier has been optimised. The performance of the DPU in tracking image motion due to spacecraft drift has also been verified and distortion maps derived to relate XMM-OM detector coordinates to the sky. Photometric calibrations have been derived for all filter elements, but work continues on colour equations and to tie these more accurately into standard systems. Similarly, preliminary throughput and wavelength calibrations have been derived for the Grisms.

Figure 3: An image of part of the Lockman Hole field taken in various XMM-OM filters as marked. The object circled in the White Light image is an AGN, referred to as R32 by Schmidt et al. (1998). The size of the images is about 3.5 arcmin across and the exposure times were 1500 s (white), 1000 s (V), 1000 s (B), 1000 s (U), 2200 s (UVW1) & 4400 s (UVW2). Faint vertical bars in some of the images are caused by charge leakage along the readout direction of the CCD from bright stars outside the illustrated field

To illustrate the capabilities of XMM-OM, we show in Fig. 3 images of part of the Lockman Hole field in the White Light filter, and in five of the six colour filters (the remaining filter, UVM2, was not used during this observation). The images contains an R=18.1 mag AGN identified in the ROSAT observation of the field, and referred to as R32 by Schmidt et al. (1998). The AGN is clearly UV bright. The XMM-OM detects approximately 12 count s^-1 from the AGN in White Light, while the count rate in the colour filters ranges from a high of 2.9 count s^-1 in U, down to about 0.25 count s^-1 in the UV filter UVW2.

To illustrate the spectral capability of XMM-OM, we show in Fig. 4 the extracted spectrum of the DA white dwarf standard BPM16274. The Balmer absorption lines can be clearly discerned.

Figure 4: Spectrum of the white dwarf BPM16274 obtained using the visual grism. The regular low-level fluctuations in the signal with wavelength are due to the modulo-8 pattern (Sect. 5.6.3), which has not been corrected in this plot