Both, Fig. 106 and Table 22, provide the expected count rate of the OM, in counts per second, for mag stars of various types, with the different filters introduced above. These numbers were obtained under the assumption of a perfect, i.e., deadtime-free and coincidence loss-free, detector, and with an aperture radius of () for optical (UV) filters.
The numbers listed in Table 22 can be used to calculate the perfect detector count rates of stars of given magnitude by the formula
where is the count rate of an mag star of the same spectral type than the target, and and are the and the count rate of the target of interest, respectively.
However, OM deadtime and coincidence losses must be taken into account. For the 20th magnitude stars in Fig. 106 and Table 22, OM coincidence losses are negligible. Losses become significant for a point source at a count rate of about 10 counts s (about 10% coincidence).
The correction is approximated by the following formula:
An optimum aperture radius of was found to be the radius at which the loss correction formula is self consistent. At this radius the accuracy is at a level of 5% between different frametimes.
The validity range of the coincidence loss correction can be obtained from eq. 4 as a function of the frametime, whose maximum value (11.04 ms) occurs for full frame exposures, high or low resolution, where the whole detector is used. In windowed exposures the frametime varies from about 5 to 10 ms. For this reason the maximum measured count rates that can be reliably corrected are in the range of a few hundreds counts per second. Higher rates (1000 cts/s) do not damage the detector, but are scientifically useless.
In Fig. 107, a comparison between formula (4) and the inflight instrument performance is shown.
Table 24 presents the limiting magnitudes derived from the photometric analysis of one of the OM calibration fields (HD5980, also observed from the ground). This can be considered as an extreme case because of the crowdedness of the field which increases the observed background by overlap of the PSF wings. This is why the obtained limits are brighter than the simulations of Table 23.
|Filter||Spectral type range|
The expected levels of different external background radiation processes in the optical/UV are tabulated in Table 25. The background count rate in the OM is dominated by the zodiacal light in the optical. In the far UV the intrinsic detector background becomes important. Images are regularly taken with the blocked filter and no LED illumination to measure the detector dark counts. The mean OM dark count rate is 4.0 counts s pixel. The variation across the detector is 9% , with a mainly radial dependence, being highest in an annulus of about 8' radius and lowest at the centre. Variations as a function of time are 7% , without any apparent trend. However, if a very bright star is in the field of view, the dark rate can be up to 60% higher than normal, despite the use of the Blocked filter.
|Background source||Occurrence||Count rate range|
|Diffuse Galactic||all directions||2.14-7.527|
|Average dark count rate||all directions|
Artifacts can appear in the XMM-Newton OM images due to light being scattered within the detector. These have two causes: internal reflection of light within the detector window and reflection of the off-axis starlight and background light from part of the detector housing.
The first of these causes a faint, out of focus ghost image of a bright star, displaced in the radial direction away from the primary image (see Fig. 108).