Both, Fig. 105 and Table 20, 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 detector, i.e., free of deadtime and coincidence loss, and without time sensitivity degradation, and with an aperture radius of () for optical (UV) filters.
The numbers listed in Table 20 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 as 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 instance using SAS. For the 20th magnitude stars in Fig. 105 and Table 20, OM coincidence losses are negligible. Losses become significant for a point source at a count rate of about 10 counts s (about 10% coincidence) for a full frame exposure.
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 of the coincidence loss correction 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) may not damage the detector, but are scientifically useless.
We give in Table 21 coincidence loss corrected rates for limit high measured rates. The table gives the maximum measurable rate for each given value of frametime (1 count per frametime) and the corresponding corrected values for the maximum rate minus 0.01 and 0.001 c/s. For these high rates we obtain correction factors of 10, and very small differences in count rate produce variations in the corrected rates of more than 20%.
|Frametime||Maximum||Corrected rate||Corrected rate|
The coincidence loss correction implemented in SAS (from version 14) sets a validity limit at a rate of 0.97 counts per frametime (source+background). Within this limit, the correction error is less than 2%. This corresponds to about 700 and 300 source+background corrected counts for frametimes of 5 and 11 ms, respectively. For values higher than 0.97, the source counts are not corrected by SAS, and therefore, the conversion to magnitudes and fluxes are not applied. The produced source list contains only the measured count rate and position for these sources.
In Fig. 106, a comparison between formula (4) and the inflight instrument performance is shown.
Table 23 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 22.
|Filter||Spectral type range|
A detailed study of the background sources in OM can be found in:
The expected levels of different external background radiation processes in the optical/UV are tabulated in Table 24. 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. 107).