Both, Fig. 90 and Table 21,
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.
Filter | B0 | A0 | F0 | G0 | G2 | K0 | M0 | WD |
V | 1555 | 1510 | 1473 | 1473 | 1495 | 1452 | 1392 | 1412 |
B | 6190 | 5272 | 3805 | 3010 | 2577 | 2289 | 1337 | 4454 |
U | 8533 | 2216 | 1486 | 1119 | 966 | 444 | 178 | 4667 |
UVW1 | 5647 | 958 | 366 | 218 | 182 | 29.1 | 17.4 | 3093 |
UVM2 | 2276 | 316 | 39.3 | 8.69 | 5.54 | 0.436 | 0.032 | 1174 |
UVW2 | 950 | 126 | 11.1 | 2.06 | 1.19 | 0.112 | 0.069 | 561 |
The numbers listed in Table 21 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. 90 and
Table 21, 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. 91, a comparison between formula (4) and the inflight instrument performance is shown.
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Filter | Spectral type | ||||
B0 | A0 | G0 | K0 | WD | |
V | 19.8 | 19.8 | 19.7 | 19.7 | 19.7 |
B | 21.0 | 20.8 | 20.2 | 19.9 | 20.6 |
U | 21.8 | 20.4 | 19.6 | 18.6 | 21.2 |
UVW1 | 21.1 | 19.2 | 17.6 | 15.4 | 20.5 |
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 | ||
B0/A0 | A1/G3 | G4/M8 | |
V | 19.5 | 19.4 | 19.4 |
B | 20.6 | 19.8 | 19.3 |
U | 20.2 | 19.4 | 18.6 |
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![]() ![]() |
Zodiacal![]() |
longitude
![]() |
1.69![]() ![]() |
Average dark count rate![]() |
all directions |
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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. 92).
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