next up previous contents
Next: Detector changes Up: Background Previous: Background - HRI

Background WFC


The total count rate over the full FOV of the WFC is in general be dominated by background except when observing the brightest XUV sources. There are a number of different sources of background which affect the sensitivity and observing constraints in different ways. These are described below. Detected count rates are quoted for the full field of view of the filters unless otherwise stated. The values given for the separate background components are still predictions, since the contribution these components make to the background measured in-orbit is still being studied.

The dark count or intrinsic background from the MCP detectors is always much smaller than the other components. The distribution of these counts is uniform over the face of the detectors and is unaffected by the filter in use.

Geocoronal and atmospheric glow in the UV & XUV due to resonance transitions of He and other species in the incident solar flux is very strong. The filters have been designed to reject this background which would otherwise swamp the WFC count rate. The rates are given in Table 7.3 gif and often dominate the total background count rate.


Filter WFC count rate (counts s tex2html_wrap_inline1894 )
S1: C/Lexan/B 10 - 20
S2: Be/Lexan 20 - 40
P1: Al/Lexan 50 - 200
P2: Sn/Al 200 - 21000
Table: 7.3   Predicted WFC count rate from geocorona and atmosphere

The rates vary as a function of line of sight through the atmosphere, the solar direction, satellite day/night and solar activity. During periods of high solar activity peak background count rates can exceed 1000 counts s tex2html_wrap_inline1894 but only for tex2html_wrap_inline2338 % of each orbit.

The particle flux associated with features like the South Atlantic Anomaly and the auroral horns is very high. On entering such regions the WFC is automatically switched off. Outside these regions the particle rate is believed to be very small. A separate, very hard particle flux arises from cosmic rays, mostly protons, giving 1-10 counts s tex2html_wrap_inline1894 dependent on ROSAT's geographic latitude. Both sources of hard flux are unaffected by the optics and will be uniformly distributed over the field.

The diffuse XUV sky background will also be seen by the WFC. Approximate values are given in Table 7.4 gif.


Filter WFC count rate (counts s tex2html_wrap_inline1894 )
S1: C/Lexan/B 20 - 60
S2: Be/Lexan 25 - 75
P1: Al/Lexan 2 - 6
P2: Sn/Al <560
Table: 7.4   Predicted WFC count rate from diffuse XUV sky background

Table 7.5 gif lists the approximate total background count rates which should be used when making feasibility calculations in connection with ROSAT observing proposals - see Chapter 12 gif. Measured background rates are in accord with these values, although there is of course considerable temporal variation.


Filter WFC count rate WFC count rate Comment
(full FOV)
counts s tex2html_wrap_inline1894 tex2html_wrap_inline2350 counts s tex2html_wrap_inline1894 arcmin tex2html_wrap_inline1930
S1 40 5.6
S2 45 6.3
P1 50 7.0
P2 tex2html_wrap_inline2356 tex2html_wrap_inline2358 tex2html_wrap_inline2360 counts s tex2html_wrap_inline1894
Table: 7.5   Predicted total WFC count rates

[1] At night

Note that the maximum WFC count rate which can normally be stored and telemetered is 200 counts s tex2html_wrap_inline1894 , although a detected rate of 1200 counts s tex2html_wrap_inline1894 can be tolerated before the detector is switched off. Average count rates above the 200 counts s tex2html_wrap_inline1894 limit imply large deadtime losses. During on-time the total background rate can vary between tex2html_wrap_inline2370  counts s tex2html_wrap_inline1894 for the survey filters and P1, but for the P2 filter, for which the background is dominated by the geocoronal lines, it ranges from tex2html_wrap_inline2374  counts s tex2html_wrap_inline1894 , with the lowest count rates occurring during satellite night. Observations requested with this filter will be scheduled at night if possible. The active field of view can be restricted by electronic windowing, if necessary to prevent excessive count rates and hence deadtimes (see § 5.3 gif).

next up previous contents
Next: Detector changes Up: Background Previous: Background - HRI

Michael Arida
Tue Jun 11 16:18:41 EDT 1996