About ROSAT	ROSAT Home Page	ROSAT Images

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  and often dominate the total background count rate.

 

S1: C/Lexan/B 10 - 20

S2: Be/Lexan 20 - 40

P1: Al/Lexan 50 - 200

P2: Sn/Al 200 - 21000

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 but only for % 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 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 .

 

S1: C/Lexan/B 20 - 60

S2: Be/Lexan 25 - 75

P1: Al/Lexan 2 - 6

P2: Sn/Al <560

Table 7.5  lists the approximate total background count rates which should be used when making feasibility calculations in connection with ROSAT observing proposals - see Chapter 12 . 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)

S1 40 5.6

S2 45 6.3

P1 50 7.0 P2

[1] At night

Note that the maximum WFC count rate which can normally be stored and telemetered is 200 counts s , although a detected rate of 1200 counts s can be tolerated before the detector is switched off. Average count rates above the 200 counts s limit imply large deadtime losses. During on-time the total background rate can vary between  counts s for the survey filters and P1, but for the P2 filter, for which the background is dominated by the geocoronal lines, it ranges from  counts s , 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 ).