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Some of the available WFC filters have a secondary transmission
band in the far-ultraviolet.
Although these are much weaker than the primary XUV bands,
problems may arise when observing objects that have a particularly
strong far-UV component in their spectrum.
The count rate detected by the WFC will then be an unresolved
mixture of XUV and far-UV photons.
In severe cases the scientific return of the observation may be
compromised.
Observers should ensure that their selected targets do not fall
into this category.
For any target the severity of the potential problem can be
assessed by referring to
Tab. 6.6.
This lists predicted count rates for each filter from an 
blackbody source at temperatures of 10000, 20000 and 
.
The expected far-UV count rate from a target can then be estimated
by scaling the 
count rate for the appropriate temperature
to the actual 
value (i.e. by multiplying the value given in
Tab. 6.6
by 
).
Note that the UV leak is largest for the S2 filter; for the other
filters the UV leak is unlikely to be a problem except for the
brightest stars.
| Temp (K) | Count rate in each filter (counts  ) | |||
| S1 | S2 | P1 | P2 | |
| 10000 |  |  | 
|  |
| 20000 |  | 0.74 | 
|  |
| 30000 |  | 2.1 | 
|  |
