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Ultraviolet sensitivity of the WFC: the ``UV leak''

 

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 Table 12.1  . This lists predicted count rates for each filter from an blackbody source at temperatures of 10000, 20000 and 30000 K. 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 Table 12.1  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.

 

S1 S2 P1 P2

C/Lexan/B Be/Lexan Al/Lexan Sn/Al 10000

20000 0.74

30000 2.1