When designing a UV filter, a high suppression of off-band transmission, particularly in the red, has to be traded with overall in-band transmission. The very high blue quantum efficiency of the HRC compared to WFPC2 makes it possible to obtain an overall red leak suppression comparable to that of the WFPC2 while using much higher transmission filters. The ratio of in-band versus total flux is given in Table 5.6 for a few UV and blue HRC filters, where the cutoff point between in-band and out-of-band flux is defined as the filter's 1% transmission point. The same ratio is also listed for the equivalent filters in WFPC2. Clearly, red leaks are not a problem for F330W, F435W, and F475W. Red leaks are more important for F250W and F220W. In particular, accurate UV photometry of objects with the spectrum of an M star will require correction for the redleak in F250W and will be essentially impossible in F220W. For the latter filter a redleak correction will also be necessary for K and G stars.
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