As the CCD is extremely sensitive to optical light as well as X-rays, an optical/UV blocking filter is required in front of the chip. The filter consists of 800 Å of Al plus 400 Å of Ti mounted on a 90% transmitting nickel mesh. The filter is mounted inside the camera.
The choice of filter materials and expected pinhole production due to micrometeoroids are discussed by Skinner & Burrows (1993). Optical photons produce only a single electron per photon in the CCD and will be screened out by our event recognition algorithm (see §4.4.15). They are not a problem provided that the count rate is low enough so that the chance of an optical photon appearing in the same pixel as an X-ray event is negligible. EUV photons are more troublesome, since they can produce several electrons per pixel. Worse, the geocoronal He II line at 304 Å is quite strong and would dominate the count rate over our field of view if we used a standard Al filter as we had originally planned. The Al/Ti filter we have chosen has reasonably good transmission to soft X-rays while providing a transmission of at optical wavelengths, 4.5% for broad-band EUV, and at 304 Å. This is sufficiently low to ensure that the soft X-ray background dominates the EUV background by an order of magnitude and that the optical background is low enough (20 counts per CCD per exposure) to have negligible impact on our data.
This extremely thin filter is quite fragile and is easily destroyed by mechanical contact. (Chance contact with the filter is almost impossible as it is only accessible when the camera door is open.) However, the blocking filter will withstand vibration. We have flown similar freestanding filters on our last three rocket flights without any damage. The flight filter has been vibrated twice and has been tested acoustically without suffering any damage.
We have performed a burst test on an identical filter in the laboratory and have shown that it can support a differential pressure of 18 T before significant damage occurs. Even at 20 Torr, only 8 mesh cells broke during this test.