Digicon

A digicon detector is a spatially resolved light detector using the photoelectric effect directly; it uses magnetic and electric fields operating in a vacuum to focus the electrons released from a photocathode by incoming light onto a collection of silicon diodes. It is a photon-counting instrument, so most useful for weak sources.

Digicon detectors were used on the original instruments for the Hubble Space Telescope, but are very rarely used in new designs, where CMOS active-pixel detectors can achieve the same performance without the need for large electric fields or complicated vacuum assemblies.

References

http://www.stsci.edu/instrument-news/handbooks/ghrs/GHRS_35.html describes (with rather ugly diagrams) the digicons used in the GHRS instrument on Hubble.