Low light level CCDs L3CCDs

A low light level CCD (L3CCD) is a completely normal CCD with an extended serial or output register added in to the design. The extended section of the serial register, the gain register, is clocked out with a higher voltage than normal (40-60 volts) allowing for a slight possibility (1-2%) that each electron transferred through each gate will create a second electron by avalanche multiplication. The extended gain (or multiplication) register thus has a total probability of a gain equal to 1.01N, where N is the number of elements in the gain register. For a typical N of ~ 500, the L3CCD produces an on-ship gain of 145.

The electrons finally pass through a normal style CCD output amplifier and are digitized. Read noise (Chapter 3) is the noise in this output electronic stage and the equivalent read noise from an L3CCD is the read noise reduced by the

Fig. 2.8. An OTA silicon wafer fresh from the oven. Thin wafers of silicon are used along with various chemical processes and photomasks to produce the integrated circuits we call CCDs. Here we see a 150 ohm-cm Si wafer onto which have been built three 12 micron pixel OTAs (the checkerboard devices), two 2.6 K by 4 K, 12 micron pixel standard CCDs, two 2K by 1K CCDs, an 800 by 800 CCD, and a number of other smaller test CCD devices. The standard CCDs were used to determine the validity of the wafer production as well as being employed in a variety of instruments at the WIYN observatory, NOAO, and elsewhere. The test devices were used to provide feedback on a variety of electronic design and configuration tests used in the OTAs. The CCDs will be cut apart and mounted separately for use. The Si wafer is 15 cm in diameter.

Fig. 2.8. An OTA silicon wafer fresh from the oven. Thin wafers of silicon are used along with various chemical processes and photomasks to produce the integrated circuits we call CCDs. Here we see a 150 ohm-cm Si wafer onto which have been built three 12 micron pixel OTAs (the checkerboard devices), two 2.6 K by 4 K, 12 micron pixel standard CCDs, two 2K by 1K CCDs, an 800 by 800 CCD, and a number of other smaller test CCD devices. The standard CCDs were used to determine the validity of the wafer production as well as being employed in a variety of instruments at the WIYN observatory, NOAO, and elsewhere. The test devices were used to provide feedback on a variety of electronic design and configuration tests used in the OTAs. The CCDs will be cut apart and mounted separately for use. The Si wafer is 15 cm in diameter.

gain factor. Thus for low noise, slow readout CCDs, L3CCDs are often called noise-free. Even at very fast readout rates (10-100 frames/second), which we will see can greatly increase the read noise, they are quite respectable. L3CCDs are still a very new product being developed for security cameras, night vision applications, and fast readout requirements. To date they come in small formats (1000 x 1000 pixels with most being much less) of pixel sizes from 20-40 microns (E2V L3CCDs) to a version with 7.4 micron pixels (TI L3CCD). The number of gain stages, N, is near 400-600 and some astronomical applications have been tested.

The ESA space mission GAIA will use L3CCDs in one of its on-board CCD cameras. L3CCDs work as nearly perfect photon counting devices and promise to be a great addition to astronomical imagers. See Mackay et al., (2002) and Jerram et al., (2001) for more information. Appendix B lists some websites with additional information on L3CCDs.

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