During an exposure the image is integrated in the SEC Vidicon section of the camera. There is no exposure meter so the length of the exposure must be estimated. The duration of the exposure is controlled by the on-board computer (OBC). The exposure length is quantized in units of 0.4096 seconds and can be modified in real-time.
At the conclusion of the exposure the camera retains the image until a read is initiated. A read consists of a raster scan of 768 X 768 pixels. The video signal is digitized into one of 256 discrete levels (0 to 255 Data Numbers, or DN) by an eight-bit analog-to-digital converter. Since there is no on-board data recorder, the signal is concurrently transmitted to the ground station in real-time as the read scan is performed. At the highest available telemetry rate, 20 kilobits/sec, the transmission of an entire image and associated engineering data takes 5.24 minutes. The read is destructive, so if something happens to the quality of the received signal or to the ground data-handling system during the read, portions of the image can be permanently lost.
After a camera has been read, residual images are erased and a reproducible electronic pedestal of 15 to 40 DN is produced by exposing the camera to a tungsten flood lamp, reading the camera with a defocussed beam, and then exposing and reading again. This sequence is called a PREP. Standard and overexposed preps are available (see Section 4.6). Technical details on the cameras are given in the IUE Camera User's Guide (Coleman et al. 1977).