Each face of the pyramid was a concave spherical surface. The pyramid divided the OTA image of the sky into four parts. Each of these detectors was housed in a cell filled with 0.1 atmosphere of argon sealed by a MgF2 field flattener. To place the desired camera into operation, the pyramid was commanded to rotate into one of two fixed orientations separated by 45 degrees. The Wide Field chips are numbered 1 through 4, the Planetary Camera chips 6 through 8.
FULL mode is the default, with full single-pixel resolution.
In this mode, the BLEV correction is determined from the bias value in one of the engineering words, but without the least significant bit. The Kelsall spots , a series of 11 pinholes along each of the common pyramid edges used for image registration, are illuminated only during special calibration observations. As a result of the primary beam aberration, the light from sources near the pyramid edges is divided between adjacent chips, and consequently the lower columns and rows of all chips are strongly vignetted. The images from each chip are oriented such that north shifts by roughly 90 degrees from chip to chip. Figure 30.3 shows the image produced after wmosaic has been used to properly align and geometrically correct the four sub-images and produce a single mosaic image.