Instrumental Parameters

The Wide-Field / Planetary Camera (WFPC), and its replacement camera, WFPC 2, have nearly identical optical layouts (Fig. 1).

In both cases light from the Optical Telescope Assembly (OTA) is reflected off a ``pickoff mirror'', passes through the shutter and filter assemblies, and is then divided into four separate channels by a ``pyramid'' mirror which also serves as a field flattener. Light from each channel is then reflected off a flat ``fold'' mirror, through a Cassegrain relay system, through a MgF field flattener, and finally onto an 800800 pixel CCD detector.

An important difference between the old and new cameras is that WFPC has four Wide-Field Camera (WFC) and four Planetary Camera (PC) imaging channels. Either the WFC or PC mode can be chosen by simply rotating the pyramid mirror. Instead, WFPC 2 has only three WFC channels and a single PC channel with a fixed pyramid mirror. But perhaps the most important difference is that WFPC 2 has a corrective figure applied to the Cassegrain relay secondary mirrors, which nullifies the spherical aberration of the OTA primary mirror.

Properties of the CCD detectors for each camera are summarized in Table 1. WFPC uses thinned back-side illuminated CCDs, whereas the WFPC 2 detectors are front-illuminated. The front-illuminated devices have much better flat-field characteristics, but a price is paid in the form of lower quantum efficiency (QE) at red wavelengths. An important advantage of WFPC 2 is its lower read-out noise. The read-out noise is a factor lower than the old camera, and more than compensates for the reduced QE.