Instrument Description

2.1 Science Objectives

The scientific objective of the WFPC2 is to provide photometrically and
geometrically accurate images of astronomical objects over a relatively wide field-of-view (FOV), with high angular resolution across a broad range of wavelengths.

WFPC2 meets or exceeds the photometric performance of WF/PC-1 in most areas. The goal is l% rms photometric accuracy, which means that the relative response in all 800x800 pixels per CCD must be known to better than 1% through each filter, and that standard calibrations be done at this level. Currently, the absolute calibration in the primary broadband photometric filters is accurate at around the 2% level, and is expected to continue to improve. Success in this area is dependent on the stability of all elements in the optical train, particularly the CCDs and filters.

The narrow point spread function is essential to all science programs being conducted with the WFPC2, because it allows one to both go deeper than ground based imagery, and to resolve smaller scale structure with higher reliability and dynamic range. Further, many of the scientific goals which originally justified the HST require that these high quality images be obtained across a wide field-of-view. The Cepheid distance scale program, for example, cannot be accomplished without a relatively wide field-of-view.

A unique capability of the WFPC2 is that it provides a sustained, high resolution, wide field imaging capability in the vacuum ultraviolet. Considerable effort has been expended to assure that this capability is maintained. Broad passband far-UV filters, including a Sodium Wood's filter, are included. The Wood's filter has superb red blocking characteristics. Photometry at wavelengths short of 3000Å is improved through the control of internal molecular contamination sources and the ability to put the CCDs through warm-up decontamination cycles without loss of prior calibrations.

While the WFPC2 CCDs have lower V-band quantum efficiency than the WF/PC-1 chips, for many applications this is more than made up for by the lower read noise, and by the intrinsically uniform flat field. For example, these characteristics are expected to increase the accuracy of stellar photometry, which was compromised by uncertainty in the flat field in WF/PC-1.