Introduction

1.3 History of WFPC2


The original Wide Field and Planetary Camera (WF/PC-1) served as the prototype for WFPC2. In many respects the two instruments are very similar. Both were designed to operate from 1150Å to 11000Å, both use 800x800 CCD detectors, and both provide spatial samplings of ~0.045" and ~0.1" per pixel. The development and construction of WF/PC-1 was led by Prof. J. A. Westphal, Principal Investigator (PI) of the California Institute of Technology. The instrument was built at the Jet Propulsion Laboratory (JPL) and was launched aboard HST in April 1990. It obtained scientific data until it was replaced by WFPC2 during the first service mission in December 1993.

Because of its important role in the overall HST mission, NASA decided to build a second Wide Field and Planetary Camera (WFPC2) as a backup clone of WF/PC-1 even before HST was launched. WFPC2 was already in the early stages of construction at JPL when HST was launched. After the discovery of spherical aberration in the HST primary mirror, it was quickly realized that a modification of the WFPC2 internal optics could correct the aberration and restore most of the originally expected imaging performance. As a result, development of WFPC2 was accelerated. Dr. J. T. Trauger of JPL is the project PI for WFPC2 and led the Investigation Definition Team (IDT).

The WFPC2 completed system level thermal vacuum (SLTV) testing at JPL in April and May 1993. Between June and November there were payload compatibility checks at Goddard Space Flight Center (GSFC), and payload integration at Kennedy Space Center (KSC). WF/PC-1 was replaced by WFPC2 during the first Servicing Mission in December 1993. WFPC2 was shown to meet most of its engineering and scientific performance requirements by testing conducted during the three month Servicing Mission Observatory Verification (SMOV) period following the servicing mission. The General Observer community has had access to WFPC2 since the start of Cycle 4 in January 1994.

WFPC2 accurately corrects the HST spherical aberration, is a scientifically capable camera configured for reliable operation in space without maintenance, and is an instrument which can be calibrated and maintained without excessive operational overhead. It incorporates evolutionary improvements in photometric imaging capabilities. The CCD sensors, signal chain electronics, filter set, UV performance, internal calibrations, and operational efficiency have all been improved through new technologies and lessons learned from WF/PC-1 operations and the HST experience since launch.

WFPC2 SMOV requirements were developed by the IDT, GSFC, and the STScI to include: verification of the baseline instrument performance; an optical adjustment by focusing and aligning to minimize coma; the estimation of residual wavefront errors from the analysis of star images; a photometric calibration with a core set of filters (including both visible and UV wavelengths); and the evaluation of photometric accuracy and stability over the full field with the core filter set. The results of these studies are documented in Holtzman, et al., 1995a and 1995b, and are summarized in this Handbook.

Despite these successes, the first years of scientific operation of WFPC2 have revealed a number of relatively minor instrumental defects that were not expected from the pre-launch testing. These include a low-level charge transfer inefficiency, a higher than expected level of scattered light around bright objects, and variable and lower than expected ultraviolet (UV) efficiency. In addition, we have come to understand the instrument more fully -- particularly in terms of its overall photometric performance, geometric distortion, scale and alignments, hot pixels, and CCD traps. All of this new information is described here.