1.1.3 Quantum Efficiency and Exposure Limits

The quantum efficiency (QE) of WFPC2+HST peaks at 14% in the red, and remains above 4% over the visible spectrum. The UV response extends to Lyman wavelengths (QE~0.1%). Internal optics provide a spherical aberration correction.

Exposures of bright targets are limited by saturation effects, which appear above ~53000 detected photons per pixel (for setting ATD-GAIN=15), and by the shortest exposure time which is 0.11 seconds. There are no instrument safety issues associated with bright targets. Detection of faint targets is limited by the sky background for broad band filters at visual wavelengths. For narrow band and ultraviolet filters, detections are limited by noise in the read-out amplifier ("read noise"), which contributes an RMS noise equivalent to ~5 detected photons per pixel.