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4.5 Residual Image

A second potential source of residual images, which occurs only in front-side-illuminated CCDs, is known as residual bulk image (RBI). Long wavelength photons can penetrate deeply enough to produce charge in the substrate. Most of this charge recombines rapidly (due to short carrier lifetimes), but some may diffuse into the epitaxial layer, where it can become trapped in epitaxial interface states. Residual images can occur as this charge is slowly released during an exposure. RBI is temperature sensitive since the bulk trapping time constants decrease with increasing temperature. The WFPC2 CCDs do exhibit RBI, but at -70 degrees C trapped charge rapidly escapes so that residual images disappear within 1000s. Driven by the WFPC2 electronics, the CCDs recover quickly from large over-exposures (100 times full well or more), showing no measurable residual images a half hour after the overexposure.

For images exposed below the saturation level there is a very weak residual image due to charge trapping and charge transfer efficiency (CTE) problem. Measurements on 1800s dark frames interleaved with 2800s exposures of a star field yield a residual flux of 0.3% +/- 0.1% of the original star flux, for stars with fluxes from 65 to 17,000 total counts. For typical star fields observed by WFPC2, these residual images are likely to be a problem only for stars that were saturated in a previous image, or for programs where long exposures in low throughput filters are taken immediately after highly exposes images. Hence, repeated exposures at the same CCD position should not lead to any appreciable systematic offset in photometry. CTE is further discussed in Section 4.11.