CCD Performance

4.11 Charge Transfer Efficiency

The WFPC2 CCDs have a small but significant charge transfer efficiency (CTE) problem which causes some signal to be lost when charge is transferred down the chip during readout. This has the effect of making objects at higher row numbers (more charge transfers) appear fainter than they would if they were at low row numbers. The effect depends on the temperature of the CCDs. At the original temperature of 76 degrees C, as much as 10-15% of the light within a 0.5" radius aperture around a star could be lost for objects at the highest rows. As a result, the CCD operating temperature was changed to 88 degrees C on 23 April 1994. Now the effect seems to have a maximum amplitude of 4% for stars with more than 1,500 total detected electrons and no measurable difference in slope is seen for stars up to 20,000 total electrons. However, the brightest stars seem to have a smaller fractional loss. It is also possible that proportional losses for fainter stars, at least with no background, may be larger. Hence, the effect is not well described by either a constant fractional or a constant additive loss per charge transfer. The effect also depends on the amount of background on the chip. There is significantly less CTE effect in the presence of even a moderate (several hundred electrons) background. A major calibration effort is underway to quantify better the dependencies on star color and brightness, and background (see Chapter 8).

The photometric calibration of the instrument presented here (and this summary of CTE) is based on Holtzman, et al. (1995b). It has been crudely corrected for CTE. All of the frames considered are short exposures with essentially no background. For data taken at 88 degrees C, a 4% correction ramp was applied to the measured 0.5" radius aperture photometry, in the sense that objects at row 800 were made brighter by 4%, but the brightness of objects at the first row was not changed. The correction was applied to bring measurements to the values they would have had in the absence of CTE, or equivalently, the values they would have had if measurements had been made at row 0. Thus, the calibration presented here applies directly to images in which CTE is insignificant (for example, faint stars with high background levels). For images with low background, a CTE correction must be applied before using the calibration presented in this handbook.

The current lack of a comprehensive understanding of CTE effects introduces one of the largest uncertainties for WFPC2 photometry. The CTE problems are caused by electron traps in the CCDs which are filled as charge passes through pixels. However, not all traps are accessible to all electrons passing through. Some traps are only accessible if there is significant charge involved. This model suggests that there will not be significant CTE losses in the presence of background, particularly for faint stars, because background electrons fill the traps before such stars pass through. For brighter stars with background there will still be some loss because their charge may access traps that are unaffected by the background that previously clocked through. Faint stars in scenes with little background may suffer from larger losses, although there is no direct evidence for this yet.

An apparently related effect surfaced during Cycle 5 as observers compared long and short exposures of the several stellar fields--there was evidence for a discrepancy between the photometric zeropoint of long and short exposures. Further investigation shows that "long vs. short" is probably a misnomer. The level of background appears to be the main parameter rather than the exposure time. The magnitude difference measured between short and long exposures is more pronounced for faint stars in large apertures, where it can reach 0.05 mag, and is essentially absent for stars with more than 1000 total counts. The dependence on aperture and magnitude appears consistent with a charge transfer efficiency problem. The offset of faint star magnitudes can be explained by a loss of 0.3 DN (2 e-) in each pixel used in the aperture. Further testing and analysis of this effect are underway.

A side effect of the CTE problem, is that residual trails can appear in exposures after a highly exposed image is read out (Figure 4.11).

Figure 4.11: Images Illustrating CTE Residual Trail.

The trail is caused by charge which is trapped during read out of the highly exposed image, which is then slowly released during subsequent exposures. The effect is most pronounced when long exposures in through low throughput filters (narrow band or UV filters) immediately follow a highly exposed image (usually a broad band filter).