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1.0 Charge Transfer Efficiency

To be read in conjunction with Section 4.11 of the WFPC2 Instrument Handbook, -Version 4.0.
During the past year, two studies were completed resulting in a better characterization of the Charge Transfer Efficiency (CTE) problem for WFPC2, based on an analysis of observations of the globular cluster Omega Cen (NGC 5139). The first study provides a set of formulae that can be used to correct for CTE loss when doing aperture photometry, based on a dataset taken on June 29, 1996 (WFPC2 ISR 97-08, Whitmore and Heyer, 1997).1 The second study found evidence that CTE loss for faint stars has increased with time (Whitmore, 1998).

The primary observational consequence of CTE loss is that a point source at the top of the chip appears to be fainter than if observed at the bottom of the chip, due to the loss of electrons as the star is read out down the chip (see Figure 1). This is called Y-CTE. There also appears to be a similar, but weaker tendency, for stars on the right side of the chip to be fainter (called X-CTE). The effects also depend on the brightness of the star and the background level. Formulae are presented in WFPC2 ISR 97-08 that reduce the observational scatter in this particular dataset from 4-7% to 2-3%, depending on the filter.

Figure 1: Ratio of count rates observed for the same star (i.e., throughput ratio) as a function of the change in Y position for stars in 4 different brightness ranges. The non-zero slope shows that a star appears brighter when it is at the top of the chip than when it is at the bottom of the chip. The effect is larger for fainter stars. See ISR 97-08 for details.

A continuation of this project based on a set of 8 observations of Cen suggests that CTE loss for WFPC2 is time dependent. The datasets cover the time range from April 28, 1994 (shortly after the cooldown) to March 23, 1998. For bright stars (i.e., brighter than 200 DN when using gain = 15; equivalent to 400 DN for gain = 7) there is only a modest increase in the amount of CTE loss as a function of time. However, for faint stars CTE loss has increased more rapidly. For example, for very faint stars (i.e., 20-50 DN at a gain of 15) the CTE loss has increased from 3% to 26% for a star at the top of the chip. There is no obvious change in the value of X-CTE.

It should be noted that these results are based on very short (14 second) exposures. In general, typical WFPC2 exposures are much longer than these short calibration images, resulting in higher background levels which significantly reduces the CTE loss and minimizes the CTE problem for most science observations. The observations used in Figure 2 below had backgrounds of ~0.1 DN/pixel.

Figure 2: Y-CTE loss as a function of time for four different ranges of the target brightness. The open circles represent measurements with exposure times longer than most of the other exposures (i.e., 14 seconds), with the normalized value shown above as a filled circle. The stars show the predictions based on the formulae in ISR 97-08 for the June 1996 data.

As an observer, there are a few different strategies for minimizing the effect of CTE loss. The first is to take longer exposures when possible, so the background is higher and the target brighter, both of which reduce CTE loss. Users thinking of dithering may wish to take this into account if they are considering shortened exposures to allow for more dither positions. When the highest possible accuracy is required another strategy would be to include a special calibration observation of Cen in your own program taken as close in time as possible to your own program target. Another strategy when only a single small target is required would be to place the target near the bottom of the CCD, though not too close so that edge effects become important. A further possible strategy is to preflash the chip to raise the background. However, tests indicate that the required level of preflash is so high that in general more is lost than gained by this method. A variation of this currently being tested is "noiseless" preflash, where a flatfield exposure is read out immediately prior to a short science exposure.2

Calibration proposal 7929 has been added to the Cycle 7 calibration plan in order to monitor CTE every 6 months.

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1 A list of references is presented on page 26.

2 More details can be found in Whitmore, 1998.

Copyright © 1998, Association of Universities for Research in Astronomy, Inc. All rights reserved.

Last updated: 06/16/98 10:38:00