Charge transfer efficiency (CTE) is a measure of how effective the CCD is at moving charge from one pixel location to the next when reading out the chip. A perfect CCD would be able to transfer 100% of the charge as the charge is shunted across the chip and out through the serial register. In practice, small traps in the silicon lattice compromise this process by holding on to electrons, releasing them at a later time. (Depending on the trap type, the release time ranges from a few microseconds to several seconds). For large charge packets (several thousands of electrons), losing a few electrons along the way is not a serious problem, but for smaller (~100 e- or less) signals, it can have a substantial effect.
CTE is typically measured as a pixel transfer efficiency, and would be unity for a perfect CCD. The CTE numbers for the ACS CCDs at the time of installation are given in Table 4.5. While the numbers look impressive, remember that reading out the WFC CCD requires 2048 parallel and 2048 serial transfers, so that almost 2% of the charge from a pixel in the corner opposite the readout amplifier is lost.
Chip |
Parallel |
Serial |
---|---|---|
WFC1 |
0.999995 |
0.999999 |
WFC2 |
0.999995 |
0.999999 |
HRC |
0.999983 |
0.999994 |
Also, the CTE numbers are significantly different for images where the pixels have a low intensity compared to those where the intensity is high.
Both the WFPC2 and STIS CCDs have been found to suffer from a significant degradation in CTE since their installation in 1993 and 1997, respectively. At the end of Cycle 11 we performed the first on-orbit calibration of the photometric losses due to imperfect CTE on ACS HRC and WFC. We used images of 47 Tucanae from a CTE calibration program to measure the dependence of stellar photometry on the number of parallel and serial transfers. The results are described by Riess et al. (ACS ISR 2003-09
) and are summarized here. For WFC, significant photometric losses are apparent for stars undergoing numerous parallel transfers (y-direction) and are ~1 to 2% for typical observing parameters, rising to ~10% in worst cases (faint stars, low background). The size of the photometric loss appears to have a strong power-law dependence on the stellar flux, as seen for other CCDs flown on HST.
The dependence on background is significant but there is little advantage to increasing the background intentionally (e.g., by post-flashing) due to the added shot noise. No losses are apparent for WFC due to serial transfers (x-direction). For HRC, significant photometric losses also arise from parallel transfer (~1% for typical observations, ~5% for worst case) but are not seen for serial transfer. Correction formulae are presented in ACS ISR 2004-06
to correct photometric losses as a function of a source's position, flux, background, time, and aperture size.
Figure 4.12 shows the predicted photometric losses for the WFC due to imperfect parallel CTE as a function of time. These curves require extreme extrapolation and should be used for planning purposes only. Four specific science applications are shown as examples: the measurement of the faint end of M31's CMD (GO 9453), the measurement of high-redshift supernovae (GO 9528), the measurement of any PSF whose brightness is the zeropoint (i.e., 1 e-/second), and photometry of a 20th magnitude star in a narrow band.
Figure 4.12: Projected CTE losses in WFC (and equivalently, the size of corrections). Science application |
Source flux (e-) |
Sky (e-) |
---|---|---|
SN Ia at peak z = 1.5 |
100 |
30 |
M31 faint-end of CMD |
40 |
100 |
PSF e-/sec, 1/2 orbit |
1000 |
40 |
F502N, 30 sec, 20th mag star |
258 |
0.1 |
When observing a single target significantly smaller than a single detector, it is possible to place it near an amplifier to reduce the impact of imperfect CTE. This is easy to accomplish by judicious choice of aperture and target position, or by utilizing POS TARG commands. However, be aware that large POS TARGs are not advisable because they change the fractional pixel shifts of dither patterns due to the geometric distortion of ACS. An alternative means to achieve the placement of a target near the amplifier is by using some of the subarray apertures. For example, WFC1-512 (target will have 256 transfers in X and Y), WFC1-1K, and WFC1-2K place the target near the B amplifier (or WFC2-2K for the C amplifier).
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