5.6 Generic Detector and Camera Properties
5.6.1 Gain Calibration
The ACS CCDs have default gain values of approximately 1 e-/DN (WFC) and 2 e-/DN (HRC) -- these settings have, in particular, been used to establish adjustments to basic quantum efficiency curves. Minor errors in the default gain values would therefore be absorbed into revisions of the QE curves. There is no compelling reason to seek refinement in the quantitative values of the default used for primary calibrations and the bulk of the science program.
The default gains do not sample the full well depths of the CCDs, falling short by about 35% (WFC), and 22% (HRC). Use of the next higher gain values of approximately 2 e-/DN (WFC) and 4 e-/DN (HRC) do provide full sampling of the roughly 85,000 e- and 165,000 e- full well depths, respectively. Since the readout noise is only marginally higher than with the default gains, and is well-sampled even at these higher values (true for WFC, not quite for HRC, but even here its much better than for any WFPC2 data ever taken), many science programs and calibration programs may logically choose these gains.
ACS gain values in use between on-orbit installation and January 2004 can be traced to ground test results. The primary tool for measuring gain values is the photon-transfer method which is described in the Appendix at
http://acs.pha.jhu.edu/instrument/calibration/results/by_item/detector/wfc/build4/gain/
This technique relies on analysis of pairs of identical exposure flat fields taken at a range of intensity levels. Over an ensemble of pixels at a given exposure level, the relation between differences of intensity values (noise) and the direct signal level depends on the readout noise and the gain. This relation can be fit at a range of intensity levels to uniquely determine these two quantities. This technique can also provide limited information on linearity (which depends on the constancy of the lamp source, or experimental techniques to control for drifts) and the saturation count level (which may be different for a uniform illumination pattern than for point sources) by determining where the photon-transfer curve departs from linearity at high signal levels. Errors of about 0.6% in the gain values are quoted for the WFC determinations, and similar values hold for the HRC as given at
http://acs.pha.jhu.edu/instrument/calibration/results/by_item/detector/hrc/build1/gain/
On the WFC, with two CCDs and the default use of two readout amplifiers per CCD, errors of 0.6% in the normalization of gains quadrant-to-quadrant would leave easily visible steps in observations of spatially flat sources. A redetermination of the WFC gains (ACS ISR 02-03), maintaining the same mean over all amplifiers at a given gain setting but using a continuity constraint across quadrant boundaries, provided an improvement in amplifier-to-amplifier gains at better than 0.1% for the default gains. Any steps of intensity seen in data across quadrant boundaries are likely to reflect minor errors in bias and overscan values (see section 5.2), rather than errors in relative gains.
In ACS ISR 04-01 on-orbit data was used to redetermine the mean absolute gain values relative to the standard adopted for the default gains on each camera, e.g., WFC GAIN=1 and HRC GAIN=2 values were retained and other gains adjusted relative to these. Through analysis of observations of the same stellar field, in the same filter and at the two gain settings it is possible to obtain accurate adjustments of these gain values relative to the default gain levels. The results discussed here supply these to better than 0.1%, removing errors that averaged about 1% for the two cameras prior to updates made in January 2004.
Absolute errors of about 0.6% could remain in the default gain values, but these are of no real consequence since redetermination of the CCD quantum efficiency would compensate for this, since the QE adjustments were based on data acquired with the default gains. Basic photometric calibrations apply with equal accuracy to data acquired at all supported gains (assuming for the non-default gains that reprocessing was done after January 6, 2004 to include the revised values).
High Resolution Camera
The prelaunch GAIN = 4 value using the default AMP C is 4.289. The revised value based on on-orbit calibrations is 4.235. For GAIN = 1 (less important since this is available-but-unsupported), the prelaunch value of 1.185 has been revised to 1.163.
Wide Field Camera
Revisions of WFC GAIN = 2 values based on on-orbit testing are for prelaunch (new): AMP A 2.018 (2.002), AMP B 1.960 (1.945), AMP C 2.044 (2.028), and AMP D 2.010 (1.994). And for the available-but-unsupported GAIN = 4 the prelaunch (new) values are: AMP A 4.005 (4.011), AMP B 3.897 (3.902), AMP C 4.068 (4.074), and AMP D 3.990 (3.996).
5.6.2 Full Well Depth
Conceptually, full well depths can be derived by analyzing images of a rich star field taken at two significantly different exposure times, identifying bright but still unsaturated stars in the short exposure image, calculating which stars will saturate in the longer exposure and then simply recording the peak value reached for each star in electrons (using a gain that samples the full well depth, of course). In practice, as discussed in ACS ISR 04-01, it is also necessary to correct for a ~10% "piling up" effect of higher values being reached at extreme levels of over-saturation relative to the value at which saturation and bleeding to neighboring pixels in the column begins.
Since the full well depth may vary over the CCDs, it is desired to have a rich star field observed at a gain that samples the full well depth, and for which a large number of stars saturate. Calibration programs have serendipitously supplied the requisite data of rich fields observed at two different exposure times.
High Resolution Camera
The HRC shows large scale variation of about 20% over the CCD. The smallest full well depth values are at about 155,000 e- and the largest at about 185,000 e-, with 165,000 representing a rough estimate at an area weighted average value. See Figure 3 of ACS ISR 04-01 for details.
Wide Field Camera
As with the HRC there is a real and significant large scale variation of the full well depth on the WFC CCDs. The variation over the WFC CCDs is from about 80,000 e- to 88,000 e- with a typical value of about 84,000 e-. There is a significant offset between the two CCDs. The spatial variation may be seen as Figure 4 in Gilliland ACS ISR 04-01.
5.6.3 Linearity at Low to Moderate Intensity
High Resolution Camera
Rich star fields observed at quite different exposure times provide a simple, direct test for linearity. In Figure 5.23 the results of such tests are shown in two ways. The first is a simple plot of aperture sum values in the long exposures versus the same stars on the same pixels in the short exposure -- no deviations from linearity are evident. For a more sensitive test the bottom plot shows the results of summing counts over all stars within a defined magnitude range in the short and long cases separately, before then taking the ratio and normalizing to the relative exposure times.
The linearity of the HRC at low and moderate intensity levels, as evidenced by comparing stars observed with exposures differing by a factor of 60, appears to be excellent.
Figure 5.23: The upper panel shows aperture sums over 9 pixels for all stars used over a range emphasizing results at low to moderate count levels. The plotted line has a slope set by the relative exposure time. In the lower panel the ratio of counts in ensembles of stars divided into factor of two intensity bins, and further normalized by the relative exposure time are shown. One sigma error bars are derived based on the ensemble signal to noise of the short exposure case. The lowest bin has 81 stars, with typical values of 400 stars per bin above this.
Wide Field Camera
As shown in Figure 5.24 counts within small apertures suggest excellent linearity down to quite low exposure levels of about 5 e- in the central pixel for stellar sources, i.e., down to a level where individual stars cannot be recognized in single exposures. The data used for this test were taken in April, 2002, only one month after on-orbit installation of ACS. CTE effects are of course a source of low-intensity non-linearity (that correlates with y-positioned detector) and should have been very small at this early date (tests verified this). By contrast, for the approximately one year after launch observations used in ISR 2003-09 (quantifying CTE more generally), the CTE impact (at middle of CCDs) at the three lowest intensity levels in Figure 5.27 are at 24, 16, and 10.3% respectively. Thus, any intrinsic non-linearities that may exist at low intensity levels are small compared to the normal losses expected from CTE growth after ACS had been on orbit for only a few months. Within the mutual error bars results for the GAIN = 2 comparisons are entirely consistent -- the low-level linearity of the WFC channel is obviously excellent.
Figure 5.24: The upper panel shows simple aperture sums for the long and short exposures for all stars of low-to-moderate intensity, the line has a slope set by the relative exposure times. The lower panel shows ratio of counts summed over all stars within intensity bins (in factor of two steps) for the 1200 second to mean 22.5 second exposures after normalization to the relative exposure time. To account for minor encirled energy differences for the very small 9 pixel (total, not radius) apertures used all points have been normalized by 1.006, the initial value for the brightest bin. The error bars show plus/minus one sigma deviations based on the total signal to noise of the short exposure sums. The number of stars per bin is typically about 400, although the lowest bin contains only 33 stars.
5.6.4 Linearity Beyond Saturation
The response of the ACS CCDs remains linear not only up to, but well beyond, the point of saturation when using a GAIN value that samples the full well depth. ISR 04-01 shows the well behaved response of the ACS: electrons are clearly conserved after saturation. This result is similar to that of the STIS CCD (Gilliland et al., 1999, PASP, 111 1009-1020) and the WFPC2 camera (Gilliland, R.L. 1994, ApJ, 435, L63-66). It is possible to easily perform photometry on point sources that remain isolated simply by summing over all of the pixels bled into with a GAIN value that samples the full well depth. Given the extra dynamic range afforded before saturation at GAIN=2 (WFC) and GAIN=4 (HRC), and the only modestly increased readout noise coupled with the potentially beneficial aspect of being able to recover photometry on saturated objects, more frequent use of these GAIN values may be appropriate.
Here the extent to which accurate photometry can be extracted for point sources in which one or more pixels have exceeded the physical full well depth is explored. Only the cases of GAIN = 4 for the HRC and GAIN = 2 for the WFC are considered, which provide direct sampling of the count levels independent of whether saturation has occurred. Ideal data for these tests consist of multiple exposures taken back-to-back on a moderate-to-rich star field with a broad range of exposure times resulting in both unsaturated and saturated data for many stars.
High Resolution Camera
Figure 5.25 illustrates linearity beyond saturation. In the lower panel peak values in the long exposure are plotted against peak values from the short exposure. Over the expected linear domain, points fall within a narrow cone centered on a line that has a slope equal to the exposure time ratio while above this the values in the long exposure saturate as expected. Deviations from lying perfectly along the line here result primarily from a 0.1 pixel offset between the images used for this test leading to different relative fractions of light falling on the central pixel. The upper panel of Figure 5.25 shows the same stars but now using identical extraction apertures in the two exposures, the vertical tick mark near 205,000 e- on the x-axis flags stars below which the central pixel remained unsaturated, above which the central pixel experienced saturation. In the aperture sum data there is no difference in the long exposure photometric quality between stars that saturate, even up to a factor of 5, and those that do not. Within the domain sampled here the accuracy of saturated star photometry is much better than 1%.
Figure 5.25: The lower panel shows peak pixel values for many stars observed for 350 seconds (long) and 60 seconds (short) with the HRC; over the range for which stars are unsaturated in both exposures there is a linear relation with slope equal to exposure time ratio. Brighter stars in the long exposure saturate at values near the full well depth. The upper panel shows the same stars using photometry sums from identical apertures in both exposures, stars above the vertical mark near 205,000 on the x-axis separates stars that are saturated in the long exposure. The photometric response when summing over saturated pixels that have been bled into remains perfectly linear far beyond saturation of the central pixel at GAIN=4.
Wide Field Camera
Figure 5.26 shows results for the WFC. Over a range of nearly 4 magnitudes beyond saturation, photometry remains linear to < 1%.
All of the above linearity results are based upon comparisons of FLT images. The conservation of flux property of drizzle leads to equally good results for linearity beyond saturation comparing long and short DRZ images. An analysis of the drizzled data sets corresponding to Figure 5.26 show equally impressive results.
Figure 5.26: The lower panel shows peak pixel values for many stars observed for 340 seconds (long) and 10 seconds (short) with the WFC; over the range for which stars are unsaturated in both exposures there is a linear relation with slope equal to the exposure time ratio. Brighter stars in the long exposure saturate at values near the full well depth. The middle panel shows the same stars using photometry sums from identical apertures in both exposures, stars above the vertical mark near 12,350 on the x-axis separates stars that are saturated in the long exposure. The upper panel shows the ratio of long to short aperture sums normalized by the relative exposure time plotted against the degree of over-saturation in the central pixel of the long exposure. The plotted line in each panel represents perfect linearity, rather than fits to the data. The photometric response when summing over saturated pixels that have been bled into remains perfectly linear far beyond saturation of the central pixel at GAIN=2.
5.6.5 Shutter Stability
The shutter mechanism operation varies for different commanded exposure times. For example, the WFC shutter management varies for the shortest exposures, e.g., at 0.5 sec for WFC it is a continuous motion, 0.6 sec is not allowed and 0.7 sec begins the standard mode of operation. Similar differences exist for the shortest HRC options. Ground-based testing is discussed in Martel, Hartig and Sirianni (2001c) and should be referred to for background on operation of the shutters.
For short exposure times, field dependent timing (shutter shading), A versus B blade shutter control dependence, stability, and timing accuracy were assessed for the HRC and WFC during ground based testing and through utilizing on-orbit data. The ground results using time variable flat-field sources were suggestive of deviations in actual versus nominal exposure times up to, but no larger than ~1% for short exposures of less than 1.0 sec.
For a full discussion of on-orbit data used to assess shutter stability see ACS ISR 03-03 (Gilliland and Hartig 2003).
Even at the shortest exposures, shutter shading measures did not exceed ~0.5% center-to-edge for either camera. This supports a previous decision to not actively invoke a shading correction in CALACS.
No systematic biases in timing reach the 0.5% level for any average over multiple pairs at any exposure time when switching between A and B shutter control. This supports not actively utilizing a record of which shutter was used.
Stability of shutter timing is a bit more problematic. For the HRC stability appears excellent, measured as rms across several exposures, the errors remain well under 0.5%. At exposure times of 1.0 sec peak-to-peak fluctuations on the HRC are at only the 0.1% level. Only at 0.1 sec do the HRC exposures exhibit peak-to-peak fluctuations up to ~1.0% (and part of this may be measurement error). The WFC shutter timing stability is not as good, but was also more difficult to quantify given a higher level of systematic errors. Again measured as an rms across several exposures, errors are well under 0.5%. However, out of 7 pairs of back-to-back 1.0 sec exposures, two of the pairs have individual components differing by > 1.3%. Where such differences exist on WFC, it is always the case that shutter timing under A control is shorter than the following B exposure. At 0.5 sec on WFC (where shutter operation is one continuous rotation) the greatest discrepancy A to B remains < 0.3%. These results suggest that if generally short WFC exposures are required (where short is taken to be < 2 sec), then 0.5 sec exposures seem stable and likely to support 1% accuracy, but exposures over the range 0.7 to <2.0 sec may experience timing fluctuations that could compromise such accuracy. This conclusion regarding WFC exposures is not regarded as robust, but is offered as that most consistent with a simple and conservative interpretation of the test data.
Adopting a threshold of 0.5% in absolute timing, only 4 exposure times over HRC and WFC required revisions from pre-launch values. The 0.1 sec HRC exposure is actually larger than this by 4.1% WFC exposures at 0.5 sec, 0.7 sec and 0.8 sec in reality differ by +1.6%, -1.0% and -0.6% respectively. For these 4 exposures, using revised times in CALACS is recommended in order to support accurate photometry; retrievals made after March 11, 2004 will have invoked use of these corrected values.
