Following the fine-alignment and focus activities of the SM3B Orbital Verification
period, the optical qualities of all three ACS channels were judged to have met their design specifications. The encircled energy values for the WFC, HRC, and SBC obtained during this time are given in Table 5.4
compares the wavelength-dependent throughputs of the ACS WFC and HRC with those of WFC3/UVIS, WFC3/IR, NICMOS/NIC3, and WFPC2.
contains the detection limits in Johnson-Cousins
V magnitudes for unreddened O5 V, A0 V, and G2 V stars, generated using the ETC
. WFC and HRC values used the parameters CR-SPLIT=2, GAIN=2, and a 0.2 arcsecond circular aperture. For the SBC, a 0.5 arcsecond circular aperture was used. An average sky background was used in these examples. However, limiting magnitudes are sensitive to the background levels; for instance, the magnitude of an A0 V in the WFC using the F606W filter changes by ▒
0.4 magnitudes at the background extremes. Figure 5.8
shows a comparison of the limiting magnitude for point-sources achieved by the different cameras with a signal to noise of 5 in a 10 hour exposure. Figure 5.9
shows a comparison of the time needed for extended sources to attain ABMAG=26.
contains plots of exposure time versus magnitude for a desired signal-to-noise ratio. These plots are useful for determining the exposure times needed for your scientific objectives. More accurate estimates require the use of the ACS ETC (http://etc.stsci.edu/etc
Both CCD and SBC imaging observations are subject to saturation at high total
accumulated counts per pixel. For the CCDs, this is due either to the depth of the full well or to the 16 bit data format. For the SBC, this is due to the 16 bit format of the buffer memory (see Section 4.3.1
and Section 4.5.2
Subsequent to the replacement of the ACS CCD Electronics Box during SM4, all
WFC images show horizontal striping noise that is roughly constant across each row of read-out in all four WFC amplifiers. This striping is the result of a 1/f noise on the bias reference voltage, and has an approximately Gaussian amplitude distribution with standard deviation of 0.9 electrons. The contribution of the stripes to the global read noise statistics is small, but the correlated nature of the noise may affect photometric precision for very faint sources and very low surface brightnesses.
During Cycle 17, STScI developed and tested an algorithm for removing the stripes
from WFC science images. The algorithm is effective when the science image is not excessively crowded such that a row-by-row background level becomes difficult to estimate. Because the stripe removal code is not universally effective, it is not currently applied as part of the ACS calibration pipeline. Instead, STScI has released the stripe removal algorithm to the community as a stand-alone task that can be run on ACS data retrieved from the HST archive. This task, acs_destripe
, has been written in Python as part of the acstools
package in the public release of STScI_Python. As a Python task, it can be run from PyRAF, any Python interpreter or even the operating system command-line, to correct post-SM4 pipeline-calibrated images (_flt.fits). Please see the ACS Web site
for details on running this code.Further details regarding the WFC striping and its mitigation are provided in the ACS ISR 2011-05
(Grogin et al. 2011).
Because the WFC bias striping noise is so consistent among the four read-out
amplifiers, and because it also manifests within the WFC pre-scan regions, STScI is working to incorporate a pre-scan based de-striping algorithm into the ACS calibration pipeline CALACS, which will be available by early 2012. This permits consistent striping-noise mitigation for all post-SM4 WFC full-frame images, including calibration images as well as arbitrary science images, given the trade-off of slightly less precise striping-noise reduction for “low-complexity” science images. This pre-scan based de-striping algorithm, as well as its implementation in CALACS, will be described in an upcoming ACS Instrument Science Report
(Anderson & Grogin, in prep.).