We refer you to Chapter 6 if you are interested in slitless spectroscopy or polarimetry.
To determine your exposure-time requirements, consult Chapter 9 where an explanation of how to calculate a signal-to-noise ratio and a description of the sky backgrounds are provided. To assess whether you are close to the brightness, signal-to-noise, and dynamic-range limitations of the detectors, refer to
Chapter 4.
Having identified a sequence of science exposures, you need to determine what additional exposures you may require to achieve your scientific goals. Specifically, if the success of your science program requires calibration to a higher level of precision than is provided by STScI calibration data, and if you are able to justify your ability to reach this level of calibration accuracy yourself, you will need to include the necessary calibration exposures in your program, including the orbits required for calibration in your total orbit request.
ACS data taken at the highest possible rate for more than a few orbits or in the Continuous Viewing Zone (CVZ) may accumulate data faster than they can be transmitted to the ground. High data volume proposals will be reviewed and, on some occasions, users may be requested to break the proposal into different visits. Consider using sub-arrays, or take other steps to reduce data volume.
In this step, you place all of your exposures (science and non-science, alike) into orbits, including tabulated overheads, and determine the total number of orbits required. Refer to
Chapter 8 when performing this step. If you are observing a small target and find your total time request is significantly affected by data-transfer overheads (which will be the case
only if you are taking many separate exposures under 339 seconds with the WFC), you can consider the use of CCD subarrays to lessen the data volume. Subarrays are described in
Section 7.3.1 and
Section 8.2.1.
If you are unhappy with the total number of orbits required, you can adjust your instrument configuration, lessen your acquisition requirements, or change your target signal-to-noise or wavelength requirements, until you find a combination which allows you to achieve your science goals. If you are happy with the total number of orbits required, you are done!
All CCDs operated in a radiative environment are subject to a significant degradation in charge transfer efficiency (CTE). The degradation is due to radiation damage of the silicon, inducing the creation of traps that impede an efficient clocking of the charge on the CCD. Since reading out the ACS WFC requires 2048 parallel transfers and 2048 serial transfers, it is not surprising that CTE effects have begun to manifest themselves since first years of ACS operation.
Special CTE monitoring programs show that CTE degradation proceeds linearly with time. For the current Cycle a star with 100 total electrons, a sky background of 10 electrons, and a placement at row 1024 (center) in one of the WFC chips would experience a loss of about 15% for aperture photometry within a 3 pixel radius. A target placed at the WFC aperture reference point, near the maximum number of parallel shifts during readout, would have approximately twice the loss. Expected absolute errors after calibration of science data, at these low-loss levels, is expected to
be of order 10% the relative loss. See Chiaberge, M. (ACS ISR 2012-05) for more details.
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. The aperture WFC1-CTE is available to mitigate CTE loss. This aperture has the same area as the WFC1 aperture except that the reference position is 200 pixels from the upper right corner of chip 1, in both the chips x- and y- direction. Therefore, WFC1-CTE is not appropriate for highly extended targets.
Recently there have been efforts to correct WFC images for CTE charge-trailing at the pixel level (Massey et al. 2010, MNRAS, 401, 371; Anderson & Bedin 2010, PASP, 122, 1035) as an alternative to photometric corrections a posteriori. The Anderson & Bedin algorithm is now part of the latest release of CALACS. Please
check for updates on the ACS Web site.
The ACS was designed with a requirement that no single straylight feature may contain more than 0.1% of the detected energy in the object producing it. This goal has generally been met, but during the extensive ground and SMOV test programs a few exceptions have been identified (
Hartig et al. 2002, Proc. SPIE 4854;
HLA ISR 2008-01) such as the WFC elliptical haloes and the F660N ghosts.
While some of these anomalies exceed the specified intensity, some judicious planning of your science observations is recommended to help alleviate their effect on your data, especially if bright sources are expected in the field of view. For instance, the impact of diffraction spikes (which for ACS lie along x and y axes) and of CCD blooming (which occurs along the y direction) due to saturation of a bright star(s), can be reduced by choosing an ORIENT which prevents the source of interest from being connected to the bright star along either of these axes, Alternatively, a suitable ORIENT could move the bright star(s) into the interchip gap or off the field of view altogether. Similarly, the impact of WFC elliptical haloes can be minimized by avoiding a bright star in the quadrant associated with amplifier D.
Subsequent to the replacement of the ACS CCD Electronics Box during SM4, all WFC images show horizontal striping which is constant across the full row (for both amplifiers) of each chip. This striping is the result of a
1/f noise on the bias reference voltage, and has a standard deviation of 0.9
e-. 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. Please see
Section 5.2.6 for additional details, and mitigation strategy. Destriping is now part of the latest CALACS release. Further information can be found in
ACS ISR 2011-05.
