In this section, we describe the sequence of steps you should follow when designing your ACS observing proposal. The sequence is an iterative one, as trade-offs are made between signal-to-noise ratio and the limitations of the instrument itself. The basic sequence of steps in defining an ACS observation are:
As you choose your science requirements and match them to the instrument’s capabilities, keep in mind that the capabilities depend on whether you are observing in the optical with the WFC, or in the far-UV with the SBC. Trade-offs are described in
Table 7.1.
For imaging observations, the basic configuration consists of detector, operating mode (
MODE=
ACCUM), and filter.
Chapter 5 presents detailed information about each ACS imaging mode.
We refer you to Chapter 6 if you are interested in slitless spectroscopy or polarimetry.
STScI provides full calibration and user support for most of ACS’s operational modes. However, there are some “available but unsupported” modes accessible to observers in consultation with an ACS Instrument Scientist. These unsupported modes include a few apertures, limited- interest optional parameters, some GAIN options, and filterless (CLEAR) operation. If your science cannot be obtained using fully supported modes, or would be much better with use of these special cases, then you may consider using an unsupported mode.
Unsupported modes should only be used if the technical requirements and scientific justifications are particularly compelling. The following caveats apply:
Phase I proposals that include unsupported ACS modes must include the following:
During the Phase II proposal submission process, use of unsupported modes require formal approval from the ACS-WFPC2 Team at STScI. To request an unsupported mode, send a brief e-mail to your Program Coordinator (PC) that addresses the above four points. The PC will relay the request to the contact scientist or relevant ACS instrument scientist, who will then decide whether the use will be allowed. This procedure ensures that any potential technical problems have been taken into account. Note also that archival research may be hindered by use of these modes. Requests for unsupported modes that do not adequately address the above four points or that will result in only marginal improvements in the quality of the data obtained may be denied, even if the request was included in your approved Phase I proposal.
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Optional parameters: SIZEAXIS1, SIZEAXIS2, CENTERAXIS1, CENTERAXIS2, COMPRESSION, AMP, FLASHEXP, WFC: GAIN=0.5, 1.0, 1.4
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Apertures: WFC2-2K, WFC2-MRAMPQ, WFC2-SMFL, WFC2-POL0V, WFC2-POL0UV
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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.
If you find that the exposure time needed to meet your signal-to-noise requirements is too great, or that you are constrained by the detector’s brightness or dynamic-range limitations, you must adjust your basic ACS configuration.
Table 7.2 summarizes the available options and actions for iteratively selecting an ACS configuration that is suited to your science and is technically feasible.
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.
At this point, if you are happy with the total number of orbits required, you are done! 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.
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. Projected to mid-2011 (about 9 years on orbit) a star with 100 electrons, a nominal sky background of 30 electrons, and a placement at row 1024 (center) in one of the WFC chips would experience a loss of about 19% for an aperture of 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.
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. We will be supporting the Anderson & Bedin algorithm as a stand-alone tool incorporated into the STSDAS software package. The ACS-WFPC2 team is also working on implementing the CTE de-trailing software in the WFC calibration pipeline in the near future.
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
