7.1 Designing an ACS Observing Proposal

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:

  • Identify science requirements and select the basic ACS configuration to support those requirements.
  • Estimate exposure time to achieve the required signal-to-noise ratio, using the Exposure Time Calculator (ETC).
  • Identify any additional target acquisition and calibration exposures needed.
  • Calculate the total number of orbits required, taking into account the overheads.
It is essential that observers proposing to use the SBC read and abide by the procedures and rules described in Section 4.6 and 7.2 to ensure the safety of the MAMA detectors.

Figure 7.1: Defining an ACS observation.

7.1.1 Identify Science Requirements and Define ACS Configuration

First, you must identify the science goals you wish to achieve with ACS. Basic decisions you must make are:

  • Nature of target
  • Filter selection

Trade-offs involving only ACS modes are described in Table 7.1. A discussion of trade-offs between different instruments can be found in Section 2.2.

Table 7.1: Science decision guide.

Decision

Choices

Trade-offs

Field of view

Camera
Filter selection

WFC: 202 × 202 arcsec2
SBC: 35 × 31 arcsec2

Spectral response

Camera
Filter selection

WFC: 3700–11,000 Å
SBC: 1150–1700 Å

Spatial resolution

Camera

WFC: ~0.05 arcsecond/pixel
SBC: ~0.032 arcsecond/pixel

Filter selection

Camera

WFC: broad, medium & narrow band, ramps
SBC: broad band

Spectroscopy

Camera
Spatial resolution
Field of view
Wavelength range

Grism (G800L): WFC
Prism (PR110LPR130L): SBC

Polarimetry

Filters

UV polarizers combine with Wheel 2 filters
VIS polarizers combine with Wheel 1 filters

Imaging

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.

Special Uses

We refer you to Chapter 6 if you are interested in slitless spectroscopy or polarimetry.

7.1.2 Available but Unsupported Modes

Please check for updates on the ACS website.


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 particular 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:

  • STScI does not provide calibration reference files for available-but-unsupported modes. It is the observer's responsibility to obtain any needed calibrations.
  • Requests to repeat failed observations taken with unsupported modes will not be honored if the failure is related to use of this mode.
  • User support from STScI will be more limited.

Phase I proposals that include unsupported ACS modes must include the following:

  • Explanation of why supported modes don't suffice.
  • A request for any observing time needed for calibration purposes.
  • Justification for added risk of use in terms of scientific payback.
  • Demonstration that the observers are able to analyze such data.

During the Phase II proposal submission process, use of unsupported modes requires formal approval from the ACS 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.

The current list of available-but-unsupported items are:

  • Targets: BIAS
  • Optional parameters: SIZEAXIS1, SIZEAXIS2, CENTERAXIS1, CENTERAXIS2, COMPRESSION, AMP, FLASHEXP, WFC: GAIN=0.5, 1.0, 1.4
  • Spectral elements: CLEAR (WFC)
  • ACQ mode: optional parameter GAIN
  • POLUV modes in conjunction with filters: F435W, F660N, F814W
  • POLV modes in conjunction with filters: F502N, F550M, F555W, F625W, F658N

7.1.3 Determine Exposure Time and Check Feasibility

Once you have selected your basic ACS configuration, the next steps are:

  • Estimate the exposure time needed to achieve your required signal-to-noise ratio, given your source brightness. (You can use the ETC for this; see also Chapter 9 and the plots in Chapter 10).
  • For observations using the CCD detectors, ensure that you do not exceed the pixel full well.
  • For observations using the MAMA detector, ensure that your observations do not exceed brightness (count rate) limits.
  • For observations using the MAMA detector, ensure that for pixels of interest, your observations do not exceed the limit of 65,535 accumulated counts per pixel per exposure imposed by the ACS 16 bit buffer.

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 large, 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.

Table 7.2: Science feasibility guide.

Action

Outcome

Recourse

Estimate exposure time.

If too long, re-evaluate instrument configuration.

Consider another filter.

Check full-well limit for CCD observations.

If full well exceeded and you wish to avoid saturation, reduce time per exposure.

Divide total exposure time into multiple, short exposures.[a]

Check bright object limits for MAMA observations.

If source is too bright, re-evaluate instrument configuration.

Consider another filter or change detectors and wavelength regime.

Check 65,535 counts/pixel limit for MAMA observations.

If limit exceeded, reduce time per exposure.

Divide total exposure time into multiple, short exposures.

aSplitting CCD exposures affects the exposure time needed to achieve a given signal-to-noise ratio because of the read noise.

7.1.4 Identify Need for Additional Exposures

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.

7.1.5 Data Volume Constraints

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 subarrays, or take other steps to reduce data volume.

7.1.6 Determine Total Orbit Request

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 337 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 that allows you to achieve your science goals. If you are happy with the total number of orbits required, you are done!

7.1.7 Charge Transfer Efficiency

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.

Special CTE-monitoring programs show that CTE degradation proceeds largely 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 28% 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 ACS ISR 2012-05 for more details.

When observing a single target that is significantly smaller than the detector footprint, 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, WFC1B-512WFC1B-1K, and WFC1B-2K place the target near the B amplifier (target will have 256, 512, and 1024 parallel transfers). The aperture WFC1-CTE is available to mitigate CTE loss and includes the entire chip 1 4096 × 2048 pixel area. However, the reference position is in the upper right corner of chip 1, 200 pixels from both the top and right edges. Therefore, WFC1-CTE is not appropriate for highly extended targets.

The ACS Team has developed a post-observation correction for CTE losses based upon the Anderson & Bedin 2010, PASP, 122, 1035 methodology. This empirical algorithm first develops a model to reproduce the observed CTE trails, then inverts the model to convert observed pixel values in any image to an estimate of the original pixel values, undoing the effects of the degraded CTE.

The original version of the algorithm, implemented in CALACS, worked very well for intermediate to high flux levels (>200 electrons). Improvements to the algorithm, which was most recently updated in 2017, have made corrections more effective at low flux levels (<100 electrons), employed more accurate time- and temperature-dependent corrections for CTE over ACS's lifetime, and do a better job of mitigating readnoise amplification. More details on this subject can be found in ACS ISR 2011-01ACS ISR 2012-03ACS ISR 2012-05 and ACS ISR 2018-04. Check the ACS website and the ACS Data Handbook for further details.

7.1.8 Image Anomalies

ACS was designed with a requirement that no single stray light feature can 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. 2003, Proc. SPIE 4854HLA ISR 2008-01) such as the WFC elliptical haloes and the F660N ghosts. SBC observations of bright objects sometimes show optical ghosts due to reflection between the back and front sides of the filter. The mean energy of the ghost is about 0.82% of the energy of the primary target (ACS ISR 2007-05).

A tool has been developed to predict and characterize scattered light anomalies due to bright stars near the field of view of an ACS/WFC observation. For more detail, see the ACS anomalies webpage, the ACS Data Handbook, and ACS ISR 2016-06. 

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 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 that 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) off the field of view altogether. Similarly, the impact of WFC elliptical haloes can be minimized by avoiding the placement of 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 exhibit a horizontal striping that is constant across the full row (for both amplifiers) of each chip. This striping is the result of 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 readnoise statistics is small, but the correlated nature of the noise can 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 CALACS pipeline. Further information can be found in ACS ISR 2011-05.

Artifacts present in WFC flat field and post-flashed dark images, including dust motes, "freckles", and "flecks", are discussed in Section 4.2.7.