STScI

Advanced Camera for Surveys Instrument Handbook for Cycle 14

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Chapter 3:
Introduction to ACS

3.1 Instrument Capabilities
3.2 Instrument Design
    3.2.1 Detectors
    3.2.2 ACS Optical Design
3.3 Basic Instrument Operations
    3.3.1 Target Acquisitions
    3.3.2 Typical ACS Observing Sequence
    3.3.3 Data Storage and Transfer
    3.3.4 Parallel Operations
3.4 Designing an ACS Observing Proposal
    3.4.1 Identify Science Requirements and Define ACS Configuration
    3.4.2 Determine Exposure Time and Check Feasibility
    3.4.3 Identify Need for Additional Exposures
    3.4.4 Determine Total Orbit Request

In this Chapter we provide an overview of the capabilities and scientific applications of ACS. We describe the optical design and basic operation of the instrument, and provide a flow chart and discussion to help you design a technically feasible and scientifically optimized ACS observing proposal.

3.1 Instrument Capabilities

The ACS is a camera designed to provide HST with a deep, wide-field survey capability from the visible to near-IR, imaging from the near-UV to the near-IR with the PSF critically sampled at 6300Å, and solar blind far-UV imaging. The primary design goal, now verified, of the ACS Wide-Field Channel is to achieve a factor of 10 improvement in discovery efficiency, compared to WFPC2, where discovery efficiency is defined as the product of imaging area and instrument throughput.

ACS has three channels, each optimized for a specific goal:

In addition to these three prime capabilities, ACS also provides:

Table 4.1, 4.2, and 4.3 provide a full list of filters and spectroscopic elements for each imaging channel.

ACS is a versatile instrument that can be applied to a broad range of scientific programs. The high sensitivity and wide field of the WFC in the visible and near-infrared will make it the instrument of choice for deep imaging programs in this wavelength region. The HRC, with its excellent spatial resolution, provides full sampling of the HST PSF at >6000Å and can be used for high precision photometry in stellar population programs. The HRC coronagraph can be used for the detection of circumstellar disks and QSO host galaxies.

3.2 Instrument Design

In this section, we provide a high-level summary of the basic design and operation of ACS, concentrating on the information most relevant to the design of your HST observing proposal. Subsequent chapters provide more detailed information on specific aspects of the instrument's performance and the design of proposals.

3.2.1 Detectors

ACS uses one or more large-format detectors in each channel:

The WFC & HRC CCDs

The ACS CCDs are thinned, backside-illuminated devices cooled by thermo-electric cooler (TEC) stacks and housed in sealed, evacuated dewars with fused silica windows. The spectral response of the WFC CCDs is optimized for imaging at visible to near-IR wavelengths, while the spectral response of the HRC CCD is optimized specifically for the near-UV. Both CCD cameras produce a time-integrated image in the ACCUM data-taking mode. As with all CCD detectors, there is noise (readout noise) and time (read time) associated with reading out the detector following an exposure. The minimum exposure time is 0.1 sec for HRC, and 0.5 sec for WFC, and the minimum time between successive identical exposures is 45s (HRC) or 135s (WFC) for full-frame and can be reduced to ~36s for subarray readouts. The dynamic range for a single exposure is ultimately limited by the depth of the CCD full well (~85,000 e- for the WFC and 155,000 e- for the HRC), which determines the total amount of charge that can accumulate in any one pixel during an exposure without saturation. Cosmic rays will affect all CCD exposures: CCD observations should be broken into multiple exposures whenever possible, to allow removal of cosmic rays in post-observation data processing; during Phase II you can use the CR-SPLIT optional parameter or dithering to do this (See Section 7.2.5).

The SBC MAMA

The SBC MAMA is a photon-counting detector which provides a two-dimensional ultraviolet capability. It can only be operated in ACCUM mode. The ACS MAMA detector is subject to both scientific and absolute brightness limits. At high local (50 counts sec-1 pixel-1) and global (>285,000 counts sec-1) illumination rates, counting becomes nonlinear in a way that is not correctable. At only slightly higher illumination rates, the MAMA detectors are subject to damage. We have therefore defined absolute local and global count-rate limits, which translate to a set of configuration-dependent bright-object screening limits. Sources which violate the absolute count rate limits in a given configuration cannot be observed in those configurations, as discussed in Section 7.5.

3.2.2 ACS Optical Design

The ACS design incorporates two main optical channels: one for the WFC and one which is shared by the HRC and SBC. Each channel has independent corrective optics to compensate for HST's spherical aberration. The WFC has three optical elements, coated with silver, to optimize instrument throughput in the visible and in the near-IR. The silver coatings cut off at wavelengths shortward of 3700Å. The WFC has two filter wheels which it shares with the HRC, offering the possibility of internal WFC/HRC parallel observing for some filter combinations (Section 8.7). The optical design of the WFC is shown schematically in Figure 3.1. The HRC/SBC optical chain comprises three aluminized mirrors, overcoated with MgF2 and is shown schematically in Figure 3.2. The HRC or SBC channels are selected by means of a plane fold mirror (M3 in Figure 3.3). The HRC is selected by inserting the fold mirror into the optical chain so that the beam is imaged onto the HRC detector through the WFC/HRC filter wheels. The SBC channel is selected by moving the fold mirror out of the beam to yield a two mirror optical chain which images through the SBC filter wheel onto the SBC detector. The aberrated beam coronagraph is accessed by inserting a mechanism into the HRC optical chain. This mechanism positions a substrate with two occulting spots at the aberrated telescope focal plane and an apodizer at the re-imaged exit pupil.

While there is no mechanical reason why the coronagraph could not be used with the SBC, for health and safety reasons use of the coronagraph is forbidden with the SBC.

Figure 3.1: ACS Optical Design: Wide Field Channel


 
Figure 3.2: ACS Optical design: High Resolution/Solar Blind Channels


 

Filter Wheels

ACS has three filter wheels: two shared by the WFC and HRC, and a separate wheel dedicated to the SBC. The WFC/HRC filter wheels contain the major filter sets summarized in Table 3.1. Each wheel also contains one clear WFC aperture and one clear HRC aperture (see Chapter 4). Parallel WFC and HRC observations are possible for some filter combinations and these are automatically added by APT in Phase II, unless the user disables this option via the PAREXP optional parameter, or if adding the parallel observations cannot be done due to timing considerations. Note that since the filter wheels are shared it is not possible to independently select the filter for WFC and HRC parallel observations.


Table 3.1: ACS CCD Filters
Filter Type Filter Description Camera
Broadband Sloan Digital Sky Survey (SDSS) B, V, Wide V, R, I Near-UV WFC/HRC WFC/HRC HRC
Narrowband H (2%), [OIII] (2%), [NII] (1%) NeV (3440Å) Methane (8920Å) WFC/HRC HRC HRC/[WFC1]
Ramp filters 2% bandpass (3700-10700Å) 9% bandpass (3700-10700Å) WFC/HRC WFC/HRC
Spectroscopic Grism Prism WFC/HRC HRC
Polarizers Visible (0º, 60º,120º) Near-UV (0º, 60º, 120º) HRC/[WFC1] HRC/[WFC1]
1Limited field of view (72" x 72") for these filters using WFC

The SBC filters are shown in Table 3.2.


Table 3.2: SBC Filters
Filter Type
Filter Description
Medium Band
Lyman-Alpha
Long pass
MgF2, CaF2, BaF2, Quartz, Fused Silica
Objective Prisms
LiF, CaF2

Calibration-Lamp Systems

ACS has a calibration subsystem, consisting of tungsten lamps and a deuterium lamp for internally flat fielding each of the optical chains. The calibration subsystem illuminates a diffuser on the rear surface of the ACS aperture door, which must be closed for calibration exposures. Under normal circumstances, users are not allowed to use the internal calibration lamps.

In addition, a post-flash capability was added to the instrument to provide the means of mitigating the effects of Charge Transfer Efficiency (CTE) degradation. We do not expect to use this facility much in Cycle 14, (except for calibration and characterization) but in later years, as radiation damage of the CCDs causes the CTE to degrade, it is possible that some users will want to avail themselves of this facility. Since we are not yet at a stage where use of post-flash is expected to be useful for any science observations, we do not provide an extensive discussion related to this.

3.3 Basic Instrument Operations

3.3.1 Target Acquisitions

For the majority of ACS observations target acquisition is simply a matter of defining the appropriate aperture for the observation. Once the telescope acquires its guide stars, your target will be within ~1-2 arcseconds of the specified pointing. For observations with the ramp filters, one must specify the desired central wavelength for the observation. For the special case of coronagraphic observations, an onboard target acquisition will need to be specified. The nominal accuracy of the onboard target acquisition process is ~6 mas, comparable to that achieved by STIS.

3.3.2 Typical ACS Observing Sequence

ACS is expected to be used primarily for deep, wide-field survey imaging. The important issues for observers to consider will be the "packaging" of their observations, i.e. how observations are CR-SPLIT to mitigate the impact of cosmic rays, whether sub-stepping or "dithering" of images is required for removal of hot pixels, and how, if necessary, to construct a mosaic pattern to map the target. HRC observations and narrowband observations with the WFC are more likely to be read-noise limited, requiring consideration of the optimum CR-SPLIT times. Observations with the MAMA detectors do not suffer from cosmic rays or read noise, but long integration times will often be needed to obtain sufficient signal-to-noise in the photon-starved ultraviolet.

A typical ACS observing sequence is expected to consist of a series of CR-SPLIT and dithered ~10-20 minute exposures for each program filter. Coronagraphic observations will require an initial target acquisition observation to permit centering of the target under the occulting mask. Observers will generally not take their own calibration exposures. See Chapter 8 for more details of observing strategies.

3.3.3 Data Storage and Transfer

At the conclusion of each exposure, the science data is read out from the detector and placed in ACS's internal buffer memory, where it is stored until it can be transferred to the HST solid state data recorder (and thereafter to the ground). The internal buffer memory is large enough to hold one WFC image, or sixteen HRC or SBC images, and so the buffer will typically need to be dumped before or during the following WFC exposure. If the following exposure is longer than ~339 seconds, then the buffer dump from the proceeding exposure will be performed during integration (see Section 9.2 for a more complete discussion).

ACS's internal buffer stores the data in a 16 bit-per-pixel format. This structure imposes a maximum of 65,535 counts per pixel. For the MAMA detectors this maximum is equivalent to a limit on the total number of detected photons per pixel which can be accumulated in a single exposure. For the WFC and HRC, the 16 bit buffer format (and not the full well) limits the photons per pixel which can be accumulated without saturating in a single exposure when GAIN = 1 for WFC, and GAIN 2 for the HRC is selected. See Chapters 7 and 8 for a detailed description of ACS instrument operations.

3.3.4 Parallel Operations

Parallel observations with the WFC and HRC are possible with ACS for certain filter combinations (See Section 8.7).

ACS can be used in parallel with any of the other science instruments on HST, within certain restrictions. Figure 3.3 shows the HST field of view following SM3B with ACS installed. Dimensions in this figure are approximate; accurate aperture positions can be found on STScI's Observatory web page under "Pointing"1 or by using the Visual Target Tuner (VTT). The ACS grism and prism dispersion directions are approximately along the V2 axis. The policy for applying for parallel observing time is described in the Call for Proposals. We provide suggestions for designing parallel observations with ACS in Section 8.7. While the ACS CCDs can be used in parallel with another instrument on HST, subject to certain restrictions described in Section 8.7, there are significant restrictions on the use of the MAMA detectors in parallel - see Chapter 2.

3.4 Designing an ACS Observing Proposal

In this section, we describe the sequence of steps you will need to take when designing your ACS observing proposal. The process 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:

Figure 3.3: HST Field of View Following SM3B


 
Figure 3.4: Defining an ACS Observation


 

3.4.1 Identify Science Requirements and Define ACS Configuration

First and foremost, of course, you must identify the science you wish to achieve with ACS. Basic decisions you will need to make are:

As you choose your science requirements and work to match them to the instrument's capabilities, keep in mind that those capabilities differ greatly depending on whether you are observing in the optical or near-UV with the CCD, or in the far-UV using the MAMA detector. Tradeoffs are described in Table 3.3


Table 3.3: Science Decision Guide
Decision Affects Tradeoffs
Field of view Camera
Filter selection		 WFC: 202 x 202 arcseconds HRC: 29 x 26 arcseconds SBC: 35 x 31 arcseconds
Spectral response Camera Filter selection WFC: 3700-11,000Å HRC: 2000-11,000Å SBC: 1150-1700Å
Spatial Resolution Camera WFC: ~50 milliarcsecond pixels HRC: ~ 27 milliarcsecond pixels SBC: ~32 milliarcsecond pixels
Filter Selection Camera WFC: broad, medium & narrow band, ramps HRC: Visible, UV, ramp middle sections
Spectroscopy Camera Spatial resolution Field of View Wavelength range Grism (G800L): WFC and HRC Prism (PR200L): HRC Prism (PR110L, PR130L): SBC
Polarimetry Filters UV polarizers combine with Wheel 2 filters VIS polarizers combine with Wheel 1 filters
Coronagraphy Filter selection Coronagraphic imaging available with HRC only

.

Imaging

For imaging observations, the base configuration is detector (Configuration), operating mode (MODE=ACCUM), and filter. Chapter 4 presents detailed information about each of ACS's imaging modes.

Special Uses

We refer you to Chapter 5 if you are interested in any of the following special uses of ACS: slitless spectroscopy, polarimetry and coronagraphy.

3.4.2 Determine Exposure Time and Check Feasibility

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

To determine your exposure-time requirements consult Chapter 6 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 7. For a consideration of observation strategies and calibration exposures, consult Chapter 8.

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 will need to adjust your base ACS configuration. Table 3.4 summarizes the options available to you and steps you may wish to take as you iterate to select an ACS configuration which is both suited to your science and is technically feasible.


Table 3.4: Science Feasibility Guide
Action Outcome Recourse
Estimate exposure time If too long -> re-evaluate instrument configuration. Consider use of an alternative 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.1 Consider use of different Gain.
Check bright-object limits for MAMA observations If source is too bright -> re-evaluate instrument configuration. Consider the use of an alternative filter or change detectors and wavelength regime.
Check 65,535 counts- per-pixel limit for MAMA observations If limit exceeded -> reduce time per exposure. Divide total exposure time into multiple, short exposures
1Splitting CCD exposures affects the exposure time needed to achieve a given signal-to-noise ratio because of the read noise.

3.4.3 Identify Need for Additional Exposures

Having identified your desired sequence of science exposures, you need to determine what additional exposures you may require to achieve your scientific goals. Specifically:

3.4.4 Determine Total Orbit Request

In this, the final step, you place all your exposures (science and non-science, alike) into orbits, including tabulated overheads, and determine the total number of orbits you require. Refer to Chapter 9 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 Chapter 8 in sections WFC CCD Subarrays and HRC CCD Subarrays and in Section 9.2.1.

At this point, if you are happy with the total number of orbits required, you're done! If you are unhappy with the total number of orbits required, you can, of course, iterate, adjusting your instrument configuration, lessening your acquisition requirements, changing your target signal-to-noise or wavelength requirements, until you find a combination which allows you to achieve your science goals with ACS.

1Pointing web page: http://www.stsci.edu/instruments/
observatory/taps.html
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