Chapter 3:
Introduction to ACS

In this chapter . . .

3.1 ACS Location in the HST Focal Plane / 9 3.2 Instrument Capabilities / 11 3.3 Instrument Design / 12 3.4 Basic Instrument Operations / 15 3.5 ACS Quick Reference Guide / 17

3.1 ACS Location in the HST Focal Plane

ACS is mounted in one of the instrument bays behind the HST primary mirror. The relative locations of the science instruments in the focal plane and their fields of view are shown schematically in Figure 3.1. When referring to the HST and its focal plane, we use a coordinate system that is fixed to the telescope and consists of three orthogonal axes: U1, U2 and U3. U1 lies along the optical axis, U2 is parallel to the solar-array rotation axis, and U3 is perpendicular to the solar-array axis. (Note: Some HST documentation uses the alternative V1, V2, V3 coordinate system for which V1=U1, V2=–U2 and V3=–U3.)

Figure 3.1: The HST field of view with the locations of the SI and the FGS apertures in the (U2,U3) focal plane. The scale in arcsec is indicated.

Figure 3.1 shows the layout of the instrument entrance apertures in the telescope focal plane as projected onto the sky.

3.2 Instrument Capabilities

Please check for updates on the ACS Web site.

ACS is a versatile instrument that can be applied to a broad range of scientific programs. The three prime capabilities of ACS are: Deep, wide-field imaging from visible to near-IR wavelengths.

High spatial resolution imaging from near-UV to near-IR wavelengths.
Solar blind UV imaging.

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

Wide Field Channel (WFC): 202 x 202 arcsecond field of view from ~3500 Å to 11,000 Å, and peak efficiency of 48% (including the Optical Telescope Assembly (OTA)). The plate scale of ~0.05 arcseconds/pixel provides critical sampling at 11,600 Å.
High Resolution Channel (HRC): 29 x 26 arcsecond field of view from ~1700 Å to 11,000 Å, and peak efficiency of 29%. The plate scale of ~0.027 arcseconds/pixel provides critical sampling at 6300 Å.
Solar Blind Channel (SBC): 34.6 x 30.5 arcsecond field of view from 1150 Å to 1700 Å, plate scale of ~0.032 arcseconds/pixel, and peak efficiency of 7.5%.

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

Grism spectroscopy: Low resolution (R ~100 @ 8000 Å) wide field spectroscopy from 5500 Å to 10,500 Å available in both the WFC and the HRC.
Prism spectroscopy: Low resolution (R = 59 @ 2500 Å) near-UV spectroscopy from 1700 Å to 3900 Å available in the HRC.
Prism spectroscopy: Low resolution (R ~100 @ 1500 Å) far-UV spectroscopy from 1250 Å to 1800 Å available in the SBC.
Coronagraphy: Aberrated beam coronagraphy in the HRC from 2000 Å to 11,000 Å with 1.8 arcsecond and 3.0 arcsecond diameter occulting spots.
Imaging Polarimetry: Polarimetric imaging in the HRC and WFC with relative polarization angles of 0°, 60°, and 120°.

3.3 Instrument Design

3.3.1 Detectors

ACS uses the following detectors in each channel:

The WFC employs a mosaic of two 4096 x 2048 Scientific Imaging Technologies (SITe) CCDs. The 15 x 15 µm pixels provide ~0.05 arcseconds/pixel spatial resolution, with critical sampling at 11,600 Å, resulting in a nominal 202 x 202 arcsecond field of view (FOV). The spectral sensitivity of the WFC ranges from ~3500 Å to ~11,000 Å, with a peak efficiency of 48% at ~7000 Å (including OTA).
The HRC detector is a 1024 x 1024 SITe CCD, with 21 x 21 µm pixels that provide ~0.028 x 0.025 arcsecond/pixel spatial resolution with critical sampling at 6300 Å. This gives the HRC a nominal 29 x 26 arcsecond field of view. The spectral response of the HRC ranges from ~1700 Å to ~11,000 Å, and it has a peak efficiency of 29% at ~6500 Å (including OTA).
The SBC detector is a solar-blind CsI microchannel plate (MCP) with Multi-Anode Microchannel Array (MAMA) readout. It has 1024 x 1024 pixels, each 25 x 25 µm in size. This provides a spatial resolution of ~0.034 x 0.030 arcseconds/pixels, producing a nominal FOV of 34.6 x 30.1 arcseconds. The SBC UV spectral response ranges from ~1150 Å to ~1700 Å with a peak efficiency of 7.5% at 1250 Å.

The WFC & HRC CCDs

The ACS CCDs are thinned, backside-illuminated full-frame devices cooled by thermo-electric cooler (TEC) stacks 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 HRC CCD spectral response is optimized specifically for near-UV wavelengths. 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 seconds for the HRC and 0.5 seconds for the WFC. Between successive identical exposures, the minimum time is 45 seconds for the HRC, and 135 seconds for the WFC, for full-frame readouts. However, this can be reduced to as little as ~35 seconds for WFC 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, see Section 4.3.6.

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. In order to preserve its functionality, the SBC MAMA detector is subject to both scientific and absolute brightness limits. At high local ( 50 counts/second/pixel) and global (> 200,000 counts/second) 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 screening limits in a given configuration cannot be observed in those configurations, as discussed in Section 4.6 and Section 7.2.

3.3.2 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 spherical aberration in the HST primary mirror. The WFC has three silver-coated optics to optimize instrument throughput in the visible and near-IR. The silver coatings cut off at wavelengths shortward of 3500 Å. 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 7.9). The optical design of the WFC is shown schematically in Figure 3.2. The HRC/SBC optical chain comprises three aluminized mirrors overcoated with MgF₂, shown schematically in Figure 3.3. 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 that focuses light 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 3.2: ACS optical design: wide field channel.

Figure 3.3: 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. Each wheel also contains one clear WFC aperture and one clear HRC aperture (see Chapter 5 for more on filters). Parallel WFC and HRC observations are possible for some filter combinations (auto-parallels) and these are automatically added by the Astronomer's Proposal Tool (APT) (http://apt.stsci.edu/) in Phase II, unless the user disables this option via the PAREXP optional parameter, or if adding the parallel observations disallowed due to timing considerations. Because the filter wheels are shared, it is not possible to independently select the filter for WFC and HRC parallel observations.

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 at present (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. Astrometry programs may particularly benefit from the use of this capability, as is discussed briefly in Section 4.3.7. 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 it.

3.4 Basic Instrument Operations

3.4.1 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 to 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 milliarcseconds.

3.4.2 Typical ACS Observing Sequence

ACS is expected to be used primarily for deep, wide-field survey imaging. Important issues for observers to consider will be the “packaging” of their observations, how observations are CR-SPLIT to mitigate the impact of cosmic rays, how sub-stepping or “dithering” of images for removal of hot pixels is implemented, 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 readnoise limited, requiring consideration of optimum number of readouts. Observations with the MAMA detectors are not affected by cosmic rays or readnoise, but long integration times will often be needed to obtain sufficient signal-to-noise.

A typical ACS observing sequence is expected to consist of a series of 10 to 20 minute dithered 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 7 for more details about observing strategies.

3.4.3 Data Storage and Transfer

At the conclusion of each exposure, the science data are 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 preceding exposure will be performed during integration (see Section 8.2 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 Chapter 4 and Chapter 7 for a detailed description of ACS instrument operations.

3.5 ACS Quick Reference Guide

Please check for updates on the ACS Web site.

Table 3.1: Instrument characteristics.

	WFC	HRC	SBC
Field of View	202"¥ 202"	29"¥ 26"	34.6"¥ 30.8"
Plate Scale	~0.05"/pixel	~0.028 ¥ 0.025"/pixel	~0.034 ¥ 0.030"/pixel
Pixel Size	15 ¥ 15 mm	21 ¥ 21 mm	25 ¥ 25 mm
Image Format	2 ¥ 2048 ¥ 4096 pixels	1024 ¥ 1024 pixel	1024 ¥ 1024 pixel
Spectral Response	~3500 Å to 11,000 Å	~1700 Å to 11,000 Å	~1150 Å to 1700 Å
Detector	SITe CCDs thinned backside illuminated, anti-reflection coated, multiphased pinned	SITe CCD thinned backside illuminated, anti-reflection coated, multiphased pinned	CsI MCP with MAMA readout
Detector Efficiency	~77% at 4000 Å ~83% at 6000 Å ~67% at 8000 Å	~33% at 2500 Å ~69% at 6000 Å ~53% at 8000 Å	~19.2% @ 1216 Å
Peak Efficiency2	48% at ~7000 Å	29% at ~6500 Å	7.5% at ~1250 Å
Readnoise	5.0 e–	4.7 e–	0 counts
Dark Current2	0.0029 e–/s/pixel	0.0058 e–/s/pixel	1.241 ¥ 10-5 e–/s/pixel
Full Well2	84,700 e–	155,000 e–	1Detector count rate linearity limit: 200,000 counts/second Pixel count-rate linearity limit: ~22 counts/second/pixel
Gain	1,2,4 and 8 e–/DN (Max 65,535 DN)	1,2,4 and 8 e–/DN (Max 65,535 DN)	n/a
Operating Temperature2	-80×C	-81×C	n/a

1Loss of linearity occurs at count rates larger than these values. For more information, please see Section 4.5.6. 2Average value for WFC1 and WFC2

Table 3.2: Calibration accuracies.

Attribute	WFC	HRC	SBC	Limiting factor
Distortion Solution Accuracy	0.1 pixel	0.1 pixel	0.25 pixel	Calibration & stability of geometric distortion1
Absolute Astrometry	0.5" to 1"	1"	1"	Guide Star Catalog uncertainties2
Absolute Photometry	3%	2%	5%	Absolute calibration, standards
Relative Photometry within an Image	1%	1%	2%	Flat-field characterization or characterization of geometric distortion.
Repeated Photometry of Same Star	0.3%	0.3%	1%	Stability of flat field
Transformation to Standard Magnitude Systems	0.02 mag SDSS 0.025 mag WFPC2 0.03 mag BVRI	0.02 mag SDSS 0.025 mag WFPC2 0.03 mag BVRI	n/a	DQE curve determination Color terms
Polarimetry	1%	1%	n/a
Wavelength Calibration	20 Å grism	12 Å grism	1 pixel prisms	Accuracy of dispersion solution
Spectrophotometry	6% (Grism)	10% (Grism)	20% (Prism)

1Anderson & King, ISR 04-15 (http://www.stsci.edu/hst/acs/documents/isrs/isr0415.pdf) 2Koekemoe, et. al., ISR 05-06 (http://www.stsci.edu/hst/acs/documents/isrs/isr0506.pdf)

Table 3.3: ACS filters.

Filter type	Filter description	Camera
Broadband	Sloan Digital Sky Survey (SDSS): F475W, F625W, F775W, F850LP	WFC/HRC
	B, V, Medium V, Wide V, I: F435W, F555W, F550M, F606W, F814W	WFC/HRC
	Near-UV: F220W, F250W, F330W	HRC
	No Filter: CLEAR	WFC/HRC
Narrowband	Ha (2%), [OIII] (1%), [NII] (1%): F658N, F502N, F660N	WFC/HRC
	NeV (3440 Å): F344N	HRC
	Methane (8920 Å): F892N	HRC/[WFC1]
Ramp filters	2% bandpass (3700-10,700 Å): FR388N, FR505N, FR656N	WFC/HRC
	2% bandpass (3700-10,700 Å): FR423N, FR462N, FR716N, FR782N, FR853N, FR931N, FR1016N, FR551N, FR601N	WFC
	9% bandpass (3700-10,700 Å): FR459M, FR914M	WFC/HRC
	9% bandpass (3700-10,700 Å): FR647M	WFC
Spectroscopic	Grism: G800L	WFC/HRC
	Prism: PR200L	HRC
Polarizers	Visible (0×, 60×,120×): POL0V, POL60V, POL120V	HRC/[WFC1]
	Near-UV (0×, 60×, 120×): POL0UV, POL60UV, POL120UV	HRC/[WFC1]
Medium Band	Lyman-Alpha: F122M	SBC
Long Pass	MgF2, CaF2, BaF2, Quartz, Fused Silica: F115LP, F125LP, F140LP, F150LP, F165LP	SBC
Prisms	LiF, CaF2: PR110L, PR130L	SBC

1Limited field of view (72" x 72") for these filters using WFC

Table 3.4: ACS polarizers.

Polarizer set	Filters	Filter comments
POL0UV, POL60UV, POL120UV	F220W F250W F330W F435W F814W	HRC NUV short HRC NUV long HRC U Johnson B Broad I
POL0V, POL60V, POL120V	F475W F606W F625W F658N F775W	SDSS g Johnson V SDSS r Ha SDSS i

Table 3.5: ACS dispersers.

Disperser	Channel	Wavelength range (Å)	Resolving power
G800L	WFC	1st order: 5500 to 10,500	100 @8000 Å
G800L	WFC	2nd order: 5000 to 8500	200 @8000 Å
G800L	HRC	1st order: 5500 to 10,500	140 @8000 Å
G800L	HRC	2nd order: 5500 to 8500	280 @8000 Å
PR200L	HRC	1700 to 3900	59 @2500 Å
PR110L	SBC	1150 to 1800	79 @1500 Å
PR130L	SBC	1250 to 1800	96 @1500 Å

Table 3.6: Useful tables and figures list.

Topics	Tables
Proposal Preparation Tips	Table 6.1: Examples of polarizer and non-polarizer exposures in a Phase II proposal. Table 7.1: Science decision guide. Table 7.2: Science feasibility guide. Table 9.1: Useful quantities for the ACS WFC. Table 9.2: Useful quantities for the ACS HRC. Table 9.3: Useful quantities for the ACS SBC.
Overheads	Table 8.1: Science exposure overheads: general. Table 8.2: ACS science exposure overhead times (minutes).
Auto-parallels	Table 7.12: Filter combinations for auto-parallels in the two cases of (i) HRC camera prime and (ii) WFC camera prime. Table 7.13: Minimum primary exposure time (seconds) to attach an auto-parallel for CR-SPLIT=n.
Aperture Parameters	Table 7.7: WFC aperture parameters. Table 7.8: Ramp filter apertures. Table 7.9: HRC aperture parameters. Table 7.10: SBC aperture parameters Table 7.11: Plate scales and axis angles for the 3 ACS channels. Figure 3.1: The HST field of view with the locations of the SI and the FGS apertures in the (U2,U3) focal plane. The scale in arcsec is indicated. Figure 7.8: Aperture and image feature orientation. Figure 7.9: ACS apertures in the V2/V3 reference frame.
Detector Characteristics	Table 3.1: Instrument characteristics. Table 4.1: CCD gain and readout noise (e- rms) under SIDE-1 operation (03/2002->06/2006). Table 4.2: CCD gain and readout noise (e- rms) under SIDE-2 operation (07/2006-01/2007). Table 4.7: SBC detector performance characteristics.
SBC Bright Object Protection Limits	Table 7.3: Absolute SBC count rate screening limits for nonvariable and variable objects. Table 7.4: Bright limit V-band magnitudes for observations with the SBC filters and prisms (no reddening). Table 7.5: Bright limit J- and F-band magnitudes of an unreddened O5 star for observations with the SBC filters and prisms. Table 7.6: Bright object protection policy for SBC observations.
Hot and Warm Pixels	Table 4.3: Creation rate of new hot pixels (pixel/day). Table 4.4: Annual permanent hot pixel growth (%).. Figure 4.12: Growth of permanent warm and hot pixel population as a function of time (WFC top, HRC bottom).
Charge Transfer Efficiency	Table 4.5: Charge transfer efficiency measurements for the ACS CCDs at installation time (Fe55 test at 1620 e–). Table 4.6: Example of science applications and their assumed parameters. Figure 4.15: Projected CTE losses in WFC (and equivalently, the size of corrections).
Geometric Distortion	Figure 10.127: The geometric distortion map for the ACS/WFC, which shows only the non-linear component to the solution. Note that this figure is rotated 180× with respect to the pipeline calibration products, where WFC2 is the lower half of the detector. Figure 10.128: The map of the effective pixel areas of the ACS/WFC chips. Figure 10.129: The geometric distortion map for the HRC. Figure 10.130: The map of the effective pixel areas of the HRC. Figure 10.131: The geometric distortion map for the ACS/SBC. Figure 10.132: The map of the effective pixel areas of the ACS/SBC.
Encircled Energy	Table 5.4: Encircled energy measurements for the ACS channels. Table 9.4: Encircled energy comparison for WFC/F850LP. Figure 4.18: MAMA point spread function. Figure 5.9: Encircled energy for the CCD channels. Figure 5.10: Encircled energy for the SBC.
PSF	Figure 5.8: Kernels representing CCD pixel functions for HRC and WFC. Table 5.10: Model ACS SBC PSFs. Figure 5.9: Encircled energy for the CCD channels.
Plate Scale	Table 7.11: Plate scales and axis angles for the 3 ACS channels.
Filters	Table 3.3: ACS filters. Table 3.5: ACS dispersers. Table 5.1: ACS WFC/HRC filters in Filter Wheel #1. Table 5.2: ACS WFC/HRC filters in Filter Wheel #2. Table 5.3: ACS SBC filter complement. Table 5.6: In-band flux as a percentage of the total flux. Table 5.7: Visible-light rejection of the SBC F115LP imaging mode.
Polarizers:	Table 3.4: ACS polarizers. Table 6.1: Examples of polarizer and non-polarizer exposures in a Phase II proposal. Table 6.2: Filters that can be used in conjunction with the ACS polarizers.
Photometry	Table 5.5: V detection limits for ACS, HRC, and SBC direct imaging. Table 9.1: Useful quantities for the ACS WFC. Table 9.2: Useful quantities for the ACS HRC. Table 9.3: Useful quantities for the ACS SBC. Table 10.1: Color corrections ABn to go from Johnson V magnitude to AB magnitude for the WFC. Table 10.2: Color corrections ABn to go from Johnson V magnitude to AB magnitude for the HRC. Table 10.3: Color corrections ABn to go from Johnson V magnitude to AB magnitude for the SBC.
Limiting Magnitudes for Direct Imaging	Table 5.5: V detection limits for ACS, HRC, and SBC direct imaging.
Spectroscopy	Table 6.3: Optical parameters of ACS dispersers. Table 6.4: V detection limits for the ACS grism/prism modes. Figure 6.15: Sensitivity versus wavelength for WFC G800L. Figure 6.16: Sensitivity versus pixel position for WFC G800L. Figure 6.18: Sensitivity versus wavelength for HRC G800L. Figure 6.19: Sensitivity versus pixel position for HRC G800L. Figure 6.21: Sensitivity versus wavelength for HRC/PR200L. Figure 6.22: Sensitivity versus wavelength for SBC PR110L. Figure 6.24: Sensitivity versus wavelength for SBC/PR130L.
Sky Backgrounds	Table 9.5: Approximate zodiacal sky background as a function of ecliptic latitude and ecliptic longitude (in V magnitudes per square arcseconds). Table 9.6: Geocoronal emission lines. Table 9.7: High sky backgrounds. Figure 9.1: High sky background intensity as a function of wavelength. Figure 9.2: Background contributions in V magnitude per arcseconds2 due to the zodiacal light, Moon, and the sunlit Earth, as a function of angle between the target and the limb of the Earth or Moon. Figure 9.3: Extinction versus wavelength.
Calibration Accuracies	Table 3.2: Calibration accuracies. Figure 4.5: Dark rate trend with time for the WFC CCDs. Figure 4.6: Dark current as a function of time for the HRC Figure 4.7: Histogram of WFC dark frames taken before and after the change in temperature.