STScI

HST Phase II Proposal Instructions for Cycle 12 (RPS2)

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Chapter 12:
Advanced Camera
for Surveys (ACS)

In this chapter . . .

 

Tables and Figures

Table 12.1: Supported Instrument Parameters for the ACS
 Table 12.3: Spectral Elements for use with ACS/WFC and ACS/HRC Configurationsa
 Table 12.4: Wavelength Ranges for the WFC/HRC Ramp Filters
 Table 12.5: Spectral Elements for use with the ACS/SBC Configuration

12.1 Introduction to the ACS

The Instrument Configurations and Operating Modes described in this chapter are used to specify exposures on the RPS2 Visit and Exposure Specifications. The legal Visit and Exposure Specifications entries are discussed in the following sections. More complete descriptions of Instrument Configurations, Modes, Apertures, Spectral Elements, etc. are available in the ACS Instrument Handbook.

Note that many of the Optional Parameters have default values. In such cases an entry as an Optional Parameter in the Visit and Exposure Specifications is necessary only if it is desired to override the default value. In the STScI proposal system, the physical units of Optional Parameter quantities are always implicit and should never be entered by the observer.

The following tables list the permitted Instrument Configurations, Operating Modes, Apertures, Spectral Elements, and Optional Parameters for the ACS.

Table 12.1: Supported Instrument Parameters for the ACS
Config Mode Aperture Spectral Elements Optional Parameters
ACS/WFC ACCUM WFC, WFC1, WFC2, WFC1-FIX, WFC2-FIX
WFCENTER,
WFC1-512,
WFC1-1K,
WFC1-2K		 See Table 12.3. CR-SPLIT,
GAIN,
SIZEAXIS2,
FLASHEXP
ACS/HRC ACCUM HRC, HRC-FIX, HRC-OCCULT0.8, HRC-CORON1.8, HRC-CORON3.0, HRC-512,
HRC-SUB1.8		 See Table 12.3. CR-SPLIT,
GAIN,
SIZEAXIS2,
FLASHEXP
ACQ HRC-ACQ See Table 12.3,
F220W & F606W,
F220W & F550M,
F220W & F502N
ACS/SBC ACCUM SBC, SBC-FIX See Table 12.5.

 

The following internal calibration targets are available for the ACS.

Table 12.2: Internal Calibration Targets for the ACS
Target Name Description1 Spectral Element
BIAS A CCD readout without an exposure (i.e., the exposure time must be specified as 0) in order to measure the detector bias. Permitted only with the ACS/WFC and ACS/HRC in ACCUM mode. GOs and GTOs should request BIAS exposures only when needed to match external subarray exposures specified via Optional Parameter SIZEAXIS2 . STScI will provide appropriate BIAS reference files for all other situations (e.g., when you specify F892N or a polarizer with a WFC exposure, or use a subarray aperture).
To ensure proper location of a subarray BIAS exposure taken to match a subarray external exposure:
  1. Specify the Aperture to be the same as that of the external exposure.
  2. Provide identical subarray Optional Parameter Specification.
  3. If a WFC external subarray exposure uses a ramp filter, specify the ramp filter also. See the ACS Instrument Handbook for more details.
DEF2
or any ramp filter in Table 12.3
1Use of Optional Parameters with internal targets:
1All internal target exposures will be taken as single exposures (i.e., an implicit CR-SPLIT=NO).
1Optional Parameters permitted with BIAS are:
GAIN, COMPRESSION,SIZEAXIS2.
1
2DEF means the spectral element currently in place for the ACS/WFC and ACS/HRC; and means an opaque position for the ACS/SBC.

 

12.2 Mode = ACCUM
Configuration = ACS/WFC

Photons are counted on the Wide Field Channel CCD as accumulated charge which is read out at the end of the exposure and converted to DN at a selectable gain. The DN are stored as 16-bit words in a data memory array. A full detector readout is 4144x4136 pixels, which includes 24 leading pixels and 24 trailing pixels of overscan per line and 40 virtual overscan lines.

12.2.1 Aperture or FOV

Allowed apertures for this mode are:

WFC, WFC1, WFC2, WFC1-FIX, WFC2-FIX, WFCENTER,
WFC1-512, WFC1-1K, WFC1-2K

Only the WFC aperture is permitted with the following spectral elements: POL0UV, POL60UV, POL120UV, POL0V, POL60V, POL120V, F892N, FR388N, FR423N, FR462N, FR505N, FR551N, FR601N, FR656N, FR716N, FR782N, FR853N, FR931N, FR1016N, FR459M, FR647M, FR914M. The actual position of the target within the WFC FOV for these filters will be determined by the STScI based on the spectral element specified and detector performance.

12.2.2 Spectral Element

See Table 12.3: Spectral Elements for use with ACS/WFC and ACS/HRC Configurationsa.Note: When F892N or a polarizer is specified, STScI will automatically assign a subarray containing the entire FOV provided by those spectral elements. The subarray is approximately one-quarter the size of the full WFC array. Those subarray parameters may not be overridden.

12.2.3 Wavelength

If a ramp filter (any spectral element beginning with the letters "FR") is specified, enter the value of the desired central wavelength in Ångstroms. Table 12.4: Wavelength Ranges for the WFC/HRC Ramp Filters gives the allowed minimum and maximum wavelength for each ramp filter.

Note: A wavelength should not be specified if a ramp filter is not being used.

12.2.4 Optional Parameters

CR-SPLIT
= 2 (default) - 8; NO

Specifies the number of sub-exposures into which the exposure is to be split for the purpose of cosmic ray elimination in post-observation data processing (see the ACS Instrument Handbook). The specified exposure time will be divided equally among the number of CR-SPLIT exposures requested. If CR-SPLIT=NO, the exposure is taken without splitting. If a pattern is also specified (see Chapter 8: "Pointings and Patterns"), the specified number of sub-exposures will be taken at each pattern point. Every internal Exposure Specification will be taken as a single exposure (i.e., an implicit CR-SPLIT=NO). Thus the CR-SPLIT parameter is not permitted to be specified for internal exposures.

Note: Hot pixels are developing on WFC at a higher than expected rate. We expect that in the course of Cycle 12 the number of hot pixels on WFC will be comparable to the number affected by cosmic rays in a typical exposure of 1200 sec. The standard CR-SPLIT approach allows for cosmic-ray subtraction, but without additional dithering, does not allow for correction of hot pixels. Hence, we recommend that users who would otherwise have used a single CR-SPLIT, now use some form of dithering instead. For example, a simple ACS-WFC-DITHER-LINE pattern has been developed based on integer pixel offsets (see Section 8.4.3). This will allow the simultaneous removal of both hot pixels and cosmic ray hits in post-observation processing. For more details refer to:
http://www.stsci.edu/hst/acs/documents/newsletters/stan0302.html


 

GAIN
= 1 (default), 2(e/DN)

Specifies the gain of the CCD electronics in e/DN.

SIZEAXIS2
= FULL (default); 16 - 2046 (pixels)

Specifies the size of a WFC subarray in detector coordinates along the AXIS2 direction. Care must be taken in specifying a WFC detector subarray to ensure that the subarray is fully contained in one of the two WFC chips that compose the WFC detector array. SIZEAXIS2 may not be specified in the following situations:

The default value of FULL will result in a full detector readout of 4144 x 4136 pixels. Subarrays will be centered at the reference position of the Aperture used for the exposure.

FLASHEXP
= NONE (default), SHORT, MID, LONGIndicates the duration of a post-exposure, pre-readout LED "flash." A flash is used to ensure that CCD traps are filled, so that image photons are not lost to traps during readout. The default value of NONE means that there will be no flash. The SHORT, MID, and LONG choices select from three combinations of flash duration and flash current as defined by the ACS Calibration Team. They are intended for use in situations in which background is high, medium, and low, respectively. See the ACS Instrument Handbook for details and guidelines on the use of the post-flash capability.

12.2.5 Number of Iterations

Enter the number of times this Exposure Specification should be iterated, each with the specified Time_per_Exposure. Note: CR-SPLIT and multiple iterations are mutually exclusive capabilities. If Number_of_Iterations > 1 on an external exposure, CR-SPLIT=NO must be specified.

12.2.6 Time Per Exposure

Enter the exposure time, in seconds, for the Exposure Specification. If Number_of_Iterations = 1, the Time_per_Exposure is divided equally among the CR-SPLIT copies, if any. If Number_of_Iterations > 1, each iteration comprises a single exposure whose duration is Time_per_Exposure.

Note that exposure time for an individual WFC exposure, after any CR-SPLIT is applied, must be an integer multiple of 0.1 second and in the range of 0.5 to 3600 sec. The value of 0.6 sec. is not allowed.

Time_per_Exposure must be 0 when the target is BIAS.

12.3 Mode = ACCUM
Configuration = ACS/HRC

Photons are counted on the High Resolution Channel CCD as accumulated charge which is read out at the end of the exposure and converted to DN at a selectable gain. The DN are stored as 16-bit words in a data memory array. A full detector readout is 1062x1044 pixels, which includes 19 leading and 19 trailing pixels of overscan per line and 20 virtual overscan lines.

12.3.1 Aperture or FOV

Allowed apertures for this mode are:
HRC, HRC-FIX, HRC-OCCULT0.8, HRC-CORON1.8, HRC-CORON3.0, HRC-512, HRC-SUB1.8

HRC must be specified if spectral element PR200L is specified.

To ensure a consistent position of the coronographic mechanism for all ACCUM exposures using Apertures HRC-CORON1.8, HRC-CORON3.0 and HRC-SUB1.8, you should request such exposures and their ACQ Mode exposure(s) within a single visit (i.e., without interleaving exposures that use other Apertures)

Spectral Element

See Table 12.3: Spectral Elements for use with ACS/WFC and ACS/HRC Configurationsa. Note: For HRC exposures using a polarizer, STScI will automatically apply corrections for small polarizer-induced shifts in the target position only when aperture HRC is specified.

12.3.2 Wavelength

If a ramp filter (any spectral element beginning with the letters "FR") is specified, enter the value of the desired central wavelength in Ångstroms. Table 12.4: Wavelength Ranges for the WFC/HRC Ramp Filters gives the allowed minimum and maximum wavelength for each ramp filter. Any HRC aperture may be used with the ramp filters.

Note: A wavelength should be not specified if a ramp filter is not being used.

12.3.3 Optional Parameters

CR-SPLIT
= 2 (default) - 8; NO

Specifies the number of sub-exposures into which the exposure is to be split for the purpose of cosmic ray elimination in post-observation data processing (see the ACS Instrument Handbook). The specified exposure time will be divided equally among the number of CR-SPLIT exposures requested. If CR-SPLIT=NO, the exposure is taken without splitting. If a pattern is also specified (see Chapter 8: "Pointings and Patterns"), the specified number of sub-exposures will be taken at each pattern point. Every internal Exposure Specification will be taken as a single exposure (i.e., an implicit CR-SPLIT=NO). Thus the CR-SPLIT parameter is not permitted to be specified for internal exposures.

GAIN
= 2 (default), 4(e/DN)

Specifies the gain of the CCD electronics in e/DN.

SIZEAXIS2
= FULL (default); 16 - 1022 (pixels)

Specifies the size of an HRC subarray in detector coordinates along the AXIS2 direction. SIZEAXIS2 may not be specified if a subarray Aperture (HRC-512, HRC-SUB1.8) is specified.
The default value of FULL for SIZEAXIS2 will result in a full detector readout of 1062 x 1044 pixels. Subarrays will be centered at the reference position of the Aperture used for the exposure.

FLASHEXP
= NONE (default), SHORT, MID, LONGIndicates the duration of a post-exposure, pre-readout LED "flash." A flash is used to ensure that CCD traps are filled, so that image photons are not lost to traps during readout. The default value of NONE means that there will be no flash. The SHORT, MID, and LONG choices select from three combinations of flash duration and flash current as defined by the ACS Calibration Team. They are intended for use in situations in which background is high, medium, and low, respectively. See the ACS Instrument Handbook for details and guidelines on the use of the post-flash capability.

12.3.4 Number of Iterations

Enter the number of times this Exposure Specification should be iterated, each with the specified Time_per_Exposure. Note: CR-SPLIT and multiple iterations are mutually exclusive capabilities. If Number_Of_Iterations > 1 on an external exposure, CR-SPLIT=NO must be specified.

12.3.5 Time Per Exposure

Enter the exposure time, in seconds, for the Exposure Specification. If Number_of_Iterations = 1, the Time_per_Exposure is divided equally among the CR-SPLIT copies, if any. If Number_of_Iterations > 1, each iteration comprises a single exposure whose duration is Time_per_Exposure.

Note that the exposure time for an individual HRC exposure, after any CR-SPLIT is applied, must be an integer multiple of 0.1 second and in the range of 0.1 to 3600 sec.

Time_per_Exposure must be 0 when the target is BIAS.

12.4 Mode = ACQ
Configuration = ACS/HRC

For HRC science observations using one of the coronagraphic apertures (HRC-OCCULT0.8, HRC-CORON1.8, HRC-CORON3.0, HRC-SUB1.8), you will need to perform a target acquisition (ACQ Mode exposure) prior to the science observations in order to accurately position the target behind the occulter. In ACQ mode, the ACS onboard flight software locates the brightest target in the acquisition subarray of the HRC FOV by determining the total flux within a sequence of overlapping "check boxes" within that subarray. The target position is located within the brightest check box and HST is repositioned to place the target at the position of the small spot (HRC-CORON0.8, HRC-SUB1.8). A slew to a different aperture for an ACCUM exposure will be separate and requires a small amount of additional time.

12.4.1 Aperture or FOV

HRC-ACQ is the only valid aperture for this Mode.

12.4.2 Spectral Element

In addition to any of the allowed filters or polarizer/filter pairs specified in Table 12.3: Spectral Elements for use with ACS/WFC and ACS/HRC Configurationsa, the filter pairs F220W & F606W, F220W & F550M or F220W & F502N are allowed for ACQ mode exposures. These paired filters should be used for bright targets that would otherwise saturate the HRC detector. The neutral density equivalents of these paired filters can be found in the ACS Instrument Handbook.

12.4.3 Wavelength

If a ramp filter (any spectral element beginning with the letters "FR") is specified, enter the value of the desired central wavelength in Ångstroms. Table 12.4: Wavelength Ranges for the WFC/HRC Ramp Filters gives the allowed minimum and maximum wavelength for each ramp filter.

Note: A wavelength should not be specified if a ramp filter is not being used.

12.4.4 Optional Parameters

There are no optional parameters available to the GO in this mode.

12.4.5 Number of Iterations

The Number_Of_Iterations must be 1 in this Mode.

12.4.6 Time Per Exposure

The procedures to determine the exposure time for ACQ exposures are given in the ACS Instrument Handbook. Observers should take care to obtain a signal-to-noise ratio of at least 40 from their sources, integrated over the checkbox. Note that the gain of the CCD electronics is 4 e/DN for ACQ exposures. The acquisition overhead is typically dominated by factors other than the exposure time itself, and observers may wish to consider adding extra pad to the exposure time to ensure the required signal-to-noise, taking care not to saturate the CCD pixels or to exceed the maximum acquisition exposure time.

Note that Time per Exposure for individual HRC exposures must be an integer multiple of 0.1 second and must be in the range of 0.1 to 300 sec.

12.5 Mode = ACCUM
Configuration = ACS/SBC

The SBC uses a MAMA detector which is a photon-counting device that provides a two-dimensional ultraviolet capability. The MAMA accumulates photons in the ACS data buffer as they are received, producing images with 1024x1024 pixels. For a more detailed description, see the discussion of SBC ACCUM mode in the ACS Instrument Handbook.

12.5.1 Aperture or FOV

The permitted apertures for this mode are SBC or SBC-FIX.

SBC must be specified if spectral element PR110L or PR130L is specified.

12.5.2 Spectral Element

See Table 12.5: Spectral Elements for use with the ACS/SBC Configuration.

12.5.3 Wavelength

No wavelength should be specified for this mode.

12.5.4 Optional Parameters

There are no Optional Parameters in this Mode.

Unlike WFC or HRC, SBC exposures with a dispersing spectral element (PR110L or PR130L) will not have an additional image exposure scheduled automatically (see Optional Parameter AUTOIMAGE under ACCUM mode for HRC and WFC). If an image exposure is desired in combination with a prism exposure, it must be entered separately. It is best to place it immediately before or after the prism exposure, and in the same visit.

12.5.5 Number of Iterations

Enter the number of times this Exposure Specification should be iterated.

12.5.6 Time Per Exposure

Enter the exposure time to apply to each iteration.

Note that Time_per_Exposure for individual SBC exposures must be an integer multiple of 0.1 second and must be in the range of 0.1 to 3600 sec.

12.6 Tabular Reference Data

12.6.1 Spectral Elements for WFC and HRC

Note that except for the polarizers, only one filter may be specified for an exposure. The polarizers must be used in combination with a filter on the other filter wheel in order to maintain focus. Thus the UV polarizers (POL0UV, POL60UV, POL120UV) must be used with a filter on wheel 2 excluding: F814W, FR914N, POL0V, POL60V, POL120V. The visible polarizers (POL0V, POL60V, POL120V) must be used with a filter on wheel 1 except for: F850LP, F892N, POL0UV, POL60UV, POL120UV.

Table 12.3: Spectral Elements for use with ACS/WFC and ACS/HRC Configurationsa
Wheel Position Name Description Allowed Configurations
1
2
 F555W Johnson V ACS/WFC, ACS/HRC
1
3
 F775W SDSS i ACS/WFC, ACS/HRC
1
4
 F625W SDSS r ACS/WFC, ACS/HRC
1
5
 F550M narrow V ACS/WFC, ACS/HRC
1
6
 F850LP SDSS z ACS/WFC, ACS/HRC
1
8
 POL0UV UV polarizer 0 deg ACS/WFC, ACS/HRC
1
9
 POL60UV UV polarizer 60 deg ACS/WFC, ACS/HRC
1
10
 POL120UV UV polarizer 120 deg ACS/WFC, ACS/HRC
1
11
 F892N methane ACS/WFC, ACS/HRC
1
12
 F606W broad V ACS/WFC, ACS/HRC
1
13
 F502N O III ACS/WFC, ACS/HRC
1
14
 G800L GRISM ACS/WFC, ACS/HRC
1
15
 F658N Ha ACS/WFC, ACS/HRC
1
16
 F475W SDSS g ACS/WFC, ACS/HRC
2
2
 F660N N II ACS/WFC, ACS/HRC
2
3
 F814W Johnson I ACS/WFC, ACS/HRC
2
4
 FR388N O II Ramp ACS/WFC, ACS/HRC
2
4
 FR423N O II Ramp ACS/WFC
2
4
 FR462N O II Ramp ACS/WFC
2
5
 F435W Johnson B ACS/WFC, ACS/HRC
2
6
 FR656N Ha ramp ACS/WFC, ACS/HRC
2
6
 FR716N Ha ramp ACS/WFC
2
6
 FR782N Ha ramp ACS/WFC
2
8
 POL0V visible polarizer 0 deg ACS/WFC, ACS/HRC
2
9
 F330W HRC u ACS/HRC
2
10
 POL60V visible polarizer 60 deg ACS/WFC, ACS/HRC
2
11
 F250W near-UV filter ACS/HRC
2
12
 POL120V visible polarizer 120 deg ACS/WFC, ACS/HRC
2
13
 PR200L HRC PRISM ACS/HRC
2
14
 F344N Ne V ACS/HRC
2
15
 F220W near-UV filter ACS/HRC
2
16
 FR914M Broad Ramp ACS/WFC, ACS/HRC
2
16
 FR853N IR Ramp ACS/WFC
2
16
 FR931N IR Ramp ACS/WFC
2
17
 FR459M Broad Ramp ACS/WFC, ACS/HRC
2
17
 FR647M Broad Ramp ACS/WFC
2
17
 FR1016N IR Ramp ACS/WFC
2
18
 FR505N O III Ramp ACS/WFC, ACS/HRC
2
18
 FR551N O III Ramp ACS/WFC
2
18
 FR601N O III Ramp ACS/WFC

 
 a. Note: Normally only one spectral element may be specified for an exposure. The exceptions are:
1) As noted in 12.4 "Mode = ACQ Configuration = ACS/HRC", three filter pairs are permitted for ACQ mode exposures of bright targets.
2) The polarizers must be used with another spectral element. This is required to maintain focus. The UV polarizers (POL0UV, POL60UV, POL120UV) must be used with a spectral element on wheel 2 except for: POL0V, POL60V, POL120V, and any inner or outer ramp filter segment (see Table 12.4). The visible polarizers (POL0V, POL60V, POL120V) must be used with a spectral element on wheel 1 except for: F850LP, F892N, POL0UV, POL60UV, and POL120UV.

12.6.2 Ramp Filter Wavelength Ranges

Table 12.4: Wavelength Ranges for the WFC/HRC Ramp Filters
Name Minimum Wavelength (Å) Maximum Wavelength (Å) WFC FOV location
FR388N 3710 4049 middle
FR423N 4049 4420 inner
FR462N 4420 4824 outer
FR505N 4824 5266 middle
FR551N 5266 5748 inner
FR601N 5748 6274 outer
FR656N 6274 6848 middle
FR716N 6848 7474 inner
FR782N 7474 8158 outer
FR853N 8158 8905 inner
FR931N 8905 9719 outer
FR1016N 9719 10609 outer
FR459M 3810 5366 middle
FR647M 5366 7574 inner
FR914M 7574 10709 middle

 

12.6.3 Spectral Elements for ACS/SBC

Table 12.5: Spectral Elements for use with the ACS/SBC Configuration
Wheel Position Name Description
3
2
 F115LP MgF2
3
3
 F125LP CaF2
3
5
 F140LP BaF2
3
6
 F150LP crystal quartz
3
8
 F165LP Suprasil
3
9
 F122M Ly-a
3
11
 PR130L CaF2 PRISM
3
12
 PR110L LiF2 PRISM

 

12.7 ACS Aperture Coordinate System

Figure 12.1 shows how the POS-TARG coordinates, X and Y, are related to the U2,U3 and detector Axis1 and Axis2 directions. The Y axis is parallel to Axis2. The X axis is normal to Y but differs from the Axis1 direction by about 5 degrees. This diagram correctly shows the WFC and HRC orientations, but it does not represent their relative sizes or positions.

Figure 12.1: ACS Coordinate system
 
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