11.2 Mode = ACCUM
Config = ACS/WFC(Unavailable for Cycle 16)

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.

11.2.1 Aperture or FOV

Allowed apertures for this mode are:

WFC, WFC-FIX, WFC1, WFC2, WFC1-FIX, WFC2-FIX, WFCENTER, WFC1-CTE, WFC1-512, WFC1-1K, WFC1-2K, WFC2-2K, WFC1-IRAMP, WFC1-MRAMP, WFC2-MRAMP, WFC2-ORAMP, WFC1-IRAMPQ, WFC1-MRAMPQ, WFC2-MRAMPQ, WFC2-ORAMPQ

An appropriate ramp aperture (names contain "RAMP") must be specified when a ramp filter (names begin with "FR") is used, and may be specified for other spectral elements.

Only apertures WFC, WFC1 or WFC2 may be used with the following spectral elements: POL0UV, POL60UV, POL120UV, POL0V, POL60V, POL120V, and F892N (unless a ramp aperture and filter are also specified).
The actual position of the target within the FOV for these spectral element/aperture combinations will be determined by the STScI based on detector performance.

The aperture WFC1-CTE is available to mitigate CTE loss. The WFC1-CTE 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 AXIS1 and AXIS2 directions. Therefore WFC1-CTE is not appropriate for highly extended targets. Observations of targets placed here will be less affected by CTE loss than those placed at other commonly used apertures that are closer to the center of the detector.

For apertures WFC1-512, WFC1-1K, WFC1-2K, WFC2-2K and the quadrant ramp apertures (names end with "RAMPQ"), the proposal processing software will assign a subarray encompassing the field of view of the aperture. Applicable overscan and bias calibrations are automatically available. See the ACS Instrument Handbook for details. These subarrays cannot be modified.

Target location on the detector is the same for a full-frame ramp aperture and the corresponding subarray readout quadrant ramp aperture.

Table 11.7 summarizes rules for Aperture and Spectral Element combinations, and whether a full-frame or fixed subarray readout is done.

11.2.2 Spectral Element

For the available ACS/WFC spectral elements, see Table 11.2.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.

11.2.3 Wavelength

If a ramp filter (any spectral element beginning with the letters "FR") is specified, enter the desired central wavelength in Ångstroms. Table 11.5 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.

11.2.4 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. (Deleted hot pixel aside since the WFC is unavailable for Cycle 16)

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

Specifies the gain of the CCD electronics in e/DN. The default has been changed to 2 in order to better sample the WFC full well. Proposals for Cycle 13 and earlier, should they need to be reprocessed, will continue to have a default GAIN value of 1.

PAREXP
= DEF (default), NONE, MULTIPLE

Specifies how ACS/HRC parallel exposures will be automatically added to ACS/WFC external exposures during proposal processing. This parameter is not allowed to be specified when Number_of_Iterations > 1, in which case no parallels will be added.

The value MULTIPLE will cause the proposal processing software to attempt to add an ACS/HRC exposure in parallel with each exposure resulting from an Exposure Specification (e.g., an HRC exposure in parallel with each CR-Split WFC exposure). If parallels cannot be added when specifically requested via PAREXP=MULTIPLE, a warning will be issued.

If the value NONE is specified, no HRC parallel will be added. The default value (DEF or no specification) is equivalent to NONE.

AUTOIMAGE
= YES (default), NO

Controls the automatic scheduling of image exposures for the purpose of spectra zero point determination of grism observations. By default, a single short image through a standard filter will be taken in conjunction with each Exposure Specification using the grism for external science observations. A value AUTOIMAGE=NO will disable the automatic scheduling of the image exposure for the Exposure Specification on which it is specified. The parameter is allowed only on external science observations using the grism.

11.2.5 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.

11.2.6 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 0.6 sec. is not allowed.