Phase 2 Proposal Instructions for Cycle 10

12.2 Mode = ACCUM
Configuration = ACS/WFC

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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, WFC1-2KOnly 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 Table12.3: 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 �ngstroms. Table12.4: 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 Chapter8: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
= 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 on one of the two WFC chips that compose the WFC detector array. SIZEAXIS2 may not be specified in the following situations:

• the Aperture is WFC, unless a ramp filter is specified
• the Aperture is WFCENTER or a subarray aperture (WFC1-512, WFC1-1K, WFC1-2K).
• the Spectral Element specification includes F892N or a polarizer.

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.