Space Telescope Science Institute   11.1 Introduction to the ACS  11.3 Mode = ACCUM Config = ACS/HRC

11.2 Mode = ACCUM
Config = 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.

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

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 spectral elements will be determined by the STScI based on the spectral element specified and 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.

The aperture WFC1-CTE may not be specified in combination with a polarizer or the Spectral Element F892N.

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 ' 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.

Note: Hot pixels are developing on WFC at a higher than expected rate. We expect that in the course of Cycle 14 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 8.4.2). 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, 2 (default)(e/DN)

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

Proposals prior to Cycle 14 will continue to have a default gain of 1 e/DN if reprocessing is necessary.

PAREXP
= DEF (default), NONE, MULTIPLE

Specifies how ACS/HRC parallel exposures will be automatically added to ACS/WFC externalexposures 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.

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.


 11.1 Introduction to the ACS  11.3 Mode = ACCUM Config = ACS/HRC
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