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.
WFC,
WFC-FIX,
WFC1,
WFC2,
WFC1-FIX,
WFC2-FIX, WFCENTER, WFC1-CTE, WFC1-512, WFC1-1K, WFC1-2K, WFC1-IRAMP, WFC1-MRAMP, WFC2-MRAMP, WFC2-ORAMP, WFC1-IRAMPQ, WFC1-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. Two readout options are allowed with ACS ramp filters: (1) full WFC readout (i.e., 4Kx4K pixels designated by "
RAMP" apertures), or (2) one WFC quadrant readout (i.e., 2Kx2K pixels designated by "
RAMPQ" apertures). Because the field of view of any ramp filter segment lies completely within one WFC quadrant, the
RAMPQ apertures are usually more practical and preferred, as described below. No other apertures (subarray or otherwise) are supported for ramp filter observations. Moreover, STScI discontinued support of user-defined subarrays for all filters and grisms after SM4.
Because all four WFC quadrants are clocked simultaneously during readout, the amount of time (2.5 minutes) to read out the full 4Kx4K frame is the same as the time to read out one 2Kx2K quadrant. Consequently, there is no advantage to using the
RAMPQ apertures from the perspective of readout time alone. However, users can record up to four images with the
RAMPQ apertures (instead of just one image with the
RAMP apertures) before triggering the data buffer dump to the solid state recorder. By dividing the buffer dump into smaller segments, the users can obtain shorter exposure without incurring large overheads during the buffer dump. This allows more efficient packing of exposures in each HST orbit.
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, 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.
Table 11.6 summarizes rules for Aperture and Spectral Element combinations, and whether a full-frame or fixed subarray readout is done.
For the available ACS/WFC spectral elements, see Table 11.2 on page 222.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.
If a ramp filter (any spectral element beginning with the letters “FR”) is specified, enter the desired central wavelength in еngstroms.
Table 11.4 gives the allowed minimum and maximum wavelength for each ramp filter.
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 on page 147), the specified number of sub-exposures will be taken at each pattern point.
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In August 2010, the number of hot pixels were measured to be 1.3% of the total number of available pixels, which is similar to the number of pixels affected by cosmic rays in a 1000 sec exposure (between 1.5% and 3%). Hot pixels are growing at a rate of 0.16% per year. 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 item #7 in ACS STAN 19-Apr-2004: http://www.stsci.edu/hst/acs/documents/newsletters/stan0404.html
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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 IMAGE=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.
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.
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.
