ACCUM is the only observing mode for the UVIS channel. Photons are detected in the WFC3 CCD as accumulated charge, which is read out at the end of the exposure and converted to DN. The DN are stored as 16-bit words in a data memory array. Detector dimensions are specified with the parallel direction first and the serial direction second (i.e. row x column). A full detector readout is 4140 rows by 4206 columns, which includes 110 columns of serial overscan (of which 50 are physical and 60 are virtual) and 38 rows of parallel overscan (all virtual). The light sensitive area of the detector is 4102 x 4096 pixels. Subarrays are comprised only of the physical pixels, which are contained within a 4102 x 4146 pixel region. Subarrays may contain light sensitive pixels and physical overscan pixels, but they do not contain virtual pixels.
The specification of the FOV controls where the target is placed and the region of the detector that is read. The target will be placed at the default location (see below) within the specified FOV, unless the POSition TARGet exposure-level Special Requirement (see POSition TARGet <X-value>,<Y-value>) is used. The full detector will be read unless the aperture name ends in "SUB".
Two types of apertures are defined in Table 14.2. The first type is designed for placing targets at the "optimum location" of a region on the detector: either the entire 4102x4096 pixel array, one of the two physical 2051x4096 CCD chips, one of the 2051x2048 quadrants of the detector, or within a predefined subarray. These are identified as UVIS, UVIS1, UVIS2, UVIS-QUAD, or any aperture with the suffix -SUB, respectively. The default location within these apertures will be routinely adjusted by STScI to reflect any changes in CCD performance (e.g., new charge transfer traps, bad columns, etc.). These apertures are appropriate for targets that are small compared to the scale size of defects in the chips.
The second set of apertures defines the "geometric center" of the region and will remain FIXED in aperture coordinates. These will not be adjusted for changes in CCD characteristics. These apertures are designated with the suffix -FIX, and should be used to specify the location of the target relative to the CCDs. These "geometric center" apertures are appropriate for pointings designed to position an extended scene within the WFC3 FOV. For UVIS-FIX, the "geometric center" is on CCD chip 1, ~10 arcseconds above the gap between the two chips.
The UVIS-QUAD, UVIS-QUAD-FIX, and UVIS-QUAD-SUB apertures are allowed only with one of the quadrant filters (see Table 14.5), and one of these apertures must be specified if a quadrant filter is used. The choice of aperture only affects the telescope pointing; it does not restrict the area of the detector that is read out, except for UVIS-QUAD-SUB which will read out only the quadrant of the detector corresponding to the filter specified. .
The UVIS aperture is required for exposures using the G280 spectral element. In this case the STScI ground system will substitute a special aperture that has approximately the same pointing but is optimized for use with the grism. If an undispersed image exposure is taken in conjunction with the grism exposure, it is recommended that the G280-REF aperture be used on the undispersed image exposure to provide an optimal pointing offset between the two exposures.
See Table 14.5: Spectral Elements for use with WFC3/UVIS Configuration.
This parameter does not apply to WFC3 observations and should be left blank.
Specifies the number of sub-exposures into which the original exposure is to be split for the purpose of cosmic ray elimination in post-observation data processing (see the WFC3 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.
Specifies the number of CCD pixels in each dimension that are binned to a single signal value as the detector is read out. If the value NONE is specified, or the optional parameter is not provided, the exposure will be read out unbinned. A value of 2 produces 2x2 binning; a value of 3 produces 3x3 binning. See the discussion of binning in the WFC3 Instrument Handbook.
BIN=2 or 3 are not permitted in conjunction with any subarray aperture (-SUB). .
Enter the number of times this exposure should be iterated, and the duration in seconds of each iteration. There are many observational situations when two or more identical exposures should be taken of the same field (e.g., to keep a bright object from blooming by keeping the exposure time short). If the Number_Of_Iterations is n, the entire exposure will be iterated n times.
The value entered for the Time_Per_Exposure is the exposure time for each iteration of the specified exposure. For instance, specifying Number_Of_Iterations = 10 and a Time_Per_Exposure of 10 seconds will produce a total exposure time of 100 seconds. This differs from the situation with a CR-SPLIT, when the total exposure time will be apportioned among shorter exposures: specifying an exposure time of 10 seconds and CR-SPLIT=2 results in two exposures of 5 seconds each.
Note: CR-SPLIT and multiple iterations are mutually exclusive capabilities. If Number_Of_Iterations > 1, CR-SPLIT=NO must be specified.
Time_Per_Exposure 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.
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