9.4
Mode = ACCUM
Config = STIS/CCD
The STIS CCD has only one operating mode. ACCUM, in addition to the target acquisition modes.
ACCUM Mode is used for both imaging and spectroscopy. Photons are counted on the 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 1062 x 1044 pixels, which includes 19 leading pixels and 19 trailing pixels of overscan per line and 20 virtual overscan lines. A CCD subarray does not include the virtual overscan or any of the physical pixels bordering the outer edges of the array, so that the maximum permitted subarray size is 1060 x 1022 pixels.
Enter the value of the central wavelength in Angstroms. Table 9.2 gives the allowed values of the central wavelength for each grating. Consult the
STIS Instrument Handbook for the associated minimum and maximum wavelength and other information pertaining to the gratings.
9.4.4
Optional Parameters
See Table 9.1 for a listing by operating mode of all Optional Parameters. Here are descriptions for those Optional Parameters for the imaging and spectroscopic modes.
CR–SPLIT= 2 (default)
– 8; NO (primary and coordinated parallel exposures)
= NO (pure parallel exposures)
Specifies the number of subexposures into which the specified exposure is to be split for the purpose of cosmic ray elimination in post-observation data processing (see the
STIS 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 as specified without splitting. If a pattern of offsets is specified (see
Chapter 8: Pointings and Patterns), the specified number of exposures will be taken at each pattern point.
CR-SPLIT=NO must always be used with pure parallel exposures in this mode. If omitted, the default value of
CR-SPLIT for pure parallels is
NO.
CR-SPLIT=NO is always assumed for internal exposures. If explicitly stated, the only permitted value for internals is
CR-SPLIT=
NO.
GAIN=1 (default),
4(e/DN)
Specifies the number of CCD pixels along the AXIS1 or dispersion (
BINAXIS1) and the
AXIS2 or cross-dispersion (
BINAXIS2) directions that are binned to a single signal value as the detector is read out. The default values result in each pixel being read out and downlinked to the ground. See the discussion of binning in the
STIS Instrument Handbook.
BINAXIS1 and
BINAXIS2 are not permitted in conjunction with CCD subarray parameter
SIZEAXIS2.
Specifies the size of a CCD subarray in detector coordinates along the AXIS2 (cross-dispersion or spatial) direction. The default value of
FULL for science exposures will result in the readout of a full detector array of 1044 pixels (including virtual overscan) in the
AXIS2 direction. The subarray will be centered on the detector coordinates of the target (which change with
POSition
TARGet offsets); however, the subarray edge will never be shifted beyond the top or bottom edge of the detector.
9.4.5
Number of Iterations and Time Per Exposure
Enter the number of times this exposure should be iterated. For instance, specifying 10 iterations and a
Time_Per_Exposure of 10 seconds will give a total exposure time of 100 seconds. There are many observational situations when two or more exposures should be taken of the same field (e.g., to keep a bright object from blooming by keeping the exposure time short).
The value entered for the Time_Per_Exposure is the total exposure time for a particular exposure specification. If the
Number_of_Iterations is
n, the entire exposure specification will be iterated
n times. However, if
CR–SPLIT is used, the total exposure time will actually be apportioned among shorter exposures: specifying an exposure time of 10 seconds and
CR–SPLIT=2 results in two exposures of 5 seconds each.
If Number_of_Iterations > 1 is specified with a pattern, the pattern is executed once with the specified number of iterations being taken at each point in the pattern. If no pattern is specified, the specified number of iterations will be taken at the single pointing.
Number_of_Iterations must be
1 for STIS pure parallels (except for
CCDFLAT; see
Section 9.8.2 on page 189).
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CR–SPLIT and multiple iterations are mutually exclusive capabilities. If Number_of_Iterations > 1 on an external exposure, CR–SPLIT=NO must be specified.
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Time_Per_Exposure must be an integral multiple of 0.1 second. If it is not its value will be truncated down to the next lower integral multiple of 0.1 sec. If the exposure time is less than 0.1 sec, it will be set to the CCD minimum value of 0.1 sec. The maximum permissible
Time_Per_Exposure for this mode is 17179.
8 seconds.
