8.3.1 CCD Modes
Observations with HST's STIS instrument can be done with the CCD detector, and the Astronomer’s Proposal Tool (APT) has parameters for specifying these observations.
Boldface type indicates the name of an APT parameter or a value for a parameter.
Black text indicates an important note.
Magenta text indicates available but unsupported parameters (requires prior approval from STScI).
Red text indicates restricted parameters (for STScI use only).
Brown text indicates text file parameters.
Items in brackets - <value> - are required values.
Items in square brackets - [<value>] - are optional.
There is 1 mode for obtaining imaging and spectroscopic data with the STIS CCD detector, and that is ACCUM.
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 × 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 × 1022 pixels.
Aperture or FOV
|No wavelength should be specified if MIRROR is the spectral element.|
See Table 8.1: Supported Optional Parameters for STIS Instrument Modes 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 (only legal value for 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 7: 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 assumed 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.
CR-SPLIT=NO must be used with Spatial Scan exposures (see Special Observation Requirements, SPATIAL SCAN).
GAIN =1 (default), 4, 2, 8 (e/DN)
Specifies the gain of the CCD electronics in e/DN.
BINAXIS1, BINAXIS2 =1 (default), 2, 4 (pixels) for either
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.
SIZEAXIS2 = FULL (default); 30–1022 (pixels)
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), unless CENTERAXIS is set to a value to force centering at a particular row; however, the subarray edge will never be shifted beyond the top or bottom edge of the detector.
Turns off the automatic scheduling of wavelength calibration exposures for external spectroscopic science exposures. A value of WAVECAL=NO will disable the automatic wavelength calibrations for the entire visit. If WAVECAL=NO is specified, the following rules will apply:
- At least one target=WAVE exposure must be added in every block of exposures with the same spectral element and central wavelength. For a list of all allowed aperture/wavelength combinations, go to:
- At least one TARGET=WAVE exposure must be added before and/or after the science exposure(s) or CCDFLAT exposure.
No more than 60 minutes may elapse between any science exposure and a corresponding WAVE exposure. Aperture selection must be the same as would have been selected for an automatic wavelength calibration exposure. Exposure time must be specified by selecting the "Use Default Time" checkbox.
Instructions for adding a GO WAVECAL exposure see 8.5.2 STIS Internal Calibration Targets.
Number of Iterations
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 Target_Name = CCDFlat).
|CR–SPLIT and multiple iterations are mutually exclusive capabilities. If Number_of_Iterations > 1 on an external exposure, CR–SPLIT=NO must be specified.|
Time Per Exposure
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.