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. |
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Format definitions
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.
Introduction
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
See Table 8.7 Supported STIS Apertures.
Spectral Element
See Table 8.2 Supported Central Wavelengths for STIS Gratings.
Wavelength
Enter the value of the central wavelength in Angstroms. Table 8.2 Supported Central Wavelengths for STIS Gratings 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.
| No wavelength should be specified if MIRROR is the spectral element. |
Optional Parameters
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.
For available but unsupported parameters
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.
WAVECAL=NO
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:
http://www.stsci.edu/hst/instrumentation/stis/proposing/go-wavecal-aperture-selection
- 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.
For available but unsupported parameters
Entering a non-default exposure time is an available option.
SIZEAXIS1 =FULL (default); 30–1060 (pixels)
Specifies the size of a CCD subarray in detector coordinates along the AXIS1 (dispersion) direction. The default value of FULL for science exposures will result in the readout of a full detector array of 1062 pixels (including physical overscan) in the AXIS1 direction.
CENTERAXIS2 =TARGET (default); 17–1009 (pixels)
Specifies the center of a CCD subarray in detector coordinates along the AXIS2 (cross-dispersion or spatial) direction. The default value of TARGET means that the subarray will be centered on the detector coordinates of the target (which changes with POSition TARGet offsets).
CENTERAXIS1 =TARGET (default); 17–1047 (pixels)
Specifies the center of a CCD subarray in detector coordinates along the AXIS1 (dispersion) direction. The default value of TARGET means that the subarray will be centered on the detector coordinates of the target.
PREFLUSH =NO (default), YES
Requests a one-hour flush of the CCD before taking the CCD ACCUM exposure on which it is specified. Note that PREFLUSH can be scheduled during SAA passage.
LAMP =TUNGSTEN
Identifies the internal STIS calibration lamp TUNGSTEN to be used for a calibration exposure. Exposures using LAMP will be scheduled as internal, non-pointed exposures and must have the target name NONE. Specifying a lamp exposure in this way turns on one bulb in the lamp, in contrast to specifying TARGET = CCDFLAT which turns on two bulbs in the lamp.
For restricted parameters
Selecting an alternative WAVE aperture is a restricted option.
LAMP =HITM1, HITM2, LINE, DEUTERIUM (no default)
Identifies the internal STIS calibration lamp to be used for a calibration exposure. Exposures using LAMP will be scheduled as internal, non-pointed exposures and must have the target name NONE. If LAMP=HITM1, HITM2, LINE, or DEUTERIUM, exposure times greater than 1000 seconds should be used with care to avoid overuse of the line lamps.
SLIT–STEP =0 (default); –31000 to 31000 (steps)
This parameter is only allowed with LAMP values of TUNGSTEN or DEUTERIUM. It allows the slit projection on the detector to be offset from the default position. SLIT–STEP is given in slit wheel resolver steps (70 per CCD pixel). Within the range of –10000 to 10000 steps, the field of view remains within the chosen aperture. For optimum flat fielding, it is necessary to break up the full flat-field integration time into multiple (3–5) exposures, and slightly offset the slit projection on the detector from exposure to exposure. This allows smoothing out of the physical inhomogeneities along the slit surfaces and allows flats to be obtained under the fiducial bars.
Note that adding the SLIT-STEP Optional Parameter causes the aperture wheel to be moved by the indicated offset, but only if the aperture wheel is moved on the current exposure. Specifying SLIT-STEP alone does not cause such a movement of the aperture wheel. The aperture wheel moves whenever the currently specified aperture differs from that of the previous exposure. If the aperture is not changed, SLIT-STEP has no effect.
AMP = A, B, C, D (no default)
Use this parameter to specify a particular CCD readout amplifier to be used for the logsheet line. Optional Parameter AMP should not be used with WAVE or CCDFLAT internal calibration targets.
POS = (no default), 3.2-0, 3.2-1, 3.2-2, 3.2-3 and 3.2-4 for Spectral Element G430M and Central Wavelength 5471 = 3.6-0, 3.6-1, 3.6-2, 3.6-3, 3.6-4, 3.6-5, 3.6-6, 3.6-7 and 3.6-8 for Spectral Element MIRROR
This parameter is used to set up a limited number of non-standard MSM positions for the G430M/5741 setting and the CCD imaging mirror (MIRVIS) that shift the position at which the image or spectrum is projected onto the STIS CCD detector. This is accomplished by tilting the optical element away from its nominal position. Once set, the new position will persist in subsequent exposures with the same setting without the need of re-entering the optional parameter. This changes only when a different POS parameter is provided or a different MSM position is commanded.
NOTE: Repeating the command unnecessarily should be avoided since it adds considerable overheads. Table 9.2 gives the allowed values of POS, cylinder positions, offsets from nominal position and optical element.
| POS Value | CYL1 | CYL2 | CYL3 | Y-offset (pixels) | OPT-ELM | central wavelengths |
|---|---|---|---|---|---|---|
3.2-0 (nominal) | 2426 | 5261 | 3338 | 0 | G430M | 5741 |
3.2-1 | 2521 | 5187 | 3264 | 411 | G430M | 5741 |
3.2-2 | 2472 | 5222 | 3299 | 204 | G430M | 5741 |
3.2-3 | 2383 | 5303 | 3380 | -208 | G430M | 5741 |
3.2-4 | 2344 | 5249 | 3426 | -418 | G430M | 5741 |
3.6-0 (nominal) | 866 | 4921 | 2739 | 0 | MIRVIS | N/A |
3.6-1 | 205 | 5714 | 3532 | -401 | MIRVIS | N/A |
3.6-2 | 269 | 5581 | 3399 | -200 | MIRVIS | N/A |
3.6-3 | 5384 | 317 | 3895 | 197 | MIRVIS | N/A |
3.6-4 | 5539 | 93 | 3671 | 393 | MIRVIS | N/A |
| POS Value | CYL1 | CYL2 | CYL3 | X-offset (pixels) | OPT-ELM | central wavelengths |
3.6-0 (nominal) | 866 | 4921 | 2739 | 0 | MIRVIS | N/A |
3.6-5 | 1503 | 4137 | 1955 | 404 | MIRVIS | N/A |
3.6-6 | 1706 | 4002 | 1820 | 205 | MIRVIS | N/A |
3.6-7 | 1328 | 4526 | 2344 | -202 | MIRVIS | N/A |
3.6-8 | 1431 | 4493 | 2311 | -403 | MIRVIS | N/A |
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.
For available but unsupported parameters