Introduction

There are 2 modes for obtaining imaging and spectroscopic data with the STIS MAMA detectors, and those are ACCUM and TIME-TAG.

Mode = ACCUM Config = STIS/FUV–MAMA, Config = STIS/NUV–MAMA

The two MAMAs are photon-counting detectors that provide a two-dimensional ultraviolet capability. ACCUM Mode should be used for observations with count rates exceeding 20,000 cts/s. Dispersion (in spectroscopic modes) runs along AXIS1; the spatial direction of the slit (cross-dispersion) is along AXIS2.

There are 1024 × 1024 physical pixels in the MAMA array, but each physical pixel is sampled by three electrodes, allowing centroiding of electron clouds to half-pixel accuracy. In ACCUM Mode, the MAMAs accumulate counts in the STIS data buffer as they are received, producing images with 2048 × 2048 “high-resolution” pixels with half physical pixel coordinates. Images can be binned either by the STIS flight software (using the BINAXIS1 and BINAXIS2 optional parameters described below) or on the ground, to produce images with 1024 × 1024 physical pixels. All optional parameters are specified in physical pixel coordinates. For a more detailed description, see the discussion of MAMA ACCUM mode in the STIS Instrument Handbook 11.1 Basic Operating Modes.

Subarrays may not be used with the MAMA detectors.

Aperture or FOV

See Table 8.7 Supported STIS Apertures.

Spectral Elements

See table 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 spectroscopic spectral element. Consult the STIS Instrument Handbook section 13.3 Gratings for the associated minimum and maximum wavelength and other information pertaining to the spectroscopic elements.

No wavelength should be specified if the 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 MAMA ACCUM mode.

BINAXIS1, BINAXIS2 =DEF (default), NO, YES (for either)

These parameters specify whether onboard rebinning is to be performed on the accumulated image in memory after the exposure is completed. MAMA photon events are stored in memory in half-pixel coordinates (high-resolution pixels). This rebinning is a summation of two high-resolution pixels in the AXIS1 or dispersion (BINAXIS1) and the AXIS2 or cross dispersion (BINAXIS2) directions to values corresponding to the physical pixels of the detector. The default values for all spectral elements and apertures are not to rebin in either direction, resulting in four high-resolution pixel values for each physical pixel.

When used with the MAMA detectors, BINAXIS1 and BINAXIS2 must have the same value.

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 as "Default Time".

Instructions for adding a GO WAVECAL exposure see 8.5.2 STIS Internal Calibration Targets. 

For restricted parameters

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When used with the MAMA detectors, BINAXIS1 and BINAXIS2 must have the same value.

LAMP =HITM1, HITM2, LINE, DEUTERIUM, KRYPTON (no default)

The internal STIS calibration lamp is 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, DEUTERIUM, KRYPTON, or LINE, exposure times greater than 1000 seconds should be used with care to avoid overuse of the line lamps.

For LAMP=HITM1, HITM2, KRYPTON, DEUTERIUM, or LINE, there are additional restrictions for the MAMAs on the legal combinations of apertures, spectral elements, and central wavelengths beyond those given in the tables. Consult the STIS instrument group.

SLIT–STEP =0 (default); –10000 to 10000 (steps)

This parameter is only allowed with LAMP values of DEUTERIUM or KRYPTON. It allows the slit projection on the detector to be offset from the default position. SLIT–STEP is given in slit wheel resolver steps (35 per MAMA pixel). Within the legal 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.

POS = (no default), 1.1-0 and 1.1-1 for Spectral Element G140L and Central Wavelength 1425

Number of Iterations

Enter the  number  of times  (Number_of_Iterations)  this exposure  should be iterated, and the exposure time (Time_Per_Exposure) per iteration.

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.

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 MAMA minimum value of 0.1 sec. The maximum permissible Time_Per_Exposure for this mode is 6553.5 seconds.

Mode = TIME–TAG Config = STIS/FUV–MAMA, Config = STIS/NUV–MAMA

TIME–TAG Mode is used for imaging and spectroscopy with high time resolution. When used in TIME–TAG Mode, the MAMA produces an event stream with a time resolution of 125 microseconds. If the incoming event rate exceeds about 300,000 counts per second, not all events get registered due to finite electronic response time. The AXIS1 and AXIS2 half-pixel coordinates (11 bits each) of each photon event, along with 9 bits of fine-resolution time data, are stored as a 32-bit word in data buffer memory. See the discussion of TIME–TAG Mode in the STIS Instrument Handbook.

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 spectroscopic spectral element. Consult  the STIS Instrument Handbook section 13.3 Gratings for the associated minimum and maximum wavelength and other information pertaining to the spectroscopic elements.

No wavelength should be specified if the 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 MAMA TIME-TAG mode.

BUFFER– TIME =40 or greater (integer seconds)

This required parameter specifies how often the STIS data buffer transfers data to the HST onboard science data recorder during the exposure. STIS has a data buffer capacity of 4 × 10⁶ photon events and can transfer data in 8-MByte blocks (half the buffer capacity).

The value of BUFFER–TIME should be equal to or less than the half-buffer capacity (2 × 10⁶ counts) divided by the estimated maximum average count rate in photons per second (see the STIS Instrument Handbook for details). If more than two buffer dumps are expected during an exposure, BUFFER-TIME must be greater than 99 sec (the time needed to complete a buffer dump) to avoid gaps in the observing sequence when both half-buffers are dumping. If both conditions cannot be met simultaneously, observers should consider using ACCUM mode.

Note that BUFFER–TIME should include expected counts from the detector dark current as well as the detected photon events, factoring in the instrument quantum efficiency. A conservative value of BUFFER–TIME is recommended (err slightly on the low side) to avoid data loss. The STIS exposure time calculator can be used to predict the appropriate buffer time. For more details, see the STIS Instrument Handbook.

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. 

Number of Iterations

Number_of_Iterations must be 1 in this mode.

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. Enter the total time of data collection as Time_Per_Exposure. An exposure time of less than 0.1 sec will be set to the minimum MAMA exposure time of 0.1 second. The maximum permissible Time_Per_Exposure for this mode is 6553.5 seconds.

If BUFFER–TIME < 99 seconds, photon events will be generated faster than data can be transferred out of the buffer during the exposure. In this case, Time_Per_Exposure should be less than or equal to 2 * BUFFER–TIME so that the exposure can complete before data transfer is necessary. A BUFFER– TIME of 99 seconds corresponds to a maximum average count rate of 20,200 counts/sec.

Note that TIME–TAG exposures have the potential to rapidly use up the HST onboard storage capacity. Caution is advised on any exposure with an exposure time greater than 30 * BUFFER–TIME, which corresponds to 6 × 10⁷ counts, or about 2 GBits (close to 20% of the solid-state recorder capacity). Please see the STIS Instrument Handbook.

Special Requirements

The special requirements RT ANALYSIS and PARallel WITH are not permitted on TIME–TAG Mode exposures.

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