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 770,000 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.
Enter the value of the central wavelength in Angstroms. Table 9.2 gives the allowed values of the central wavelength for each spectroscopic spectral element. Consult the
STIS Instrument Handbook for the associated minimum and maximum wavelength and other information pertaining to the spectroscopic elements.
See Table 9.1 for a listing by operating mode of all Optional Parameters. Here are descriptions for those Optional Parameters for
MAMA TIME-TAG mode.
If the predicted total number of events from a TIME–TAG exposure exceeds the STIS data buffer capacity of 4 x 10
6 photon events, data must be transferred to the HST onboard science data recorder during the exposure. Transfers of data from the STIS buffer during an exposure will be made 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 x 10
6 counts) divided by the estimated maximum average count rate in photons per second (see the
STIS Instrument Handbook). If the actual count rate is less than expected and fewer than 2 x 10
6 counts are received during this time, the 8-MByte blocks will contain fill data.
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. For more details, see the
STIS Instrument Handbook.
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 x 10
7 counts, or about 2 GBits (close to 20% of the solid-state recorder capacity). Please see the
STIS Instrument Handbook.
