STIS MAMA detectors can be used in ACCUM or TIME-TAG modes, as described in Chapter 11 of the STIS Instrument Handbook
. In TIME-TAG mode, the position and detection time of every photon is recorded in an event list. Detection times are recorded with 125 microsecond precision, although events from bright sources may be buffered for as long as 128 milliseconds prior to assignment of a detection time.
For TIME-TAG datasets, the HST
archive returns all normal data products and also an event list in a file with a _tag suffix. The _tag file is a FITS file with two binary table extensions. The first FITS extension is a binary table named EVENTS, and contains four columns named TIME, AXIS1, AXIS2, and DETAXIS1. The last FITS extension is a binary table named GTI (good time intervals), and indicates time intervals when events could have been detected.
The TIME column in the EVENTS extension of a _tag file contains the time when each event was recorded, relative to the start time (MJD) of the exposure given in the TEXPSTRT
keyword of the primary FITS header. Each relative time is stored as an integer number of 125 microsecond hardware clock ticks, but application of the column SCALE FACTOR converts relative times to seconds. Application of the column SCALE FACTOR is automatic when using astropy.table
task to read _tag files.
The AXIS1 column in the EVENTS extension of a _tag file contains the pixel coordinate along the spectral axis where each event was recorded plus a correction term to remove Doppler shifts introduced by the orbital motion of HST
. The correction term depends on optical element and the projected orbital velocity of HST
, which varies over the course of an observation. In ACCUM mode, this Doppler compensation is applied during an observation, but in TIME-TAG mode raw positions are downlinked and Doppler compensation is applied during ground processing.
TIME-TAG observations with STIS have a temporal resolution of 125 microseconds. The effects on the observed times of both the orbital motion of the Earth and the HST
may need to be corrected relative to the solar system barycenter. The IRAF odelaytime
task creates a file of observation time events corrected for light delay from three sources: 1) general relativistic effects (up to 2 milliseconds), 2) displacement of the telescope from the center of the Earth (up to 20 milliseconds), and 3) displacement of the Earth from the solar system barycenter (up to 500 sec). The inputs to odelaytime
are the TIME-TAG data and the ephemeris files. For details regarding the odelaytime
task and parameters, the reader is referred to STIS ISR 2000-02
To find and retrieve the orbital ephemeris file or files needed, users will have to use the StarView interface. To do this from the StarView webpage (https://starview.stsci.edu
), click on the arrow next to ALL
under the Screens
tab on the left of the screen, and then click on the row with the title of Engineering
. Several search boxes will appear on the right part of the screen. In the Archive Class
box, type ‘orb’ (without the quotes) but leave all other search boxes empty. Now click the Search button on the top, and the results with all existing ORB instances will be presented as a table. Select (by clicking) the row which has a Data Start Time
preceding but closest to the observation time of the TIME-TAG data of interest. Users may want to sort the table by clicking on the Data Start Time
column title (once for ascending, and twice for descending orders of date) to aid with the selection. The selected row will be highlighted. Clicking on the Download Cart
on the top right of the page will show a drop-down menu. Click on Download Selected
, and the browser will open a new window/tab displaying the standard HST Retrieval Options page of the MAST. Select and fill in the required information. Make sure to unselect the Calibrated
checkbox (which is selected by default), and to select the ‘fit’ extension from the lower right part of the page under “… or select or enter one or more extensions:
”. Finally, hit the Send retrieval request
button and the .fit file will be available for download via ftp.
We note that files with Data Start Time
before 2000-05-02 will have file extensions of .orx. To retrieve them, follow the procedure above, but in the last stage, select the ‘orx’ extension instead of ‘fit’. The odelaytime
task only takes .orx files as an input, so the .orx files must be converted to .fit files using the IRAF hstephem
task as follows:
These files can be then be used with the odelaytime
was developed to convert the TIME-TAG table (EVENTS tables) into one or more sub-exposures using a user specified time increment. The result is similar to the ACCUM mode data. The default start time here is the first value in the GTI table. The time increment value can be adjusted so that each sub-exposure has reasonable signal-to-noise. Details regarding the inttag
task and its parameters can be found at https://stistools.readthedocs.io/en/latest/inttag.html