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11.4 TRANSFER Mode Pipeline

11.4.1 TRANSFER Mode Dataset

The dataset for a TRANSFER Mode observation includes all of the acquisition phases described in "Target Acquisition and Tracking" on page 9-16, as well as the transfer scans described in "Transfer Scans" on page 9-21. Each TRANSFER mode observation consists of a number of scans specified in the original proposal. Simultaneous guide star data cover the entire observation.

Automatic pipeline processing of TRANSFER mode data is limited to locating each scan in the astrometer's data file, editing out bad data arising from garbled telemetry, and determining the median position and standard deviations of the guide stars within the guiding FGSs during each scan.

The pipeline generates three ASCII files for every scan, one for each FGS. Each file contains the 40 Hz raw star selector A,B encoder values and the photon counts/25msec of the four PMTs. The guide star data are provided for (optional) de-jittering of the astrometer's IFOV. Each of these files begins with a small header containing keywords whose values pertain globally to the observation or specifically to the scan, such as the filter used or the universal time at the start of the scan. The HST state vector is also included.

11.4.2 Mapping TRANSFER Mode to POSITION Mode

Planned upgrades to the TRANSFER mode pipeline to support the mapping of the TRANSFER mode results onto POSITION mode plates, along with POSITION mode observations made in the same visit, include the following steps:

Differential velocity aberration correction to the astrometer and the guide stars. This step requires access to the definitive HST orbit file because the HST state vector evolves considerably over the course of the TRANSFER mode observation, which can be up to 40 minutes long.
Computation of guide star x,y centroids to establish the fiducial position for the dominant guide star.
Translational de-jittering of the astrometer IFOV at 40 Hz.
Location of the individual scans in the observation.
Cross-correlating, shifting, binning, and co-adding the individual scans.
Polynomial curve fitting to the co-added scans. The residual of each scan to this fit is evaluated to locate individual scans which might be contaminated by HST jitter. (Step 3 would not remove a recentering event in which the guider was not tracking its guide star during some extreme HST jitter crises). Such scans are identified and disqualified, and steps 5 and 6 are iteratively repeated until no further disqualifications occur.
Time tagging and recording the shifts determined from the cross-correlation. These data are used to generate a drift model for the TRANSFER mode data when the observation is in a visit with POSITION mode observations.
Generation of output files that log the corrections made, identify the scans which have been disqualified, specify the polynomial coefficients of the fit, and present the drift model and guide star centroids and standard deviations. This step also includes creation of individual scan files which contain the star selector A,B values and the counts from the four PMTs for each scan. These files have names such as F42N0102M.nSm where n = 1,2,3 to identify the FGS and m=1,2,3,... to specify the scan -number.

11.4.3 Limitations of the TRANSFER Mode Pipeline

The pipeline cannot carry out further processing of TRANSFER mode data because the header and data files made available to the pipeline do not contain sufficient information. For example, the header files do not specify if the observation is of a single calibration reference star, or of a binary system, or an extended object, each of which needs different additional processing. The B-V color of the target needs to be specified for the analysis of observations of binary stars or extended objects so that the appropriate single star reference S-curve can be retrieved from the calibration library. These activities will be discussed in Chapter 13.

Observers should consult the STScI FGS web pages for updates to the status of the pipeline upgrades.

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