7.2 Exposure-Level Processing
The term “exposure-level processing” refers to pipeline corrections that are applied to the individual FGS observations. These are discussed in this section.
7.2.1 Initial Pipeline Processing
Regardless of the observing mode, several activities are carried out during the initialization of the astrometry pipeline. This begins with the usual file management, data quality assessments, and the determination of the required reference files and their availability status. At this early stage, the data are inspected to determine the identification of the astrometer FGS, its mode of operation, and the availability of guide star data from the guiding FGSs. The astrometer’s data are inspected to evaluate the outcome of the Search, CoarseTrack, and FineLock target acquisitions, while the guide star data are inspected to identify the guiding mode (i.e., was the spacecraft guided by one or two guide stars, and were the guide stars tracked in FineLock?).
If the astrometry target acquisition failed, the FGS flags and status bits are inspected to determine the cause. In this case, data processing proceeds as far as possible (in the event that the observation was a partial success), output files are generated and populated appropriately, and pipeline processing of the observation terminates.
7.2.2 Observing Mode Dependent Processing
After pipeline initialization and data quality assessments, successful observations (i.e., those that acquired the target), are processed according to the FGS observation mode: Position or Transfer.
Position mode
The goal of exposure-level Transfer mode pipeline processing is to determine the centroid of the IFOV while the FGS tracked the object in FineLock. A collateral objective is to analyze the individual PMT data, both to determine the small angle corrections that need to be applied to the centroids as well as to provide photometric information about the target and the sky background.
The guide star data are analyzed in the same way as the astrometer data, over the identical intervals of time. For example, the guide star centroids and average photometry are computed over the time the astrometer was in FineLock.
The corrections applied to the FGS data are as follows:
- The FineLock centroids are computed by finding the median, from the 40 Hz data - of the X,Y location of the IFOV (computed from the Star Selector A,B encoder angles). PMT data are averaged for astrometer and guiding FGSs.
For the astrometer only, the PMT data are evaluated to determine the fine angle adjustments to the centroids.
The Optical Field Angle Distortion (OFAD) calibration is applied to remove distortions of the sky in the FOV.
Differential velocity aberration correction is applied to the adjusted FineLock centroids of the astrometer and guiding FGSs.
Steps 1 and 2 are carried out in calfgsa, while steps 3 and 4 are performed in calfgsb. Please see Figure 7.1 and Figure 7.3, the flow chart descriptions of calfgsa and calfgsb respectively.
Transfer mode
During a Transfer mode observation, the data retrieved from the astrometric FGS will include PMT counts and star selector positions from the slew of the IFOV to the target object, the Search and CoarseTrack target acquisition, and the individual Transfer scans. Corresponding data acquired from the guiding FGSs will include FineLock tracking of the guide stars.
The astrometer’s data are analyzed to evaluate the background counts, if available (see Chapter 7), and to locate and extract the individual scans. For each scan, the guide star centroids are computed and corrected for differential velocity aberration. Output files are generated with the appropriate information.
The data from the individual scans are used to compute the Transfer Function over the scan path. The quality of each scan is evaluated for corruption from high amplitude, high frequency spacecraft jitter, and, if unacceptably large, the scan is disqualified from further analysis.
The remaining scans are cross correlated, shifted as needed, binned as desired, and co-added to enhance the signal to noise ratio. The co-added Transfer Function can be smoothed if need be. The analysis tool which performs these functions is available from STScI. It is currently implemented as a standalone executable (FORTRAN + C) in the UNIX environment.
Although not part of pipeline processing, the analysis of observations of binary stars and extended objects will be briefly described here for completeness.
Binary Star Analysis
The Transfer Function of a binary system will be deconvolved, by use of the standard reference S-Curves of single stars from the calibration database, into two linearly superimposed point source S-Curves, each scaled by the relative brightness and shifted by the projected angular separation of the binary’s components. This provides the observer with the angular separation and position angle of the components as well as their magnitude difference. These results will be combined with those obtained from observations at different epochs to compute the system’s relative orbit.
Extended Source Analysis
For observations of an extended source, such as the resolved disk of a giant star or solar system object, the co-added Transfer Function will be analyzed to determine the angular size of the object. This involves application of a model which generates the Transfer Function of synthetic disks from point source S-Curves from the calibration database.
Transfer mode observations are processed by calfgsa to the point of locating and extracting the individual scans and computing the guide star centroids. Support for additional processing - including the automation of the data quality assessment (identifying those scans which have been unacceptably corrupted by space craft jitter, for example) and the cross correlation and co-adding of the individual scans - are available as data analysis tools. Upgrades to calfgsa will be noted on the FGS web page. Figure 7.2 displays the processing steps performed by the current version of calfgsa for Transfer mode observations..