This section contains a discussion of the detailed corrections that are applied to
Position mode FGS astrometry data by calfgsa
. A flowchart is provided in Figure 3.3
to help illuminate data processing with calfgsb
(please see the flowchart given by Figure 3.1
for an overview of calfgsa
related processing of Position Mode data).
The pipeline produces output files that log these corrections, the associated standard
deviations about the centroids, and the photometry averages from the four PMTs. calfgsa
performs the majority of these corrections. However, corrections 6 through 8 are re-done by calfgsb
which uses the most up to date value of coefficients for the OFAD correction and the star selector angles to (x, y)
conversion (contained in reference files not accessed by calfgsa
At this point no further processing on the individual observations are possible. The
next step is to combine the measurements of the individual targets to correct for Position Mode-mode jitter and FGS drift.
The goal of this segment of the pipeline (within calfgsb
) is to map all of the positional measurements of the individual targets onto a fixed but arbitrary coordinate system. It involves Position Mode de-jittering
and application of the drift correction.
The pipeline accounts for spacecraft jitter during the visit by establishing a fixed
but arbitrary reference frame determined by the (x, y)
centroids of the guide stars (within the guiding FGSs) from the first exposure in the observing sequence. The HST pointing control system uses the position of the dominant guide star to fix HST's translational position and that of the roll guide star to fix HST's orientation. The output products of the pipeline processing of the individual observations include the (x, y)
centroids of the guide star positions evaluated over the same time interval as the astrometer centroids. During the course of the visit any change in the (x, y)
centroids of the dominant guide star within its FGS is interpreted to be HST translational jitter and is removed from both the astrometer and the guide star maintaining HST roll. Next, any motion of the roll guide star with respect to the dominant guide star perpendicular to the line between them is interpreted as uncompensated roll of HST about the dominant guide star. The pipeline then removes this roll from the astrometry data. Typically the size of the de-jittering correction is less than a millisecond of arc when averaged over the visit but can be as large as 3–5 mas for any given observation (eg., when HST transits from orbital night to orbital day).
De-jittering is not performed at a 40 Hz rate because that would introduce noise
into the dataset. Instead the time-averaged centroids of the guide stars are computed for the same time interval that the astrometer was in FineLock/DataValid. The positions of the guide stars in the first exposure, corrected for differential velocity aberration, define the reference frame for the remainder of the visit. So, for example, if the dominant guide star (x, y)
centroids measured during the N
th astrometry observation differed from those in the first observation by (dx, dy
) = (1 mas,1 mas), then the appropriate conversion to d
V3 is applied to the roll star and the astrometer's local (x, y)
centroids. This procedure creates a fixed but arbitrary coordinate system for the entire visit.
After the FGS data have been de-jittered, there will remain an apparent motion of
those astrometry targets which were observed more than once within the observing sequence. These check stars provide the data required for the drift correction, which assumes that the astrometer is a rigid body which both translates and rotates in the HST focal plane during the course of the visit and corrects the measured positions of the stars in the visit for contamination by this motion.
The time-tagged positions of the check stars are used to generate a model for this
drift, and the time-tagged positions of all the stars in the visit are adjusted by application of the model. Three separate models can be applied:
The choice of model depends upon the number of check stars available and the
number of times each is observed. Clearly if there is only one check star in the visit the rotation model cannot be applied. Also, if check stars are not observed frequently enough (three times or more), the quadratic models might not be reliable. The pipeline applies all three models, providing three sets of corrected centroids to the data. It is the responsibility of the user to decide which set is the best. The output of the fitting program includes fit residuals and χ2
. Inspecting these values is the best way to determine which model yielded the best result.
The size of the drift correction is typically 2 to 6 mas under two-FGS guidance. The
amount of drift appears to be related to the intensity of the bright Earth projected down the V1 boresight during target occultations. This intensity, and hence check star drift (generally), is highest for targets in HST’s orbital plane and lowest for those at high inclination.
When only one FGS is used for guiding, the telescope is not roll-constrained.
Under such circumstances the check stars can reveal very large motions, up to 60 or 70 mas over the course of the orbit (5 to 10 mas is more typical). Nevertheless, this drift can be successfully removed from the astrometry data, provided the proposal contained an adequate check star scenario. For example, the overlay of the plates from two separate Position Mode visits, each measuring ~ 20 stars in an astrometric star field distributed throughout the pickle yielded an rms residual of about 1 mas, even though one of the visits had one-FGS guiding and check-star drifts on the order of 30 mas.