|FGS Instrument Handbook|
Visit-level processing refers to those corrections that are applied to the individual exposures in order to map each onto a common reference frame. Since the FGS observes the targets sequentially, not simultaneously, any motion of the spacecraft or the FGS’s FOV during the course of the visit will introduce uncertainties in the measured positions of the objects. The corrections discussed here restore the cohesiveness of the reference frame.7.3.1 Position ModePosition mode observations during the course of a visit must be corrected for two sources of error which render the FOV somewhat unstable: low frequency HST oscillations and residual drift of the FOV across the sky.The mapping of the individual astrometry observations onto a common reference frame begins with the analysis of the guide star data. As part of the exposure-level processing, guide star centroids are computed over the same time interval as for the astrometry targets. Generally HST is guided by two guide stars. The so-called “dominant” guide star is used by the pointing control system (PCS) to control the translational pitch and yaw of the telescope. The “sub-dominant” guide star, also referred to as the roll star, is used to maintain the spacecraft’s roll or orientation on the sky. Any change in the guide star centroids over the course of the visit (after corrections for differential velocity aberration) is interpreted by the FGS Astrometry Pipeline as an uncorrected change in the spacecraft’s pointing.The pipeline defines an arbitrary fiducial reference frame based upon the location of the guide stars in the first exposure of the visit. Relative changes in the position of the dominant guide star for subsequent observations are assumed to be a translational motion of the HST focal plane. The pipeline “corrects” the position of the sub-dominant guide star and the astrometric target star. Any change in the angle defined by the line connecting the two guide stars and the spacecraft’s V2 axis is interpreted as a rotation of the focal plane, and is removed from the astrometry data.The correction of the astrometry centroids for vehicle motion (as determined by changes in guide star positions) is referred to as pos-mode dejittering. Transient corrections can be as large as 3 to 5 mas, such as when HST enters orbital day, but the adjustments are typically small—less than 1 mas. This underscores the excellent performance of HST’s pointing control system under the guidance of the FGSs.“Drift”, as discussed in Chapter 5, is defined as the apparent motion of the astrometer’ s FOV on the sky during the course of the visit as detected by the astrometry targets that are observed more than once during the visit (the check stars). Drift must be removed from the measured position of all astrometry targets. This is accomplished by using check star data to construct a model to determine the corrections to be applied. If at least two check stars are available and were observed with sufficient frequency (i.e., at least every seven minutes), a quadratic drift model (in time) can be used to correct for both translation and rotation of the FOV. The availability of only one check star will limit the model to translation corrections only. If the check stars were observed too infrequently, then a linear model will be applied.If no check stars were observed, the drift cannot be removed and the astrometry will be contaminated with positional errors as large as 15 mas.It is important to note that the drift is motion in the astrometer which remains after the guide star corrections have been applied. Its cause is not well understood, but with proper check star observing, the residuals of the drift correction are tolerably small (i.e., sub-mas).With the application of the guide star data for pos-mode dejittering and the check stars to eliminate drift, the astrometry measurements from the individual exposures can be reliably assembled onto a common reference frame to define the visit’s plate.The visit level corrections to Position mode observations, i.e., pos-mode de-jittering and the drift correction are performed by calfgsb (Figure 7.3, CALFGSB Position Mode Processing).7.3.2 Transfer ModeTransfer mode observations typically last about 20 minutes (or more), much longer than Position mode exposures (1 to 3 minutes). Therefore, it is far more likely that low frequency spacecraft jitter and FOV drift will have occurred during the Transfer mode exposure. These do not introduce uncorrectable errors since low frequency FOV motion is implicitly removed from the data by cross correlating the Transfer Function from each individual scan. However, relating the arbitrary coordinate system upon which the Transfer Function is mapped to the system common to the reference stars is an important and necessary prerequisite in linking the Transfer mode observation to the Position mode data.Transfer mode data analysis, as discussed in the exposure level section, involves the cross correlation of the Transfer Functions from each of the individual scans. The first scan is arbitrarily designated as the fiducial; all other scans in the Transfer Function are shifted to align with that of the first (this automatically eliminates jitter and drift local to the observation). Therefore, in order to restore some level of correlation with the other observations in the visit, the guide star centroids are evaluated over each scan, and, along with the shifts, are recorded.The cross correlation of the individual scans removes the drift of the FOV from the Transfer mode data. However, this is a relative correction, local to only the Transfer mode observation. By recording the shift corrections applied to the individual scans, the visit level pipeline has visibility to the drift that occurred during the Transfer mode observation.The presence of a small roll error of the Koesters prism about the normal to its entrance face (see “Transfer Mode Scale as a Function of HST Roll Angle”) introduces a bias in the location of interferometric null as measured by Position mode when compared to the same location in Transfer mode. This bias must be accounted for when mapping of the results of the Transfer mode analysis onto the visit level plate defined by the Position mode measurements of the reference stars. This bias is removed by applying parameters from the calibration database.