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FGS Instrument Handbook for Cycle 24 |
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FGS science data is processed at three distinct levels: the exposure, the visit, and the epoch. Each of these levels is subject to different sources of error. Table 5.1, FGS1r Position Mode Calibration and Error Source Summary summarizes the accuracies of Position mode calibrations. Most of the errors listed are statistical. Multi-epoch observations reduces the impact of these errors in the science data.
± 30 mas ~1 mas relative shift for δ(B-V) = 1 The location of a star in the FOV during a Position mode observation is determined by identifying the median of the 40 Hz SSA or SSB samples (while the target is being tracked in FineLock). The median measurement is robust against most spacecraft jitter, short-interval transients and telemetry dropouts. If faint targets (V > 16.0) are observed, the photometric noise results in a large noise equivalent angle. Spacecraft jitter and photometric noise contribute to the standard deviations about the median of up to 2 mas per axis for V < 14.5 and up to 3 mas per axis for V > 15.0. However, the repeatability of the centroid measurement (over smaller intervals of the exposure) is the true assessment of the precision of the measurement, typically 0.7 mas and 1.5 mas for targets where V < 14.5 and V > 15.0 respectively.The effect of PMT sensitivity on FGS observations is discussed in Appendix A:Target Acquisition and Tracking. In order to accommodate the differences between the two PMTs along each axis, the FGE computes an average difference (DIFF) and average sum (SUM) of their photometric response to the star over the first few FESTIMES in the WalkDown. These values are used in the calculation of the Fine Error Signal. The results are accurate for bright (V < 14.0) objects but become unreliable for fainter targets, a result of the short integration period and increasingly noisy photon statistics. The pipeline gathers photometric data over the entire WalkDown (typically 80 times as many samples) to achieve a better signal-to-noise and more reliable values of DIFF and SUM. These are used to recompute the Fine Error Signal and adjust the (x,y) centroids in post-observation data reduction.Differential velocity aberration arises as a result of small differences in the angle defined by the HST velocity vector and the line of sight to targets in the FGS FOV. The HST PCS guides for zero differential velocity aberration (DVA) at one position in the FOV. The positions of targets elsewhere in the FOV must be corrected for DVA. Calibration errors in the relative alignment of the FGSs, catalog position errors of the guide stars, and ephemeris errors all contribute—though negligibly—to the errors in the differential velocity aberration correction. The actual adjustment to the target’s positions can be as large as ± 30 mas (depending on the target and velocity vector geometry) but are corrected by post-observation data processing to an accuracy of ± 0.1 mas.The OFAD residuals for FGS1r are smaller than those of FGS3 due to the design of the calibration test. The FGS3 data were acquired at a time when the roll of HST was restricted to be within 30 degrees of nominal for the date of the observations. The FGS1r test executed when the target field (M35) was close to the “anti-sun” position, i.e., when HST could be rolled over a full 360 degrees. Figure 5.1 shows an overlay of the pointings used for the FGS3 calibration, while Figure 5.2 shows the same for the FGS1r calibration. The freedom to rotate the field of view maximized the apparent effect of the distortions, making them more easily measured compared to the FGS3 test.The accuracy of the astrometric catalog generated from ground based observations is insufficient for calibrating an FGS as a science instrument. As part of the OFAD calibration, it was necessary to derive an accurate star catalog. This requires that selected stars be observed at several HST pointings. In Figure 5.1 and Figure 5.2, the bold symbols denote stars that were observed as part of the calibration. For the FGS1r calibration, special care was taken to maximize the number of pointings which measured every star.The five-element corrector group (see box in Figure 2.1) is a collection of refractive elements tasked with the removal of astigmatism and the final collimation of the beam. It’s refractive properties introduce subtle changes to angle of propagation of the beam as a function of the spectral color of the source. This change causes the apparent position of the star in the FOV to shift slightly, an effect referred to as lateral color. The positional error introduced by lateral color is relevant when comparing the relative positions of two targets of extreme colors: for example, a color difference of δ(B – V) = 1 between two targets could introduce a ∼1 mas positional shift. An in-orbit assessment of lateral color associated with FGS1r was performed in December 2000 and again in December 2001 (and will be repeated in December 2002). A dedicated on orbit calibration of the lateral color shift in FGS1r was first performed in December 2000. A field of stars containing a blue star (A0) and a red star (M3) was observed at three HST roll angles that differed by about 60 degrees (the field was near anti-sun, so HST roll angle was unconstrained). The three-orbit test was repeated in December 2001. Since then the lateral color calibration has been monitored every two years. The results of these tests are available from the FGS Web site at:The pre-SM3B solar panels caused high frequency, large-excursion jitter, as HST transitioned to and from orbital day and night. These disturbances ranged in amplitude from 50 to 150 mas and lasted up to several tens of seconds. If particularly frenzied, a temporary or total loss of lock of the guide stars would result. An example of the jitter during the onset of a day/night transition is shown in Figure 5.3. The large vibrations increase the standard deviations of FineLock tracking in the three FGSs by up to a factor of eight over the pre-transition values. Fortunately, such instances were rare.This plot shows the relative position of a guide star in FGS2 along the HST V3 axis as a function of time. The large disturbance at about 157 seconds occurred as HST transitioned from orbit night into daylight. This was typical until the new solar panels were installed in March 2002 (SM3B). Significant jitter is no longer present at day/night or night/day transitions.FGS drift was discussed in Chapter 7 with regards to observation strategy, i.e., the use of check stars to track apparent motion of the FOV during the visit so it can be removed during post-observation processing. There are two different classes of drift, depending on whether one or two FGSs guided the HST during the visit. With two FGSs guiding, drift is identified as a slow but correlated wander of the targets observed more than once during the visit. The amount of drift appears to be related to the intensity of the bright Earth entering the telescope during target occultations. Accordingly, the drift is highest for targets in HST’s orbital plane (~ 10 mas) and lowest for those at high inclination (~ 2 mas).When only one FGS is used to guide the telescope, the drift is typically 20 mas over the course of the visit. The single guide star controls the translational motion of the spacecraft while the HST roll axis is constrained by the gyros. Gyro-induced drift around the dominant guide star ranges from 0.5 to 5 mas/sec, and is typically of order 1 mas/sec. Note the gyro drift is a spacecraft roll, and does not represent the translational motion of a target at the FGS (which will typically be ~0.01mas/sec). Over the course of a visit, the roll drift error measured by the astrometer can build up to 40 mas or more (but is typically less than 20 mas).Regardless of the size of the drift, it can be characterized and removed by applying a model to the check star motions, provided the visit includes a robust check star strategy: a check star observation every 5–6 minutes (described in Chapter 7). At a minimum, two check stars measured three times each are needed to model translational and rotational drift.For a target star (or any reference stars) brighter than V = 8.0 to be included as part of an FGS observation, it must be observed with the neutral density attenuator F5ND. As a result of the differing thicknesses of F583W and F5ND, and possibly a wedge effect between the two filters, the measured position of the bright target in the FOV will shift relative to the (fainter) reference stars. A cross-filter calibration is required to relate these observations, as relative positional shifts may be as high as 7 mas. Also, further evidence from FGS3 indicates these shifts are field dependent. If the effect is uncorrected, a false parallax will occur between the science and reference targets as the star field is observed at different orientations in the FOV. Since it would be prohibitive to calibrate the cross-filter effect as a function of field location, FGS1r cross-filter calibrations will be restricted to the center of the FOV. For reference, the uncertainty after the FGS3 cross-filter calibration is ~0.5 mas.For FGS3, the plate scale and OFAD exhibits a temporal dependence on an average time scale of ~4 months and a size of several tens of milli arcseconds (predominately, a scale change). The evolution of the FGS3 OFAD revealed that the variability is probably due to the slow but continued outgassing (even after 10 years!) of the graphite epoxy structures in the FGS. A long-term stability monitoring test is executed bi-monthly to help measure and characterize the distortion and relative plate scale changes and thus update the OFAD. Post-calibration residuals are on average ± 1 mas along the X-axis and Y-axis. Better performance (of order ± 0.5 mas) is achieved in the central region of the FOV.The science data that has accumulated since the beginning of cycle 8 can be fully calibrated with the OFAD calibration. Any temporal evolution since the beginning of cycle 8 is back-calibrated away by use of the long term monitoring observations that have been executing all along. Check the FGS Web pages for updates with regard to the OFAD calibrations.The errors associated with several of the corrections described above will not manifest themselves until data from individual visits are compared. The most dominant source of Position mode error are the OFAD and changes in the plate-scale. The derivation of a plate scale solution is described in the FGS Data Handbook. In general, for regions near the center of the pickle, residuals are smaller than 1 mas if the reference star field is adequately populated.