4.6.1 Summary of Transfer Mode Error Sources

The Transfer mode corrections and calibrations are:

• Exposure Level:

-Background subtraction.
-Star Selector Encoder correction (7 LSB).
-Cross-correlating and co-adding individual scans.
-Availability of a suitable point-source reference S-Curve (color response).

• Program Level:

-Roll dependence of the FGS plate scale.

Each of the corrections and calibrations are discussed briefly in Chapter 5 and more thoroughly in the FGS Data Handbook. Those corrections that could result in an enhanced observation strategy are discussed here.

4.6.2 Drift Correction

As discussed in the Position mode section, targets observed multiple times per Position mode visit typically drift across the FGS by about 6 to 12 mas when two FGSs guide the telescope. This drift is also apparent in Transfer mode observations, but the cross-correlation of S-Curves prior to binning and co-adding automatically removes the drift. Each single-scan S-Curve is shifted so that the particular features of the S-Curve used for the cross correlation coincides with that of the fiducial S-Curve. The reliability of implicitly removing the drift is only as good as the accuracy of the cross correlation procedure, which, for bright objects (V < 14.5) is accurate to < 1 mas. Analysis is underway to determine the procedure’s accuracy for fainter objects.

4.6.3 Temporal Variability of the S-Curve

Measurements of the standard star Upgren69 over the lifetime of FGS3 indicated a 10 – 18% variability of the S-Curve morphology on orbital timescales. The amplitude of these changes have important consequences on the analysis of binary star observations when the separation of the components is less than 30 mas and the magnitude difference exceeds 1.6. These temporal changes also affect analyses of extended source observations. The cause of this relatively high frequency variability in FGS3 has not been determined.

FGS1r appears to be stable at the 1% level over periods of many months to perhaps years (as of July 2002). There appears to have been a slow evolution of the y-axis S-curve however, as shown in Figure 4.5. The changes along the X-axis have been much less. On the assumption that this evolution is due to changes in the alignment of the interferometer with respect to HST’ OTA as water vapor outgasses from the instrument’s graphite epoxy composites, it is expected that FGS1r will become more stable as time goes on (the rate of outgassing slows with time in orbit). STScI will continue to monitor FGS1r’s S-Curves so that this evolution can be calibrated and its effect on science data minimized.

4.6.4 Background and Dark Counts Subtraction

For programs with isolated targets, background is not an issue. For programs with targets embedded in nebulosity, knowledge of the background is required. In order to obtain a background measurement, it is necessary that there be at least two targets in the observing sequence, separated by at least 60˝. The background will be measured during the slew of the IFOV from one target to the next. If necessary, the proposer should specify a false target at some location in the FOV at least 60˝ from the science object, and observe it in Position mode for approximately 30 seconds with the same filter as the science target. Care must be taken to avoid observing “background: objects brighter than V = 8.0 with F583W.

Dark counts become important for stars fainter than V=14.5. STScI has calibrated the FGS1r dark counts. Therefore there is no need to acquire such data as part of a science observing program. Likewise, the dead time correction is needed for stars brighter than about V=9, but STScI has already acquired the data needed to support this correction.

4.6.5 Empirical Roll Angle Determination

The science data headers contain the commanded HST roll angle, not a measured angle. The errors that contribute to a difference between the commanded and actual roll include: the relative guide star positional error, the FGS-FGS alignment error, and errors in the predicted ephemeris. The actual roll angle is calculated from the guide star telemetry by the observatory monitoring system, and is reported in the STScI Observation Logs that accompany the science data. More information is available on the following Web page:
http://www.stsci.edu/hst/observing/program-information

The error in the calculated roll angle is estimated to be about 0.04 degrees. If a more accurate determination is needed, the position angle of the observed binary with respect to the local reference frame can be measured via Position mode observations of the target and a reference star (or two reference stars if the target cannot be acquired in Position mode—see Section 4.2.5) should be included in the visit along with the Transfer mode exposures. Please confer with STScI for help designing the visit and calculating the roll from Position mode measurements.

4.6.6 Exposure Strategies for Special Cases: Moving Targets

For Transfer mode observing, a moving target represents a special case. The flight software which enables HST to track a moving target has not been implemented for FGS observations. Nonetheless, the FGS is quite capable of acquiring a moving target provided that the object’s angular speed is less than 80 mas/sec. However, during the observation, the scan path is not adjusted to accommodate for the object’s motion. The target’s fringe will be displaced in each subsequent scan until it moves completely out of the scan path. A method to work around this problem is to specify several observations of the object during the visit. For example:

1. The target should be observed in several short exposures rather than in one or two long exposures. The FGS would re-acquire the target with each new Search, CoarseTrack acquisition regardless of the target’s motion. The target list should contain enough entries to cover the swath of sky traversed by the moving target, e.g., if the motion takes the object 10 arcsec across the FOV, then at least two sets of target coordinates should be specified.
2. The number of individual observations (entries in the exposure logsheet) and number of scans in an observation will be dictated by the object’s angular speed.
3. Plan the observation when the target is moving its slowest, e.g., at opposition (if possible).
4. Take advantage of rolling the telescope to adjust the angle between the target motion vector and relevant FGS reference directions (­please contact STScI for assistance).
5. Choose the exposure times to be as short as possible.