Ideally, reference stars should have the following characteristics:
Table 4.1 is a listing of the FGS1r filters, their calibration status and applicable brightness restrictions.
(Refer back to
Figure 2.11 for the filter transmissions as a function of wavelength.)
Photon statistics dominates the noise in the measured position of stars fainter than V ~ 13.0. To track fainter objects, the Fine Error Signal must be integrated for longer periods. Table 4.1 lists the default FESTIMES for various target magnitudes. The default FESTIMES, determined from the Phase II target magnitude, are appropriate for most observations, and are set to ensure that photon noise, when converted into the Noise Equivalent Angle (NEA), does not exceed a predefined angular error threshold. The NEA is given by the relation
The NEA is used by the proposal processing tool (APT) to set the default FESTIME time. The parameter
C is the total count rate expected from the target summed over all four PMTs,
B is the background count rate, and
t is the FESTIME. The NEA is plotted as a function of magnitude and FESTIME in
Figure 4.1.
C as a function of filter and magnitude for FGS1r is given by:
The constant f-factor is a function of the filter and the target’s spectral color.
Table 4.3 provides the
f-factor for each combination of filter and color. The default FES times used by the proposal processing software for Position mode measurements are listed in
Table 4.1.
Background noise includes cosmic ray events, particle bombardment during passages through the South Atlantic Anomaly (SAA), and scattered light falling in the 5 x 5˝ IFOV. Cosmic ray events are suppressed by special circuitry and the FGS is prohibited from operating while transiting regions of heaviest impact from the SAA. Table 4.4 gives the typical dark + background counts for FGS1r in 0.025 seconds. Typically these values appear to be valid for all observations of isolated targets (suggesting that the dark counts dominate the background contribution). If the background counts for a specific observation are needed for the analysis of the observation, such as when the source is embedded in significant nebulosity or in a crowded star field, it can be obtained from the photometry gathered during the slew of the IFOV to (or away from) the target position. These data extracted by the FGS pipeline package CALFGSA from the FITS files that input are cleaned of spikes from “interloping stars” and can be used to estimate the background levels during post-observation data reduction.
Table 4.4 lists the average dark+background counts/25 msec for each of the FGS1r PMTs. These data were serendipitously gathered over a 45 minute interval from a failed science observation (the target was not acquired due to a guide star problem). These data have proved invaluable for the analysis of Transfer mode observation of faint stars (V>15).
where fa and fb are the intensities of the brighter and fainter components, respectively. A similar expression, but with
lb in the numerator, is appropriate for the faint star S-curve (see
Figure 4.3 for examples).
Even significant loss of fringe visibility does not pre-dispose the object from being successfully observed in Position mode. To be acquired in FineLock, an object’s Fine Error Signal (see
Appendix A:Target Acquisition and Tracking) must exceed a fringe detection threshold (see
Figure A.2). The threshold is set on the basis of the target’s V magnitude, as entered in the proposal, to accommodate the acquisition of faint targets. (The fainter the target the more effectively the background and dark counts reduce the fringe amplitude, hence lower detection thresholds must be applied.) If the GO were to state the V magnitude of a binary system or extended source to be sufficiently faint, (regardless of its true value), then the observed fringes will exceed the (lower) detection threshold, and the FGS will successfully acquire the object. However, if a false magnitude is specified, one should also manually set the FESTIME (an optional parameter) to the value appropriate to the object’s true magnitude. Otherwise, the observation’s overheads will be excessively long.
Some binary systems are not reliably observed in Position mode, even with the adjustment to the fringe detection threshold. Objects in this category include those with components exhibiting small magnitude differences (
Δm < 1) and angular separations greater than 60 mas but less than 800 mas (as projected along an interferometric axis). In these cases, either star may be acquired. There have been cases where one component was acquired on the X-axis while the other was acquired on the Y-axis. Such data are still useful, but care must be applied in the post- observation data processing.
There is a class of binary stars which cannot be observed in Position mode. In a FineLock acquisition (see
Appendix A:Target Acquisition and Tracking), the WalkDown to FineLock is a finite length path (approximately 0.810") beginning at a point which is “backed off” a fixed distance from the object’s photocenter. If the fringes of both stars lie outside this path, then neither will be encountered and the FineLock acquisition will fail. The condition for such a
failure is the following,
where X is the location of the system’s photocenter,
ra and
rb are the distances from the photocenter to the fringes of the components “a” and “b” respectively,
la and
lb are the flux from each component,
xs is the starting position of the WalkDown, and
xl is the length of the WalkDown. If the position of the binary along either the X or Y axis is known to meet this failure requirement, Position mode observations of this system should not be attempted.
For sources against bright backgrounds, the fringe visibility function is reduced by I / (
I +
B) where
I is the point source flux and
B is the background flux. The proposer should contact the STScI Help Desk for assistance with such observations.
