When targets are selected for FGS Position mode observations, several options and requirements should be considered. These options are described below.
The bright limit for FGS1r is V = 8.0 without the neutral density filter in place. With the F5ND filter, objects of V = 3.0 or fainter can be observed. The faint limit is V ~ 17.0.
FGS target acquisition in Position mode will be unreliable if the target has a neighbor of comparable or greater brightness within a radius of 10 arcseconds. In essence, the FGS's IFOV - a 5" x 5" box - expects to encounter the target star within the search radius. Companions of similar brightness within this search radius may be mistakenly acquired instead of the target. However, for magnitude differences m > 1, companions within ~ 6" will not affect the acquisition of the brighter target. Note that binary stars with component separations less than about 0.5" can be successfully acquired in Position mode, regardless of the
m. Refer to the discussion under Section 4.2.5 for further details regarding the acquisition of binary systems in Position mode.
The target field consists of the science target and reference stars. Observations of the reference stars will be used to define the local reference frame for relative astrometry. Since the optical field angle distortions are calibrated most accurately in the central region of the FOV, the pointing of the spacecraft (via POS_TARG commands - Chapter 6 for more details) should be specified to place the target field (as much as possible) in this area.
If the visit also includes Transfer mode observations of an object, the spacecraft pointing should be chosen to place the object at the FOV center, as this is the only location calibrated for Transfer mode. If the target field geometry requires the Transfer mode observations be executed at other locations in the FOV, special calibrations will be needed. Proposers should consult STScI's Help Desk for assistance.
Ideally, reference stars should have the following characteristics:
Check stars, which are a subset of the target list, are observed several times over the course of an orbit (visit). Two or more check stars, distributed across the field, provide the information needed to characterize the drift of the FGS's FOV on the sky (which is typically about 4 mas over the course of the visit). Each check star should be observed at least three times. The best check stars are brighter than 14th magnitude to minimize exposure time, and should include the science object for the highest accuracy astrometry.
Table 4.1 is a listing of the FGS1r filters, their calibration status and applicable brightness restrictions. (Refer back to Figure 2.8: for the filter transmissions as a function of wavelength.)
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Only the F583W filter will be calibrated for Position mode for the full FGS FOV. Filter F5ND will be calibrated only at selected locations within the FOV |
Occupying the fifth slot on the wheel is the PUPIL. It is not a filter but rather a 2/3 pupil stop. Use of the PUPIL significantly reduces the degrading effect of spherical aberration (which does not necessarily improve Position mode performance) but collaterally alters the field dependence of the distortions. Consequently, the OFAD calibration for the F583W filter cannot be applied to PUPIL observations. In addition, PUPIL observing attenuates the object's apparent brightness by nearly a full magnitude, which sets the faint limiting magnitude at about V=16 while making observations of stars fainter than V = 14.5 excessively time consuming.
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. Figure 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 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.
V Magnitude |
FESTIME (seconds) |
---|---|
8-12 |
0.025 |
13 |
0.050 |
14 |
0.1 |
15 |
0.4 |
16 |
1.6 |
17 |
3.2 |
Filter |
B-V |
|||
---|---|---|---|---|
+1.78 |
+0.60 |
+0.040 |
-0.24 |
|
F583W |
1.000 |
1.000 |
1.000 |
1.000 |
PUPIL |
0.491 |
0.491 |
0.491 |
0.491 |
F5ND |
0.010 |
0.010 |
0.010 |
0.010 |
F550W |
0.356 |
0.354 |
0.331 |
0.331 |
F605W |
0.860 |
0.700 |
0.624 |
0.575 |
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).
FGS1r PMT |
Average Background + Dark Counts per 0.025 sec |
---|---|
Ax |
3.623 |
Ay |
1.566 |
Bx |
3.658 |
By |
5.893 |
The exposure time is the minimum time that an object will be tracked in FineLock. Based the rate at which the measured location (or centroid) of a star converges (from analysis of FGS1r data) Table 4.5 lists the recommended exposure times as a function of target magnitude. We note that:
Magnitude |
phase2 exposure time (in sec) |
---|---|
8-14 |
10 |
15-17 |
25 |
Multiple or extended sources in the FGS's IFOV will result in a reduction of the amplitude of the observed interferometric fringes (relative to that of a point source). This occurs because light from multiple sources in the IFOV do not interact coherently (the observed rays originate from different angles on the sky). Therefore, multiple point source fringes will be superimposed upon one another, each scaled by the relative brightness of the source and shifted by its relative angular displacement on the sky. The result is a composite Transfer Function with reduced fringe visibility.
The fringe visibility reduction for the brighter component of a binary system with an angular separation along the X or Y axis greater than about 80 mas (i.e., when the individual S-Curves are fully separate) is given by:
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).
For projected angular separations less that 80 mas, the Transfer Function will be a blend of the merged point source S-Curves. The resultant fringe visibility will depend on the relative brightness and the angular separation of the components (i.e., Fr is more difficult to predict).
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) 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 (m < 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 A1), 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,
It is recommended that a proposer contact the STScI Help Desk for assistance with Position mode observations of binary systems.
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.
Crowded fields create two problems for FGS observations:
The proposer should consult the STScI Help Desk for assistance with such observations.
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Proposers should document-in the proposal-the logic for selecting a FESTIME or entering a false apparent magnitude of a target. |
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |