Fine Guidance Sensor Instrument Handbook for Cycle 14
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1.1 PurposeChapter 1:
Introduction
1.2 Instrument Handbook Layout
1.3 Two-gyro Guiding
1.4 The FGS as a Science Instrument
1.5 Technical Overview
1.5.1 The Instrument
1.5.2 Spectral Response
1.5.3 The S-Curve: The FGS's Interferogram
1.5.4 FGS1r and the AMA
1.5.5 Field of View
1.5.6 Modes of Operation
1.6 Planning and Analyzing FGS Observations
1.6.1 Writing an FGS Proposal
1.6.2 Data Reduction
1.7 FGS2r
1.7.1 HST's 2nd Replacement FGS
The precision pointing required of the Hubble Space Telescope (HST) motivated the design of the Fine Guidance Sensors (FGS). These large field of view (FOV) white light interferometers are able to track the positions of luminous objects with ~1 millisecond of arc (mas) precision. In addition, the FGS can scan an object to obtain its interferogram with sub-mas sampling. These capabilities enable the FGS to perform as a high-precision astrometer and a high angular resolution science instrument which can be applied to a variety of objectives, including:
- relative astrometry with an accuracy approaching 0.2 mas for targets with V < 16.8;
- detection of close binary systems down to ~8 mas, and characterization of visual orbits for systems with separations as small as 12 mas;
- measuring the angular size of extended objects;
- 40 Hz relative photometry (e.g., flares, occultations) with milli-magnitude accuracy.
The purpose of this Handbook is to provide information needed to propose for HST/FGS observations (Phase I), to design Phase II programs for accepted FGS proposals (in conjunction with the Phase II Proposal Instructions), and to describe the FGS in detail.
1.1 Purpose
The FGS Instrument Handbook is the basic reference manual for observing with the FGS. It describes the FGS design, properties, performance, operation, and calibration. The Handbook is maintained by the Observatory Support Group at STScI, who designed this document to serve three purposes:
- To help potential FGS users decide whether the instrument is suitable for their goals, and to provide instrument-specific information for preparing Phase I observing proposals with the FGS.
- To provide instrument-specific information and observing strategies relevant to the design of Phase II FGS proposals (in conjunction with the Phase II Proposal Instructions).
- To provide technical information about the FGS and FGS observations.
The
HST Data Handbookprovides complementary information about the analysis and reduction of FGS data, and should be used in conjunction with this Instrument Handbook. In addition, we recommend visiting the FGS World Wide Web pages for frequent updates on performance, calibration results, and methods of data reduction and analysis. These pages can be found at:http://www.stsci.edu/hst/fgs/1.2 Instrument Handbook Layout
To guide the proposer through the FGS's capabilities and help optimize the scientific use of the instrument, we have produced the FGS Instrument Handbook, the layout of which is as follows:
- Chapter 1: Introduction, describes the layout of the FGS Instrument Handbook and gives a brief overview of the instrument and its capabilities as a science instrument.
- Chapter 2: FGS Instrument Design, details the design of the FGS. Specific attention is given to the optical path and the effect of optical misalignments on FGS observations. The interferometric Transfer Function is described in detail, along with effects which degrade Transfer Function morphology. Descriptions of apertures and filters are also presented here.
- Chapter 3: FGS Science Guide, serves as a guide to the scientific programs which most effectively exploit FGS capabilities. The advantages offered by the FGS are described together with suitable strategies to achieve necessary scientific objectives. A representative list of publications utilizing the FGS for scientific observations is included for reference.
- Chapter 4: Observing with the FGS, describes the detailed characteristics of the two FGS observing modes - Position mode and Transfer mode - as well as the observational configurations and calibration requirements which maximize the science return for each mode.
- Chapter 5: FGS Calibration Program, describes sources of FGS errors, associated calibrations, and residual errors for Position and Transfer mode observations. A discussion of calibration plans for the upcoming Cycle is also included.
- Chapter 6: Writing a Phase II Proposal, serves as a practical guide to the preparation of Phase II proposals, and as such is relevant to those researchers who have been allocated HST observing time.
- Chapter 7: FGS Astrometry Data Processing, briefly describes the FGS astrometry data processing pipeline and analysis tools. The various corrections for both Position and Transfer mode observations are described along with the sequence in which they are applied.
In addition to the above chapters, we also provide two appendices:
- Appendix 1: Target Acquisition and Tracking, describes the acquisition of targets in both Position and Transfer modes. The target acquisition scenario may have implications for observations of moving targets or targets in crowded fields.
- Appendix 2: FGS1r Performance Summary, describes the evolution of FGS1r during its first three years in orbit and the adjustment of the AMA to improve performance in this instrument.
1.3 Two-gyro Guiding
HST currently operates with three rate sensing gyros in the guiding control loop. At some point in the future additional gyroscopes on HST may fail, thus making it necessary to observe with only two gyroscopes instead of the usual contingent of three. This would have significant impacts on the scheduling of observations -- both the number of minutes available for observation per orbit, and the times of year when a given target can schedule, would be significantly reduced. It is also possible that the guiding would be less accurate -- there is a potential for a high-frequency (many Hz) pointing jitter to be superposed on the normal HST tracking, which would degrade the effective PSF. There is also some possibility that target positioning on the detectors would become less accurate. Tests of two-gyro guiding are planned for early 2005, where many of these impacts should become better defined. Additional tests would also be conducted in the event that gyro failures make two-gyro guiding necessary.
Further discussion of how these impacts will affect the observatory and the instruments can be found in a separate Handbook, the HST Two-Gyro Handbook for Cycle 14. See the Two-Gyro Handbook for detailed information. All text in this FGS Handbook assumes three-gyro control.
HST is expected to transfer into two-gyro mode at some point in the future; since gyro failures are unpredictable this event may occur as early as Cycle 14. Therefore proposers are requested to provide information on two-gyro observing on their programs; see the
Call for Proposalsfor details.While we expect two-gyro guiding to have little affect on the quality of FGS astrometry science data (after pipeline processing removes the drift and jitter witnessed by the guiding FGSs), there will be a major impact to the schedulability of parallax programs. Under three-gyro guiding it is possible to observe a target field at the two epochs of maximum parallax factor (six months apart). This will not be the case under two-gyro guiding; the target field can be observed over a segment comprising only about 30% of the parallactic ellipse near one of the epochs of maximum parallax factor. The reader is encouraged to consult the
Two-Gyro Handbookfor details.1.4 The FGS as a Science Instrument
The FGS has two modes of operation: Position mode and Transfer mode. In Position mode the FGS locks onto and tracks a star's interferometric fringes to precisely determine its location in the FGS FOV. By sequentially observing other stars in a similar fashion, the relative angular positions of luminous objects are measured with a per-observation precision of about 1 mas over a magnitude range of 3.0 < V < 16.8. This mode is used for relative astrometry, i.e., for measuring parallax, proper motion, and reflex motion. Multi-epoch programs achieve accuracies approaching 0.2 mas.
In Transfer mode an object is scanned to obtain its interferogram with sub-mas sampling. Using the fringes of a point source as a reference, the composite fringe pattern of a non-point source is deconvolved to determine the angular separation, position angle, and relative brightness of the components of multiple-star systems or the angular diameters of resolved targets (Mira variables, asteroids, etc.).
As a science instrument, the FGS is a sub-milliarcsecond astrometer and a high angular resolution interferometer. Some of the investigations well suited for the FGS are listed here and discussed in detail in Chapter 3:
- Relative astrometry (position, parallax, proper motion, reflex motion) with single-measurement accuracies of about 1 milliarcsecond (mas). Multi-epoch observing programs can determine parallaxes with accuracies approaching 0.2 mas.
- High-angular resolution observing:
- Absolute masses and luminosities:
- Measurement of the angular diameters of non-point source objects down to about 8 mas.
- 40Hz 1-2% long-term relative photometry:
- 40Hz milli-magnitude relative photometry over orbital timescales.
1.5 Technical Overview
1.5.1 The Instrument
The FGS is a white-light shearing interferometer. It differs from the long-baseline Michelson Stellar Interferometer in that the angle of the incoming beam with respect to the HST's optical axis is measured from the tilt of the collimated wavefront presented to the "Koesters prism" rather than from the difference in the path length of two individual beams gathered by separate apertures. Thus, the FGS is a single aperture (single telescope) interferometer, well suited for operations aboard HST. The FGS is a two dimensional interferometer; it scans or tracks an object's fringes in two orthogonal directions simultaneously. As a science instrument, the FGS can observe targets as bright as V=3 and as faint as V=17.5 (dark counts dominate for V>18 targets).
1.5.2 Spectral Response
The FGS employs photomultiplier tubes (PMTs) for detectors. The PMTs-four per FGS-are an end-illuminated 13-stage venetian blind dynode design with an S-20 photocathode. The PMT sensitivity is effectively monotonic over a bandpass from 4000 to 7000A, with an ~18% efficiency at the blue end which diminishes to ~2% at the red end.
Each FGS contains a filter wheel fitted with 5 slots. FGS1r contains three wide-band filters, F550W, F583W (sometimes called CLEAR), F605W, a 5-magnitude Neutral Density attenuator (F5ND), and a 2/3 pupil stop, referred to as the PUPIL. Not all filters are supported by standard calibrations. Transmission curves of the filters and recommendations for observing modes are given in Chapter 2 and 4 respectively.
1.5.3 The S-Curve: The FGS's Interferogram
The FGS interferometer consists of a polarizing orthogonal beam splitter and two Koesters prisms. The Koesters prism, discussed in Chapter 2, is sensitive to the tilt of the incoming wavefront. Two beams emerge from each prism with relative intensities correlated to the tilt of the input wavefront. The relation between the input beam tilt and the normalized difference of the intensities of the emergent beams, measured by pairs of photomultiplier tubes, defines the fringe visibility function, referred to as the "S-Curve". Figure 1.1 shows the fringe from a point source. To sense the tilt in two dimensions, each FGS contains two Koesters prisms oriented orthogonally with respect to one another. A more detailed discussion is given in Chapter 2.
1.5.4 FGS1r and the AMA
During the Second Servicing Mission in March 1997 the original FGS1 was replaced by FGS1r. This new instrument was improved over the original design by the re-mounting of a flat mirror onto a mechanism capable of tip/tilt articulation. This mechanism, referred to as the Articulated Mirror Assembly, or AMA, allows for precise in-flight alignment of the interferometer with respect to HST's OTA. This assured optimal performance from FGS1r since the degrading effects of HST's spherically aberrated primary mirror would be minimized (the COSTAR did not correct the aberration for the FGSs). This topic is discussed in detail in Chapter 2.
Figure 1.1: FGS Interferometric Response (the "S-Curve")
1.5.5 Field of View
The total field of view (FOV) of an FGS is a quarter annulus at the outer perimeter of the HST focal plane with inner and outer radii of 10 and 14 arcmin respectively. The total area (on the sky) subtended by the FOV is ~ 69 square arcmintues. The entire FOV is accessible to the interferometer, but only a 5 x 5 arcsec aperture, called the Instantaneous Field of View (IFOV), samples the sky at any one time. A dual component Star Selector Servo system (called SSA and SSB) in each FGS moves the IFOV to a desired position in the FOV. The action of the Star Selectors is described in detail in Chapter 2, along with a more detailed technical description of the instrument. Figure 1.2 shows a schematic representation of the FGSs relative to the HST focal plane.
Figure 1.2: FGSs in the HST Focal Plane (Projected onto the Sky)
1.5.6 Modes of Operation
The FGS has two modes of operation: Position mode and Transfer mode.
Position Mode
The FGS Position mode is used for relative astrometry, i.e. parallax, proper motion, reflex motion and position studies. In Position mode, the HST pointing is held fixed while selected FGS targets are sequentially observed (fringes are acquired and tracked, see appendix A1) for a period of time (2 < t < 120 sec) to measure their relative positions in the FOV. Two-dimensional positional and photometric data are continuously recorded every 25 msec (40 Hz). The raw data are composed of a Star Selector encoder angles (which are converted to FGS X and Y detector coordinates during ground processing) and photomultiplier (PMT) counts. Figure 1.3 is a schematic of the FGS FOV and IFOV. The figure shows how Star Selectors A and B uniquely position the IFOV anywhere in the FGS FOV.
Transfer Mode
In Transfer mode, the FGS obtains an object's interferograms in two orthogonal directions by scanning the Instantaneous Field of View (IFOV) across the target (typically in 1" scan lengths). Transfer mode observing is conceptually equivalent to imaging an object with sub-milliarcsecond pixels. This allows the FGS to detect and resolve structure on scales smaller than HST's diffraction limit, making it ideal for detecting binary systems with separations as small as 8 mas with ~ 1 mas precision.
Figure 1.3: FGS Star Selector Geometry
1.6 Planning and Analyzing FGS Observations
1.6.1 Writing an FGS Proposal
Chapter 3 and 6 are of particular use in designing and implementing an FGS proposal. Chapter 3 provides information on a variety of scientific programs which exploit the unique astrometric capabilities of the FGS. Chapter 6 provides guidelines on how to design the Phase II proposal. In Chapter 6 we provide examples of observing strategies and identify special situations where further discussions with STScI are recommended.
1.6.2 Data Reduction
The
HST Data Handbookprovides a detailed description of the FGS data and related data reduction. Chapters 5 and 7 in this Instrument Handbook contain useful summaries of that information. Chapter 5 provides a discussion of the accuracies and sources of errors associated with FGS data in addition to a detailed description of the calibration program planned for the upcoming Cycle. Chapter 7 describes the set of software tools which are available to observers to reduce, analyze and interpret FGS data. Please check the FGS web pages for details on these tools.1.7 FGS2r
1.7.1 HST's 2nd Replacement FGS
After the second servicing mission (SM2) the original FGS1 was returned for refurbishment to its manufacturer, Raytheon Optical Systems Inc. (ROSI, currently BFGoodrich Space Flight Systems). Its (worn out) star selector shaft bearings were replaced and, like FGS1r, an AMA was installed. During the December 1999 servicing mission (SM3a) this repaired unit, redesignated as FGS2r, replaced the original, mechanically worn FGS2. As part of the orbital verification activities ROSI engineers, using observations of the standard star Upgren69, adjusted the AMA to align FGS2r's internal optics with the OTA to optimize the instrument's interferometric response.
With two AMA optimized FGSs now on board HST, the question of whether FGS1r or FGS2r should be designated as the science instrument might arise. The amplitude and morphology of both the FGS1r and FGS2r S-curves vary with location in the FOV due to "beam walk", for which the AMA can not compensate (discussed in chapter 2). But FGS1r demonstrates optimal fringe visibility and morphology along both its x and y axis simultaneously at the center of its FOV, while FGS2r's x and y axis S-curves are not simultaneously optimal anywhere in its FOV. Therefore, FGS1r has significantly better angular resolution and remains the clear choice as the science instrument (FGS1r's performance as an astrometer is not impaired by the FOV dependent fringes).
There is also the consideration of the orientation of an FGS's FOV in HST's focal plane. For efficient astrometric determination of parallaxes, targets and reference stars are observed at times of maximum parallax factor. Due to HST sun angle constraints, FGS1r's orientation at such times is more favorable than that of FGS2r.
STScI does not regard FGS2r as a science instrument and has no plans to calibrate it as such. It has been calibrated only to the level needed for reliable performance as a guider. It will be monitored over time to verify that these calibrations remain within tolerance as the instrument desorbs water from its graphite epoxy composites.
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