Linda Abramowicz-Reed

Fine guidance sensors aboard the Hubble Space Telescope: the scientific capabilities of these interferometers

The fine guidance sensors (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing interferometers that offer a unique capability to astronomers. The FGSs photometric dynamic range, fringe visibility, and fringe tracking ability allow the instrument to exploit the benefits of performing interferometry form a space-based platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated as the Astronomer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 arc minutes of its field of view, (2) determine the true relative orbits of close faint binary systems, (3) measure the angular diameter of a giant star, (4) search for extra-solar planets, (5) observe occultations of stars by solar system objects, as well as (6) photometrically monitor stellar flares on a low mas M dwarf. In this paper we discuss this unique instrument, its design, performance, and the areas of science for which it is the only device able to successfully observe objects of interest.

The fine guidance sensors aboard the Hubble Space Telescope: The scientific capabilities of these interferometers

The fine guidance sensors (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing interferometers that offer a unique capability to astronomers. The FGSs photometric dynamic range, fringe visibility, and fringe tracking ability allow the instrument to exploit the benefits of performing interferometry form a space-based platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated as the Astronomer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 arc minutes of its field of view, (2) determine the true relative orbits of close faint binary systems, (3) measure the angular diameter of a giant star, (4) search for extra-solar planets, (5) observe occultations of stars by solar system objects, as well as (6) photometrically monitor stellar flares on a low mas M dwarf. In this paper we discuss this unique instrument, its design, performance, and the areas of science for which it is the only device able to successfully observe objects of interest.

Optimization of the Hubble Space Telescope fine guidance sensor target-acquisition parameters

To achieve the highest accuracy boresight pointing performance the Hubble Space Telescope uses attitude feedback from the Fine Guidance Sensors (FGS). There are three FGSs on board HST. During normal operations, one sensor monitors spacecraft pitch and yaw, another monitors roll and the third is the redundant unit. Each FGS senses wavefront tilt interferometrically and converts that tilt into spacecraft pointing error. The presence of spherical aberration affects the signal from the instrument causing a reduction in acquisition and tracking performance on targets whose magnitudes are fainter than 14. This paper documents the efforts to optimize uplinkable, FGS parameters in order to increase the probability of target acquisition that is better than 98 percent over the entire field of view. To this end, the paper describes the Monte Carlo simulator used in deriving the optimized values for the FGS acquisition and discusses methods for testing the new parameters prior to on-orbit verification. It reports on improvements predicted by the acquisition simulator and evaluates on orbit performance with the optimized values. In addition, the paper discusses the commissioning of FGS 1R, installed during the February 1997 servicing mission, with regard to operational options predicted by the simulator. It also reports on how well the new FGS, with its on board alignment capability, is working with the new acquisition parameters determined by the simulator.

On-orbit performance of the fine guidance sensor FGS-1R

During the second servicing mission of the Hubble Space Telescope (HST), a newly refurbished fine guidance sensor (FGS-1R) was installed into the telescopes Radial Bay No. 1. The successful replacement of the existing FGS-1, whose degraded Star Selector Servo bearings were affecting the scheduling and acquisition of science data, was critical to continued success of the observatory. In addition to solving the bearing problem, the refurbished FGS-1R also provided an innovative approach to minimize the effects of spherical aberration on the interferometric signal generate by the FGS, hereafter referred to as an s-curve. Rather than try to remove the aberration from the wavefront over a very large field of view, FGS-1R was given the capability to realign the beam to the Koesters prism. A symmetric error, such as spherical aberration, which is divided perfectly at the beam center and folded onto itself, will have the effects of the aberration canceled. To this end, FGS-1Rs FOld Flat No. 3 was retrofit with an Actuated Mechanisms Assembly (AMA), which allows on orbit correction of the beam alignment. This paper gives an overview of the theory of operation of the FGS, and characterizes the effects of spherical aberration on s-curve modulation. It discusses the theory of operation of the AMA, and how it is used to optimize the optical alignment. It describes the analysis tools and methods used to transform on orbit data into required adjustments of the AMA. Finally, it presents the result of the on orbit optimization of s-curve modulation, and briefly discusses some of the challenges faced in refurbishing the next FGS.

The fine guidance sensors aboard the Hubble Space Telescope

The fine guidance sensors (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing interferometers that offer a unique capability to astronomers. The FGSs photometric dynamic range, fringe visibility, and fringe tracking ability allow the instrument to exploit the benefits of performing interferometry form a space-based platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated as the Astronomer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 arc minutes of its field of view, (2) determine the true relative orbits of close faint binary systems, (3) measure the angular diameter of a giant star, (4) search for extra-solar planets, (5) observe occultations of stars by solar system objects, as well as (6) photometrically monitor stellar flares on a low mas M dwarf. In this paper we discuss this unique instrument, its design, performance, and the areas of science for which it is the only device able to successfully observe objects of interest.

Experience of integrated interferometric guidance and astrometry

The three fine guidance sensors on-board the Hubble Space Telescope are the first white-light amplitude shearing interferometers to be used for space platform guidance, control, and astrometry. Two fine guidance sensors (FGS) under fine lock control now maintain spacecraft pointing precision to within 7 milliseconds of arc rms over the majority of each orbit. Fine guidance sensor control optimization techniques have yielded significant improvement in tracking stability, integrated performance with the pointing control system, loss-of-lock statistics and astrometric accuracy. We describe the optical interferometer, based on the Koesters prism design. We include a discussion of the instrument calibration status, the FGS fine lock performance design enhancements, pointing control system design enhancements, and ground software techniques appropriate to jitter removal in astrometric data. The combination results in marc sec precision astrometry.