During the second servicing mission of the Hubble Space Telescope (HST), a newly refurbished fine guidance sensor (FGS-1R) was installed into the telescope's 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 Koester's 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-1R's 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 (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing interferometers that offer a unique capability to astronomers. The FGS's 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.
To achieve the highest accuracy boresight pointing performance the Hubble Space Telescope uses attitude feedback from the Fine Guidance Sensors (FGS). There are three FGS's 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.
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 Koester's 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.
The Hubble Space Telescope (HST) is an orbiting astronomical observatory, designed to operate as close as possible to ground based instrumentation, given the limitation of operating in a low earth orbit. The spacecraft design had to accommodate an absolute pointing accuracy of 0.01 arc seconds, a relative pointing stability of 0.007 arc seconds rms, the capability to maneuver 90 degrees in 18 minutes, and operate autonomously in a safemode control scheme for up to 72 hours. Furthermore, the design had to provide for a flexible, stored command methodology, and real-time command capability. This paper briefly reviews the spacecraft engineering hardware and software design. A detailed critique of the on-orbit performance of the spacecraft is provided. Enhancements and work-around, which have enabled HST to continue implementation of a successful science plan, are explained.
The three Fine Guidance Sensors (FGS's) on board the Hubble Space Telescope have been operated extensively since the observatory was launched in April, 1990. The FGS's, each capable of measuring angle as small as 0.003 arc-seconds (15 nanoradians), provide required fine pointing information to the Space Telescope's pointing control system, and are intended to serve as astrometry instruments. On-orbit data have shown that the acquisition, pointing and tracking performance of the FGS's in most cases meets, and of these sometimes exceeds, requirements. The versatility of the FGS digital control electronics to adapt to the unexpected conditions imposed on the sensors by the telescope spherical aberration and by solar panel jitter will be discussed. There is encouragement from both on-orbit tests and analytical studies that the FGS's can accommodate the current telescope characteristics. Improvements to guide star acquisitions within the FGS's and to target acquisitions within science instruments have been accomplished with the internal distortion calibration of each FGS and with the alignment calibration between sensors. Techniques used in the calibration process and the resulting improvements in acquisitions will be presented.