1997 HST Calibration Workshop Space Telescope Science Institute, 1997 S. Casertano, et al., eds. Astrometry with the FGS in POSITION Mode and TRANSFER Mode: Observing Strategies, Pipeline Processing and Data Reduction Ed Nelan, Olivia Lupie, and Lauretta Nagel Space Telescope Science Institute, 3700 San Martin Drive Baltimore, MD 21218 We present an overview of HST astrometry, data analysis techniques, Abstract. calibrations, errors, and pipeline processing steps for FGS astrometry data. 1. Introduction Each Fine Guidance Sensor (FGS) on the HST is an optical-mechanical white-light inter- ferometer that can sense 1–3 milliarcsec (mas) angular displacements of a point source over a large dynamic range: 3 < V < 15. For fainter objects down to V < 17, the accuracy degrades to more than 2 mas. The FGS can also resolve structure in non-point sources at the 15 mas level. The light gathering power of the HST and the dynamic range of the FGS make it an unparallelled science instrument for many important astronomical investigations. Detecting and resolving multiple star systems and planetary companions, delineation of ob- jects in crowded fields, measuring angular diameters, parallax, proper motion, positional surveys, occultation studies and photometry are among its many uses. FGS3 is currently serving as the HST astrometer. For many scientific endeavors, the FGS continues to exceed ground-based efforts in sensitivity and resolution. The optical path through an FGS is complex because the beam must pass through mul- tiple optical elements. The relative alignment of all these components and the wavelength dependencies introduced by their refractive or reflective properties limits the resolution and magnitude sensitivity of the FGS. Most of the FGS calibration procedure consists of empir- ical and semi-empirical subtraction of the instrument signature necessitating observations of standard stars in all modes and various spectral ranges. This paper briefly describes the modes of the FGS and discusses the data analysis techniques and pipeline, calibrations, and the removal of instrument signatures. 2. Instrument Overview Three Fine Guidance Sensors on the HST each have a total field of view that extends radially 10 arcminutes to 14 arcminutes; each is a quarter annulus in the HST focal plane with a total area of 69 square arcminutes. The instantaneous field of view (IFOV) is smaller, 5 × 5 arcsec. Only photons in this 5 × 5 arcsec aperture fall on the PMTs at any one time. The IFOV can be placed at any position in the annulus. For extensive detail of the optical components and light path of the FGS, we refer the reader to the latest version of the HST Data Handbook (Voit 1997), Chapters 9, 10, 11, and 12, to be released later this year. Here, we briefly review those pertinent aspects of the instrument which will be important in discussions of instrument effects discussed later in this paper. At the heart of the FGS are the polarizing beam splitter (PBS), the 2 Koesters prisms, and the four photomultiplier tubes (PMTs). The optical elements upstream from the PBS narrow the IFOV to 5 ′′ × 5 ′′ and present to the PBS a collimated beam. The PBS performs 449
450 Nelan et al. Figure 1. The X and Y Axes S-Curves of the single standard star Upgren 69 at the center of the FGS3 annulus and acquired in filter f583W. a 50/50 intensity division of the incident light and emits two mutually orthogonal plane polarized beams. These beams fall opon the face of the appropriate Koesters prism. The Koesters prism is a fused silica pyramid with two halves separated by a dielectric interface. The prism senses tilt about an axis which is in the plane of the dielectric and parallel to the entrance face of the prism. Small rotations of the Star Selector A and B assemblies vary the tilt of the wavefront. When the component of the wavefront’s propa- gation vector which is perpendicular to the plane of the dielectric is zero, a condition of interferometric null results and the relative intensities of the two emergent beams are ideally equal Two Koesters prisms are needed for sensing the tilt of the wavefront in orthogonal directions. For a star at a given position in the FGS detector space, there is a unique set of SSA and SSB rotation angles which brings that star’s wavefront to zero tilt at the face of each Koesters prism. Therefore, the position of the star in detector space can be measured with high precision. 3. The S-Curve The relative intensities of the beams emerging from the Koesters prism assembly correlate with the wavefront tilt angle and therefore respond to the rotations of the SSA and the SSB assemblies that cause the IFOV to scan across a star. The PMT output during such a scan provides the characteristic interferometric pattern. The normalized difference of the PMTs on a given channel against the IFOV position produces a figure called the S-Curve. For each point along the X axis, the S-Curve is given by the following with a similar expression for Y: S x = ( A x − B x ) / ( A x + B x ) where A x and B x are the PMT counts from PMTs A and B on the X channel integrated over 25 milliseconds. When the IFOV is more than 100 mas from the location of the interferometric null, the PMTs record nearly equal intensities. A zero point crossing between the peaks of the S-Curve occurs near interferometric null. But, because of slight differences in the PMT sensitivites and in the paths traversed by the two beams, the zero point crossing is slightly shifted from the true null. A correction is made in the data reduction pipeline for this effect. The S-Curve in FGS3 of a single standard star Upgren 69 is shown in Figure 1. The Y axis S-Curve is nearly perfect as can be seen from the morphology and amplitude. The
451 FGS Astrometry X axis however is degraded, a result of misalignment of Koesters prism with respect to the wavefront’s tilt axis. 4. Modes of Observing The two observing modes of the FGS are POSition mode and TRANSfer mode. An FGS in Position mode acquires an object in fine lock and tracks it for an extended period of time. An FGS in Transfer mode acquires an object and scans the IFOV back and forth along a 45 degree diagonal path in detector space to sample the interference pattern and generate an S-Curve. Position Mode Observing : This mode can be used to determine the parallax, proper motion, and/or reflex motion of a given object. A typical Position mode observing program, regardless of the scientific goal, measures the (x,y) detector space positions of several objects concurrently observable in the FGS’s total FOV. An FGS can observe only one object at a time, so each of the objects are visited and tracked in a sequence specified in the proposal. To begin the visit, the IFOV is slewed to the predicted position of the first target. The target acquisition phases are initiated and once the interferometric null is located, Fine lock tracking ensues (described below) and data are obtained according to the exposure time in the proposal. This process repeats until the FGS has observed all stars in the visit. Transfer Mode Observing : This mode can be used to resolve the components of multi- star systems and measure the angular dimensions of extended objects. In a Transfer mode observation the FGS acquires the object, but instead of attempting to keep the FGS’s IFOV at or near interferometric null as in a Position observation, the FGS steps its IFOV is back and forth across the object along a 45 degree diagonal path in detector space to sample the entire S-Curve and its immediate vicinity. Each swath across the object is referred to as a scan. The number of scans, size of each step, and length of each scan are specified in the proposal. Typically, a Transfer mode observation will consist of 20 or more scans, each 1 . ′′ 4 long, with a STEP SIZE of 1 mas. Mixed Mode Observing : It is sometimes possible to determine the parallax, proper motion, and/or reflex motion of a multiple star system resolvable by the FGS in Transfer mode. If the FGS can both resolve a binary system and measure its parallax, then the absolute masses of its components would be determined. To accomplish this task, the FGS observes the target in Transfer mode and other nearby stars in Position mode. A mixed mode observing strategy would include a series of Position mode observations of the reference stars and a Transfer mode observation embedded somewhere in the sequence. Although the post-observation analysis of mixed mode observing data can be challenging, the potential scientific returns have made it an increasingly popular way to use the FGS as an astrometer. 5. A Note About Target Acquisition The target acquisition phases are described in detail Voit (1997). The stages are briefly discussed here because the target acquisition data are included in the data set delivered to the general observer and also because some of those data are used for background subtraction and calibration of the science data. The target acquisition consists of three stages: Search, Coarse track, and Fine lock. Search : This phase consists of an outward spiral of the IFOV from the expected location of the target which proceeds until the PMT counts from a 5 × 5 arcsec patch of sky falls within a specified range. When this occurs, the instrument proceeds to the next phase. CoarseTrack : The FGS determines the photocenter of the light by comparing the photon counts from the four PMTSs as the IFOV nutates in a 5 arcsec circular path around the
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