<title>POINTS: high astrometric capacity at modest cost via focused design</title>

Abstract

POINTS (Precision Optical INTerferometer in Space) would perform microarcsecond optical astrometric measurements from space, yielding submicroarcsecond astrometric results from the mission. It comprises a pair of independent Michelson stellar interferometers and a laser metrology system that measures both the critical starlight paths and the angle between the baselines. The instrument has two baselines of 2 m, each with two subapertures of 35 cm; by articulating the angle between the baselines, it observes targets separated by 87 to 93 deg. POINTS does global astrometry, i.e., it measures widely separated targets, which yields closure calibration, numerous bright reference stars, and absolute parallax. Simplicity, stability, and the mitigation of systematic error are the central design themes. The instrument has only three moving-part mechanisms, and only one of these must move with sub-milliradian precision; the other two can tolerate a precision of several tenths of a degree. Optical surfaces preceding the beamsplitter or its fold flat are interferometrically critical; on each side of the interferometer, there are only three such. Thus, light loss and wavefront distortion are minimized.POINTS represents a minimalistic design developed ab initio for space. Since it is intended for astrometry, and therefore does not require the u-v-plane coverage of an imaging instrument, each interferometer need have only two subapertures. The design relies on articulation of the angle between the interferometers and body pointing to select targets; the observations are restricted to the "instrument plane." That plane, which is fixed in the pointed instrument, is defined by the sensitive direction for the two interferometers. Thus, there is no need for siderostats and moving delay lines, which would have added many precision mechanisms with rolling and sliding parts that would be required to function throughout the mission. Further, there is no need for a third interferometer, as is required when out-of-plane observations are made.An instrument for astrometry, unlike those for imaging, can be compact and yet scientifically productive. The POINTS instrument is compact and therefore requires no deployment of precision structures, has no low-frequency (i.e., under 100 Hz) vibration modes, and is relatively easy to control thermally. Because of its small size and mass, it is easily and quickly repointed between observations. Further, because of the low mass, it can be economically launched into high Earth orbit which, in conjunction with a solar shield, yields nearly unrestricted sky coverage and a stable thermal environment. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/26/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx SPIE Vol. 2807 / 49 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/26/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

A weak equivalence principle test on a suborbital rocket

“…(See, for example, the Itek flexure design in Fig. 3 of Reasenberg et al [1996].) This design is intended to impose a minimum time-varying stress on the TFG plate and thus minimize changing plate distortion.…”

“…The TFG plate is connected to the metering structure by means of three flexures, each constraining two degrees of freedom and likely made of titanium. (See, for example, the Itek flexure design in figure 3 of Reasenberg et al (1996).) This design is intended to impose a minimum time-varying stress on the TFG plate and thus minimize changing plate distortion.…”

Analytical Study of Optical Wavefront Aberrations Using Maple

“…Modern high-precision optical systems, such as space astrometric interferometers (e.g. FAME: Johnston et al 1997; POINTS: Reasenberg et al 1996; GAIA: Loiseau and Malbet 1996, Loiseau and Shaklan 1996; Lindegren and Perryman 1996; SIM: SIM97), can require optical path tolerances in the sub-nanometer (1 nm = 1 0 ,9 m) to picometer (1 pm = 1 0 ,12 m) regimes over total path lengths on the order 10 m. Such tolerances place extreme requirements on optical analysis programs. Two questions are of paramount importance: 1) to which specific perturbations is a system most sensitive?…”

“…AESOP was developed to support the analysis effort involved in determining critical sensitivities to optic misalign-ments in a proposed dual interferometric astrometric telescope, POINTS (Reasenberg et al 1988(Reasenberg et al , 1995a(Reasenberg et al , 1995b(Reasenberg et al , 1996. POINTS consists of a pair of independent Michelson stellar interferometers and a laser metrology system that measures both the critical starlight paths and the angle between the two interferometer baselines.…”