Analysis of Structural and optical Interactions

I’hc Precision Optical Intcrfcromctcr in Space (POIN’1’S) is a free flying, space-hascd, as~romctric i ntc.rfcrmnc.try mission cmplo ying an inslrumc.nt with two 2-nvclcr bascl inc, intcrfcromctcrs whose baselines form an angle of 90°~ 3°. POIN’1’S will measure the angular clislancc. bclwccn two stars roughly 90° aparl to a precision of 5 microarcscconds @as). POIN”l’S is currently unrlcr joint study al California Institu[c of 3’cchnology’s JCL Propulsion 1 Amatory (JP1.) and the Smithsonian Astrophysical Obscrvalory (SAO). The interfcromctcrs usc a fringe tracking, cxlcndcd Kalman F’iltcr in order to cstimam the rclalivc ang]c bc[wccn the intcrfcromclcr optical axis and the line conncc(ing the spacecraft and the star. In order for this cxtcndcd Kalman filter to track the fringes, the fringe cohcrcncc of each intcrfcromctcr must ho at least 95% (corresponding to changes in optical path diffcrcncc of roughly tcn nanornctcrs) during the filter update Wriod, given the propagation of disturbances through the spacecraft structure to lhc optical support slructurc. POINHi uscs a full apcrlurc laser mcwology systcm (FAM) to measure any changes in the starlight optical path diffcrcncc (OPD) internal to each intcrfcromctcr, and this mcasurcmcnt is used m aclivcl y correct for the changes in optical path d i ffcrcncc. This paper addresses a disturbance analysis of the preliminary POIN”I’S spacecraft and instrument design in ordc.r to dctcrminc whether and to what cxtcrrt any isolation and/or added structural damping is nczcssary to mccl the, }OIN”I’S instrument rcquircrncnts. Ttm analysis was performed using the Intcgra[cd Modeling of Optical Systc,ms (1 MOS) integrated modeling tool. 1 MOS is an integrated software environment wherein structural, optical, and control systcm modeling can bc performed, 1,incarizcd optical models, structural finite clcmcnt models, and disturbance characterization models were dcvclopcd and integrated in IMOS. Starlight fringe cohcrcncc was u.scd as a metric to quantify the performance of the POIN1’S instrument. I,incar optical pcrkrrbation analysis gave insight into the sensitivity of the performance of the intcrfcromctcrs to perturbations of the positions and orientations of the. optical clcmcnts, Finite clcmcnt modal analysis yielded structural modes, rnodcshapcs, modal costs, and }Iankcl singular values. These models were integrated with the disturbance models allowing for generation of frequency response funclions. The result of this analysis is end-to-end disturbance characterizations (starlighl fringe cohcrcncc. as a function of reaction wheel speed, for example). 2. I’OINTS Spacecraft Description The baseline spacecraft configuration for POINI’S is shown in Fig. 1 [1]. POINTS consists of a spacecraft bus, the astromctric instrument, and a two-axis gimbal. The gimbal separates the instrnmcnt and the spacecraft bus. ‘1’hc bus holds the majority of the spacecraft hardware, (reaction wheels, command and data handling electronics, batlcrics, ctc,), whereas the instrument (contained in the large hatbox enclosure) contains the starlight in[crfcrornctcrs, the metrology intcrfcromctcrs, and other associated hardware. I’hc spacecraft bus is used to shield the instrument from the sun, in order to provide a more stable thermal environment for the instrument. The two-axis gimbal is rc~uircd in order to c.nablc the instrument to view star pairs across the entire sky (cxccpt for those near the sun) [2]. In order to allow for a larger set of targc,t star pairs, the angle between the intcrfcromctcrs must bc articulated. The range of articulation will bc roughly i 3°. POINTS will measure the angular separation of two stars by viewing thcm wilh the starlight intc.rfcromctcrs for several minutes. In the baseline configuration, the instrument is pointed in three rotational dcgrccs of freedom (dof) by the inner (spin-axis) gimbal axis, the outer (tip-axis) gimbal axis, and by rolling the spacecraft about the direction to the Sun (SW Fig.]). 2.1 Structural Design The optical bench is the structure inside the. instrument cnclosurc. that supporLs the optical clcmcnts, Iasc.rs, dcoxlors, and metrology hardware. 3hc baseline optical bench design is an aluminum metering truss structure with a cross sczlion of 50cm square. The truss is constructed of thin walled tubes with a cross section of onc inch (2,54 cm) and a wall thickness of 50mils (1 .27mm). The two instrument metering trusses (one for each starlight intcrfcromctcr) arc both mounted on a tubular central column structure, This central column has a diameter of 50cm and a wall thickness of onc inch (2.54 cm) [3]. The spacecraft bus structure uscs a thin (50n~il/1 .27nml), riveted aluminum skin with stiffeners. The gimbal yoke is assumed to have a similar construction (thin aluminum skin wilh stiffeners) [31. ‘J’hc instrument cnckwre is nccdcd primarily to provide benign thermal and contamination environments. The instrument cnclosurc wiilcithcr bcastiff honeycomb sandwich construction covcrcd with multi-lay crinsulation (M I. I), oranopcn framework of bar clcmcnls supporting Ml.]. The cnclosurc design is as ycl undccidcd. 2.2 Slew mechanisms In order to SICW [hc inslrumcn[ bctwccn star pairs, the a[ti[udc and articulation control systcm (AACS) uscs rcaclion wheels, located in the spacecraft bus, to supply torque to the space.craf[. Elcc[romagnclic motors (either slcppcr motors or dc brushlcss moIors) arc USC(I 10 actuate the instrument in the gimbal. An articulation mechanism (AM) is used to change the angle bctwcc.n the intc.rfcromctc.rs. The AM will be a stepper motor driven lead screw actuator connecting the top and bottom intcrferomctc.r oplical support structures. During targcl star observations, both the gimbal and the articulation mechanism will be lcckcd in place. 2.3 ‘1’arget star mcasurcrncnt During target star observations, the POINTS instrument measures the angle bctwccn the, two stars by measuring the an.glc bctwccn the intcrfcromctcr optical axis (normal to the intcrfcromctcr baseline) and the line joining the spacccmf[ and each star (the star line) for each inlcrfc.romctcr and by measuring the angle bctwccn the intcrfcromclcr optical axes. I“his mcasurcmcnt schcmc is shown in Fig.2. Each starlight intcrfcrometcr measures the angle bctwccn its optical axis and its target star line (its star angle, 6). A set of laser gauges, r sp!” AI,,