S P A C E I N T E R F E R O M E T R Y M I S S I O N N E W S L E T T E R Number 7, July 20, 1999 CONTENTS 1. SIM Project Management 2. SIM Preparatory Science Program 3. Astrometric Grid Workshop 4. Microdynamics Workshop 5. SIM Instrument Design Selection 6. Thermal Opto-Mechanical (TOM) Testbed 1. SIM PROJECT MANAGEMENT SIM's project manager for the last eighteen months, JPL's Chris Jones, has been promoted to leading the Mars Surveyor and Deep Space Systems Program Offices at JPL. A new SIM project manager is expected to be selected before the end of August 1999. We look forward to welcoming him/her to the exciting world of SIM. One of JPL's two Industry Partners, TRW in Redondo Beach, CA, SIM's partner for the development of the spacecraft, interferometer precision structure, and assembly, test and launch operations (ATLO), has also experienced a change in leadership. TRW's previous program manager, Jerry Glicksman, has retired and TRW has selected Joe Payne, the AXAF/Chandra Spacecraft Manager as his replacement. We welcome Joe to the team. 2. SIM PREPARATORY SCIENCE PROGRAM The Preparatory Science Program, now in its second year, seeks to support astronomical investigations related to the selection of stars for the SIM astrometric grid. The program supports other long lead-time science related to SIM. Picking suitable grid stars for SIM presents some very interesting challenges; possible approaches include theory, modeling, and ground-based observation of samples of stars. A draft of the second NASA Research Announcement (NRA 99-OSS-04) is on the SIM Web site. The official release date is July 23, 1999, and proposals will be due at NASA HQ on ** October 22, 1999** . For details, see: http://sim.jpl.nasa.gov/research-opps.html 3. ASTROMETRIC GRID WORKSHOP We are planning to host a Workshop in Pasadena on the topic of grid star selection for SIM. As the above item indicates, this topic is of great importance to the scientific productivity of SIM. To date, the issue has generated a lot of discussion, and significant disagreements on the best approach. The Workshop is intended for astronomers interested in direct involvement in this debate and in solving the grid star selection problem. The format will probably be short presentations, leading into extended and detailed debate of the different strategies. Tentative dates for the Workshop are January 20-21, 2000, for 2 full days. There will be no registration fee. If you are interested in participating, please send an e-mail to me (Steve Unwin) - fringes@huey.jpl.nasa.gov, with "Grid workshop" as the subject line, and indicating whether the proposed dates are suitable. 4. MICRODYNAMICS WORKSHOP The Microdynamics workshop, held at JPL June 23-24, 1999, is an open technical information exchange and work-in-progress meeting in the field of structural microdynamics as they apply to precision space optical systems. The Microdynamics working group includes members from various NASA centers, universities and industry. Microdynamics technologies investigate the nonlinear response of interface mechanisms such as hinges, latches, and joints at sub-micron levels of motion, as well as the broadband disturbances they may induce, such as snapping and popping. The main focus is the technology development effort of the Space Interferometry Mission (SIM) and the Next Generation Space telescope (NGST) Microdynamic tasks, which encompasses microdynamic design, analyses, ground testing and flight experiments. A working group meeting followed the workshop, and it was decided that next year's focus would be to release the Microdynamics Design Guideline, to propose and validate impulsive disturbance modeling approaches, to investigate the effect of microdynamics on the instrument control design, and to assess the influence of transient microdynamic disturbance propagation. The Workshop presentations are posted at http://sim.jpl.nasa.gov/microdynamics/presentations.html 5. SIM INSTRUMENT DESIGN SELECTION The SIM team is currently studying two designs for the interferometer, both based upon the AIM architecture. These two designs are functionally equivalent in that they are both capable of meeting all SIM science requirements. The two designs have different benefits and drawbacks. The two designs are known by the names of SIM-Classic (seven siderostats on a boom with an external metrology truss) and SIM-SOS (two movable collector pods with no external metrology truss). The team has just completed a detailed look at the SIM-Classic design and reported the results to its technical advisory committee. A similar study and report was completed earlier for the SIM-SOS design. The SIM team is currently studying the differences between these two designs, aimed at gathering enough information to make a selection between them by November 1999 which is based upon the best mix of cost, schedule, risk and operability. 6. THERMAL OPTO-MECHANICAL (TOM) TESTBED One of the new testbeds coming on-line within the SIM technology program is the Thermal Opto-mechanical (TOM) Testbed. TOM is aimed at exploring the response of optical figure to small changes in thermal conditions. This is a critical area for SIM. Since the SIM metrology system samples only a small portion of each collecting aperture, sub-nanometer changes to optical figure across the apertures during the course of an observation would result in misleading estimates of the optical path excursions seen by starlight. SIM's design solution is to maintain very tight (< 10 mK) thermal control of time varying gradients across the collecting optics. Thermal-optical- mechanical modeling indicates that these small mirror temperature excursions will insure acceptably small distortions in optical figure. The TOM Testbed's job is to prove that this is the case. TOM will proceed in three major steps. Test #1 is nearing completion. This is a thermal-only experiment where a Pyrex mirror in a thermal vacuum tank is exposed to time-varying thermal loads and its temperature response is recorded. These data are compared to predictive thermal models. Test #2 introduces optical figure measurement so that mirror temperature changes can be experimentally correlated with changes in figure. Test #2 uses a relatively high CTE test optic so that mechanical response will be exaggerated (compared to SIM) leading to high SNR measurements and easier model comparison. Test #3 introduces a flight-traceable low-CTE telescope as the test optic and a test environment closely emulating on-orbit conditions. Recently the TOM Test #1 Final Review and Test #2 Design Review were held at the Lockheed-Martin Advanced Technology Center in Palo Alto. Test #1 monitored mK temperature changes in a Pyrex test specimen mirror subjected to thermal perturbations in a thermal vacuum chamber (Lockheed Martin "Blue Chamber"). Test objectives were to verify temperature sensor performance and thermal modeling capability in the mK regime. Both objectives were met in impressive fashion. The temperature sensors were shown capable of sub-mK resolution (although they needed to be recalibrated for DC offsets of up to 10's of mK's after being installed in the test article). The thermal modeling accurately predicted temperature gradients (both through and across the mirror) to about 10 mK. What is important to SIM is the prediction of temporal changes in thermal gradients and it was postulated that modeling results should be even better (in terms of number of mK, not percentage error) for these variables. This will be tested in the coming week in an additional experiment in the Test #1 configuration. --------------------------------------------------------------------------- Steve Unwin, Editor unwin@huey.jpl.nasa.gov You are subscribed to the list 'sim-announce'. To unsubscribe from this list, please go to the 'The Store' link on the SIM web page at: http://sim.jpl.nasa.gov