On the Space Infrared Telescope Facility and the Stratospheric Observatory for Infrared Astronomy Letter Report (1994) / Chapter Skim
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REPORT OF THE TASK GROUP ON THE SPACE INFRARED TELESCOPE FACILITY ANDTHE STRATOSPHERIC OBSERVATORY FOR INFRARED ASTRONOMY I INTRODUCTION In the 1991 National Research Council report, The Decade of Discovery in Astronomy and Astrophysics, the Astronomy and Astrophysics Survey Committee characterized the 1990s as "the Decade of the Infrared." The Bahcall report (after the Committee Chair, John Bahcall)
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for infrared astronomy and [to] previous recommendations of the Space Studies Board's Committee on Space Astronomy and Astrophysics and earlier astronomy and astrophysics survey committee reports." The charge specified further that "[t]
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The TGSS concludes that, despite reductions in scientific scope that have resulted from NASA's current cost ceiling for new science missions, SIRTF remains unparalleled in its potential for addressing the major questions of modern astrophysics highlighted in Chapter 2 of the Bahcall report. The TGSS is unanimous in its opinion that SIRTF still merits the high-priority ranking it received in the Bahcall report.
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Liquid helium requirements are much lower because of the improved thermal environment in solar orbit, the significant improvements in telescope and instrument power dissipation, and a decrease in planned facility lifetime from 5 to 2.5 years. Moreover, the telescope will be launched warm, with a potential for cost savings not only in dewar design and fabrication but also in testing and integration.
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4 – 5.3 m 100 InSb 5 – 15 m 100 Si:As 15 – 40 m 100 Si:Sb 12 – 24 m 600 Si:As 20 – 40 m 600 Si:Sb 55 - 100 20 Ge:Ga TABLE 2 Parameter Titan Version (Bahcall Current Report) Wavelength range 2 – 700 m 3 – 180 m Lifetime 5 yrs 2.5 yrs http://www7.nationalacademies.org/ssb/sirtfsofialtr.html (5 of 13)
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(2" @ 6.5 m) Planetary tracking High-speed, Stepwise Average data rate 120 kbps 40 kbps Mode Full observatory Key project Important Simplifications Parameter Titan Version (Bahcall Current Concept Report)
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TABLE 3 Changes in SIRTF Capabilities Improved SIRTF Capabilities Deleted SIRTF Capabilities Availability of Si:Sb arrays-improved quantum efficiency, larger format in the 20- to40- m range Greater reliability through simplified hardware Solar orbit instead of high Earth orbit results in: a. greater observing efficiency-shorter life b.
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However, the reduced data rate and shorter lifetime demand careful approaches to planning and executing the science program in order to maximize scientific productivity while assuring community involvement. The traditional "observatory" paradigm originally envisaged for SIRTF, in which scientific programs evolve as a wide spectrum of users learn and test the capabilities of the system, is no longer applicable.
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By focusing on these important areas in which SIRTF observations can make unique contributions, the SIRTF team has greatly simplified the instrument design and operating modes and has vastly reduced mission costs. The four programs provide a sharp scientific focus that is entirely consistent with the high priority objectives identified in the Bahcall report.
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Figure 1 is a comparison of the relative astronomical capabilities of the rescoped SIRTF and ISO and, when compared with Figure 4.2 in the Bahcall report, highlights the dramatic improvement in SIRTF's detection capability since the time of that report's release. The relative astronomical capability is a figure of merit combining point-source sensitivity, array size, facility lifetime, and efficiency in the following relation: Relative astronomical (facility lifetime)
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It appears to the TGSS that the proposed Key Projects program is an excellent way of involving the whole astronomical community in SIRTF. This program and other mechanisms for promoting and coordinating participation by a broad user community are essential for maximizing scientific returns from a shorter mission.
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An aft-mounted telescope appears to meet all of the performance specifications and scientific objectives envisioned for SOFIA at the time of the Bahcall report. The SOFIA project team has identified several additional studies that are needed prior to final selection of the model of 747 aircraft and its procurement (in particular, further wind-tunnel tests of aft-mounted cavity configurations)
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The TGSS concludes that SOFIA, with frequent flight opportunities for a broad range of state of-the-art instrumentation programs, remains a uniquely powerful facility for science and continues the airborne program's role of developing technology for future space missions, for training experimentalists, and for educational outreach, as envisaged in the Bahcall report. 1 The CAA is a joint activity of the National Research Council's Space Studies Board and the Board on Physics and Astronomy.
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