Design Considerations for Large Detector Arrays on Submillimeter-wave Telescopes

The emerging technology of large (~ 10,000 pixel) submillimeter-wave bolometer arrays presents a novel optical design problem---how can such arrays be fed by diffraction-limited telescope optics where the primary mirror is less than 100,000 wavelengths in diameter? Standard Cassegrain designs for radiotelescope optics exhibit focal surface curvature so large that detectors cannot be placed more than 25 beam diameters from the central ray. The problem is worse for Ritchey-Cretien designs, because these minimize coma while increasing field curvature. Classical aberrations, including coma, are usually dominated by diffraction in submillimeter-wave single dish telescopes. The telescope designer must consider (1) diffraction, (2) aberration, (3) curvature of field, (4) cross-polarization, (5) internal reflections, (6) the effect of blockages, (7) means of beam chopping on- and off-source, (8) gravitational and thermal deformations of the primary mirror, (9) the physical mounting of large detector packages, and (10) the effect of gravity and (11) vibration on those detectors. Simultaneous optimization of these considerations in the case of large detector arrays leads to telescopes that differ considerably from standard radiotelescope designs. Offset optics provide flexibility for mounting detectors, while eliminating blockage and internal reflections. Aberrations and cross-polarization can be the same as on-axis designs having the same diameter and focal length. Trade-offs include the complication of primary mirror homology and an increase in overall cost. A dramatic increase in usable field of view can be achieved using shaped optics. Solutions having one to six mirrors will be discussed, including a possible six-mirror design for the proposed South Pole 10m telescope.Comment: in proceedings "Radio Telescopes" SPIE Astronomical Telescopes and Instrumentation, 30 March 2000, Munich. SPIE code 4015-46. 12 pages 4 figures Revised to fix typos, figur

Millimeter and Submillimeter Observations from the South Pole

During the past decade, a year-round observatory has been established at the geographic South Pole by the Center for Astrophysical Research in Antarctica (CARA). CARA has fielded several millimeter- and submillimeter-wave instruments: AST/RO (the Antarctic Submillimeter Telescope and Remote Observatory, a 1.7-m telescope outfitted with a variety of receivers at frequencies from 230 GHz to 810 GHz, including PoleSTAR, a heterodyne spectrometer array), Python (a degree-scale CMB telescope), Viper (a 2-m telescope which has been outfitted with SPARO, a submillimeter-wave bolometric array polarimeter, ACBAR, a multi-channel CMB instrument, and Dos Equis, a HEMT polarimeter), and DASI (the Degree-Angular Scale Interferometer). These instruments have obtained significant results in studies of the interstellar medium and observational cosmology, including detections of the 1 degree acoustic peak in the CMB and the Sunyaev-Zel'dovich effect. The South Pole environment is unique among observatory sites for unusually low wind speeds, low absolute humidity, and the consistent clarity of the submillimeter sky. The atmosphere is dessicated by cold: at the South Pole's average annual temperature of -49 C, the partial pressure of saturated water vapor is only 1.2% of what it is at 0 C. The low water vapor levels result in exceptionally low values of sky noise. This is crucial for large-scale observations of faint cosmological sources---for such observations the South Pole is unsurpassed.Comment: 9 pages, contribution to 2K1BC symposium "Experimental Cosmology at Millimeter Wavelengths", ed. M. De Petris and M. Gervas

Low-G fluid transfer technology study, executive summary

For abstract, see N76-24502

Space LOX vent system

The research of the program to design and build a prototype vent system capable of exhausting only vapor to space from an all liquid or two-phase mixture of oxygen, while operating under low or zero-gravity conditions is reported. Work performed during the detail design phase of the program was concerned with the finalization of vent system performance, development of component specifications, solicitation of vendor bids, selection of components and overall system package design. The compact system preliminary design defined for the comparisons was reviewed in light of a desirability to demonstrate complete tank mixing at one-g. Also, performance of the system at low-g conditions with a full tank and maximum temperature stratification or maximum pressure rise between vent cycles was investigated. It was found that under these extreme conditions, not previously considered, that use of a larger pump mixer than previously defined would be desirable. In addition, to simplify ground testing with only a small weight penalty, the exchanger vent pressure was increased from 5 psia to 22 psia nominal. This resulted in less than a 0.2% increase in system weight

Dust cloud evolution in sub-stellar atmospheres via plasma deposition and plasma sputtering

Context. In contemporary sub-stellar model atmospheres, dust growth occurs through neutral gas-phase surface chemistry. Recently, there has been a growing body of theoretical and observational evidence suggesting that ionisation processes can also occur. As a result, atmospheres are populated by regions composed of plasma, gas and dust, and the consequent influence of plasma processes on dust evolution is enhanced.Aim. This paper aims to introduce a new model of dust growth and destruction in sub-stellar atmospheres via plasma deposition and plasma sputtering.Methods. Using example sub-stellar atmospheres from DRIFT-PHOENIX, we have compared plasma deposition and sputtering timescales to those from neutral gas-phase surface chemistry to ascertain their regimes of influence. We calculated the plasma sputtering yield and discuss the circumstances where plasma sputtering dominates over deposition.Results. Within the highest dust density cloud regions, plasma deposition and sputtering dominates over neutral gas-phase surface chemistry if the degree of ionisation is ≳10−4. Loosely bound grains with surface binding energies of the order of 0.1–1 eV are susceptible to destruction through plasma sputtering for feasible degrees of ionisation and electron temperatures; whereas, strong crystalline grains with binding energies of the order 10 eV are resistant to sputtering.Conclusions. The mathematical framework outlined sets the foundation for the inclusion of plasma deposition and plasma sputtering in global dust cloud formation models of sub-stellar atmospheres

Endless tape cartridge Patent

Tape cartridge with high capacity storage of endless-loop magnetic tap