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Answers to Frequently Asked Questions about the SPST
FAQ Contents
Don't array instruments like the ALMA and the SMA make single-dish submillimeter telescopes obsolete?Interferometric array instruments like the ALMA and single-dish telescopes like the SPST have very different, but complementary, capabilities. The SPST will have a usable field of view which is 30' in diameter, whereas the field of view of the ALMA is about 12" in diameter, or 2 × 104 smaller in area. For mapping areas of sky in the search for protostars or protogalaxies, the SPST with a large bolometer array detector is about 100 times faster than the ALMA. On the other hand, the ALMA has much higher resolution and is by far the superior instrument for studying small sources with known positions. The situation is analogous to the 48-inch Schmidt camera and the 200-inch telescope on Mt. Palomar, where the wide-field instrument gives the big picture and discovers objects for study by the large, high-resolution telescope. The SPST science goals also include the study of primary and secondary CMB radiation at arcminute scales using broadband prime-focus instrument packages. The NASA Cosmic Microwave Background Future Missions Working Group has found that this important range of spatial scales in the CMB (which probably includes the peak in the power distribution of secondary CMB anisotropies) falls in the gap between the capabilities of the Planck satellite and the ALMA and SMA. Back to Top
Granted a 10 meter diameter instrument has larger sky coverage than the ALMA, but for the proposed science isn't one better off with an interferometer like the CBI, where the whole array is contained within a 10m mount?The CBI is a telescope that has a 13-element interferometer array mounted on a single 10 m diameter mount. At present the CBI operates at wavelengths near 1 cm, but we imagine that the CBI might be upgraded to submillimeter-wave operation or that an entirely new submillimeter-wave telescope with the configuration of the CBI could be built. The strength of a CBI-like instrument is rejection of common-mode sky noise in the synthesized map. The hypothetical "sub-mm CBI" would have the same resolution as the SPST, and would synthesize a field of view about 2' in diameter. This field of view is 50 to 100 times smaller in area than that of the SPST. As a survey instrument, the SPST is four orders-of-magnitude faster than this hypothetical interferometer, and is sensitive to low spatial frequencies that the interferometer filters out. The SPST rejects sky noise through choice of site. Deep observations from Mauna Kea by Smail and collaborators and by Holtzapfel and collaborators, combined with site testing at Mauna Kea and at the Pole indicate that the SPST science goals are feasible at the Pole. The high speed and optimization of the SPST make it the instrument of choice. Back to Top
For the protogalaxy search problem, isn't one better off working with a larger telescope at lower frequencies?The most economical frequency at which to search for protogalaxies depends on a competition between two effects:
At present, the best results are obtained at frequencies near 350 GHz by Smail and collaborators. The sky at the South Pole is so good that comparable techniques can be used at frequencies near 820 GHz (the 350µm atmospheric window). This increase in frequency brings several advantages:
This last point will prove to be important once this observational program is working well and detecting thousands of potential protogalactic objects. The submillimeter-wave colors of all low redshift (z < 2) objects are essentially the same. At redshifts near z ~ 3, the far-infrared dust peak at a rest-frame wavelength of ~ 100µm is shifted into the 350µm atmospheric window, and the 350µm/450µm flux ratio decreases. High redshift objects can therefore be distinguished by broadband photometery at 350µm and 450µm. Back to Top
Isn't it politically irresponsible to push for a new submillimeter-wave telescope when we all need to get behind the ALMA?All submillimeter-wave astronomers, including the members of the SPST consortium, agree that the ALMA is the top priority for ground-based submillimeter astronomy in the next decade. The SPST proposal, however, addresses a different and equally timely issue: How can the National Science Foundation Office of Polar Programs most productively allocate Antarctic Science support funding? This funding is mandated by Congress for expenditure on Antarctic science, and therefore does not compete with funding for the ALMA. The question is timely, because the Office of Polar Programs is currently rebuilding South Pole Station. Plans for new instruments to be placed at the new station must be considered now. This presents the astronomical community with the exciting possibility of building a user-facility instrument whose wide-field capabilities will greatly enhance the productivity of the ALMA, and whose capabilities as a CMB instrument fill the resolution gap between the ALMA and Planck. Back to Top
Wouldn't it be better to co-locate this instrument with the ALMA?The short answer is that the South Pole is by far the better site, especially with regard to the most critical factor for this type of instrument, sky noise. The ALMA cannot be located at the South Pole because that project is too large for the Antarctic Research Support infrastructure. The South Pole is fully developed as an observatory site for an instrument the size of the SPST: all the infrastructure needed to support the SP10m is under construction or already in place. The SPST, together with the JCMT and CSO, provide single-dish coverage of all the sky accessible to the ALMA. Back to Top
Why a new off-axis design, when highly optimized on-axis designs already exist?The SPST science goals require:
These requirements are incompatible with existing submillimeter telescope designs. The individual antennas making up interferometric arrays like the ALMA and SMA are optimized to provide a high aperture efficiency for only a few beams in the center of the field of view---a completely different requirement. The SPST design is an offset Gregorian similar to the Green Bank Telescope (GBT), because the design requirements are similar. (The SPST is 10× smaller than the GBT and operates at 30× shorter wavelengths). Issues relating to optical optimization of the SPST are discussed in Stark et al. 1998. Back to Top
South Pole Station is closed eight months a year---wouldn't it be more productive to place the telescope at a more accessible site, even if the sky were worse?There is no transport in or out of South Pole Station between mid-February and mid-October. This enforces a strict schedule discipline on projects located at the Pole, and requires careful planning and operational management. The researchers at the Center for Astrophysical Research in Antarctica (CARA) have learned how to operate the AST/RO, VIPER, and DASI telescopes year-round. The operating plans for the SPST call for three winter-over scientists forming part of the 50-person complement of the new South Pole Station. The telescope will be designed for reliability and modular substitution of spare parts. With careful management of personnel, maintenance, and spare parts, the SPST should maintain a high level of operational reliability. The sky at the South Pole is so good that on average, observations in the 350µm and 450µm bands proceed 4.7 times faster than they would at Chajnantor in the Atacama desert, so one year of successful operation at Pole is equivalent to four or five at Chajnantor. Back to Top
Isn't it difficult to recruit scientists to spend a year a the South Pole?CARA recruits excellent researchers for winter-over positions. Typically these are three year Post Doctoral research positions. The first year is spent in training for the winter-over, including a summertime trip to the Pole. The second year is the winter-over year. The third year is spent back in the "real world", writing papers and job applications. The winter-over scientists have full participation in a wide variety of astronomical observations proposed by others, and often propose and carry out their own projects as well. The winter-over scientist job requires responsibility and individual initiative. A substantial monetary bonus is paid during the winter-over year. South Pole Station is an excellent, if isolated, place to work: the winter-over station staff form a close and supportive team, and substantial efforts are made to ensure the comfort, safety, and well-being of all the winter-over staff. Some CARA winter-over scientists have wintered more than once. The winter-over experience is usually described as challenging but very positive. Most CARA winter-over scientists go on to professional positions that make extensive use of their hard-won instrumental expertise. Back to Top
What are the plans to make the SPST available as a user-facility instrument?The SPST proposal states that the fraction of the telescope time supported by the Office of Polar Programs (probably a majority of the time) will be available to all on a proposal basis. Allocation of the fraction of the telescope time supported by the International Partners has not yet been decided. The great strength of this telescope is its wide-field, deep survey capability. As a matter of policy, the Office of Polar Programs (OPP) does not support development of astronomical instruments and the current proposal to the OPP does not include new instrumentation. The proposal therefore suggests that a large fraction, perhaps 70% of the total observing time, be devoted to large "key projects" proposals, and that these "key projects" would involve the construction and installation of major instrument packages. The proposers of "key projects" would work at the telescope as circumstances required. "Key projects" would be proposed and reviewed every three years. The remainder of the telescope time would be allocated to "observing projects" using existing instrument packages. These would not involve a trip to the telescope and would be carried out by remote or programmed observing. One Antarctic Astronomy tradition that differs from other observatories is that winterover scientists have authorship participation in observations they carry out for others. The AST/RO and SPIREX telescopes have operated as a user facility instruments available to all on a proposal basis since 1997. Back to Top
Isn't it extremely windy at the South Pole?It is extremely cold (-80 C) but not windy. This is an advantage for a radio observatory site, since radiotelescopes must be designed to withstand the maximum likely winds. The top wind speed recorded at the South Pole during a thirty year period is only 57 miles/hour---this is the lowest "highest recorded wind speed" of any U.S. Weather Service station. But complete calm at the South Pole is rare. Usually there is a breeze of 6 to 8 m s-1 blowing from the geographic center of the Antarctic continent (near Dome A). This "catabaric wind" accelerates downhill, and is often very swift indeed when it gets to the coast of the Antarctic continent. Weather at the South Pole is bi-modal, depending on whether or not there is a "kink" in the Antarctic Vortex. This effect can be seen in the opacity data. When there is a kink, the wind directions shifts and freshens, bringing marine air from the Antarctic Peninsula to the Pole. Relatively high winds are therefore correlated with poor observing weather. Back to Top
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