To derive the specifications for the ALMA instrument, especially in the = calibration area, we shall start with science examples which illustrate = the use of ALMA for various categories of projects. Some of these = examples will be typical of some research, others will be very specific = and perhaps extremely demanding on the performances.=20 Case 1: Measurement of redshifts in high-z deep field sources. This requires searching for broad (500-1000 km/s) and weak emission = line on top=20 of a weak continuum, e.g. CO in a high-z deep field source. Such = searches are best done in the mm domain.=20 Case 2: Searching for weak and broad (a few 100 km/s) emission or = absorption on top of a strong continuum, such as searching for CO around radio-loud quasars = or C+ or=20 other fine structure lines in moderate to high z sources.=20 Case 3: Measuring isotopic ratios across the history of the universe = from absorption lines in front of quasars. These bright background = sources provide a unique opportunity to detect very rare isotopes = through extremely optically thin lines. Opacities of the relevant = isotopic lines may differ by a factor of 20 or more. This would be performed mostly in the mm domain, = where the quasars are strongest, but also because this technique is = appropriate for the lowest excitation lines only. Case 4: Measuring accurate isotopic ratios from emission lines in our = galaxy or nearby ones. For example, this could include measuring rare isotopes of N, Al, Mg in = a sample of evolved stars throughout our Galaxy. Case 5: In order to investigate how molecular clouds form in galaxies, one will propose to map extragalactic, early-phase molecular clouds in a nearby galaxy, e.g., M51, in the 492 GHz CI 3P1-3P0 line. = This is extended emission, which will require single-dish sampling, with contamination by emission = from the error pattern of the single-dish beam. Similar examples will = also apply to the CO lines. Case 6: Measuring the polarization morphology in an isolated protostar = or a circumstellar disk. The polarization percentage may be range between 0.5 and 5 %, and a S/N = of 5 to 10 is required to determine the position angle to 11-6 degrees. = This would be done near 260 to 300 GHz, to optimize the sensitivity. Case 7: Measuring the polarization morphology in a protostellar cluster, = or confused environment. The polarization percentage may become small at the egdes, down to 0.1 = %. Case 8: Measuring the polarization percentage (and morphology) as = function of frequency in an isolated protostar, to constrain the dust = properties. 3 frequency ranging from 100 to 900 GHz may be required in = such an experiment. Case 9: Use the linearly polarized emission from CO lines to probe = magnetic field geometries in protostellar outflows (other lines, e.g. HCN, are also possible). At high resolution (10s of AU in nearby regions), these data could test mechanisms for generation of outflows (i.e. X-winds vs. disks, if the theorists will make testable predictions); in more distant sources, the magnetic field geoemetries of the outflows can be = determined=20 using data coupled with the Goldreich-Kylafis effect to interpret the=20 polarization angles. Different configurations are predicted for dipolar=20 and quadrupolar outflows. The polarization percentage is expected to = decrease with J transitions, with CO 3-2 around 1-2 % in single-dish observations. Case 10: Consider a protostellar envelope around a young stellar object with a power-law density profile n ~ r^p. How accurate can the slope p be determined with a few % absolute calibration accuracy = between different bands? Modeling is required in analysing this problem. Simulations of the CS 7-6/5-4 ratio (Band 7/Band 6) for an ALMA = resolution of 1" showed that=20 the power slope could be determined to 0.05 with an uncertainty in the = ratio of order 5 %. Such a precision could allow to distinguish between a pure power-law and = an inside-out collapse type model with such high accuracy. Case 11: Temperature, Water Vapor, and Winds from the atmosphere of = Mars. Observations in the 230 GHz band that cover the 225.9 GHz HDO line=20 and the 230.5 GHz CO(2-1) line, can be used to simultaneously measure the temperature structure (from CO) and water vapor distribution (from HDO). The CO line is strong in absorption, so relatively easy to see. = The HDO line, on the other hand, is an extremely weak feature in emission on the limbs, and even weaker in absorption on the disk (maybe 1% in emission, 0.2% in absorption) over 200 MHz or so. This must be=20 measured against the strong continuum of Mars (200 K). In addition, the surface continuum of Mars will show radial polarization = (due to the brewster angle effect) which could be used to measure the surface dielectric properties. The CO line emission, of course is unpolarized and can be used as a calibration check. Case 12: In addition to the specific examples ALMA should be an accurate = imager, providing both high dynamic range and high fidelity when the = sources are strong enough.=20 This imposes accurate phase and relative amplitude calibrations.=20 Case 13: Same as case 12, for fields larger than the primary beam. Case & Continuum & Line & Bandwidth & Resolution & Frequency & = Polarization & Size & Resolution =20 1 & < 1 mJy & 2 mJy & 500 km/s & 50 km/s & 80 - 250 GHz & None = & 1" & none 2 & 10 mJy & 0.2 mJy & 200 km/s & 25 km/s & 350-700 GHz & None = & 1" & none 3 & 100 mJy & 0.2 mJy & 3 km/s & 1 km/s & 80 - 250 GHz & = None & 1" & none 4 & small & 1 mJy & 20 km/s & none & 80 - 250 GHz & None = & 1" & none 5 & small & a few K & 100 km/s & 3 km/s & 230 - 490 GHz & None = & 300" & 1" 6 & 300 mJy & - & 8 GHz & - & 260 - 300 GHz & 0.5 % & 2-10" = & 0.2-1"=20 7 & 300 mJy & - & 8 GHz & - & 260 - 300 GHz & 0.5 % & 30" = & 1"=20 8 & 100-1000 mJy & - & 8 GHz & - & 100 - 900 GHz & 0.5-5 % & 2-10" & = 0.2-1" 9 & - & a few K & 10 km/s & 2 km/s & 230 - 345 GHz & 1-5 % & 10" & = 0.1"=20 10 & & a few K & 1 km/s & 0.1 km/s & 245 - 345 GHz & None = & 10" & 1" 11 & 200 K & 0.4 - 20 K & 200 km/s & 0.03 km/s & 235 GHz & 1-5 % & = 10" & 0.2 -1" 12 High Fidelity imaging (small field of view) 13 High Fidelity imaging (large field of view) The above examples were contributed by the ALMA Scientific Advisory = Committee. They do not constitute an exhaustive description of the type = of projects which ALMA may be able to perform. To derive and justify the ALMA requirements on calibration, including = specifications of the necessary devices and requirements on the = observing strategies to perform the calibration, a detailed analysis is required. For each example an observing strategy = will be proposed, and the performances of the ALMA calibration devices = required to achieve the science goal derived.=20 The relative amplitude and phase calibration accuracy are essentially = derived from the high fidelity requirements, as is the pointing accuracy = of the antennas. The other examples drive the bandpass calibration = accuracy, the polarization specification and the absolute amplitude=20 accuracy. ------=_NextPart_000_0005_01C2CEBF.E9E883C0--