Dear ASAC members, Here is as attachment a request for Science Examples to help us refining the precision requirements for the Bandpass and Polarisation calibrations. Time is getting short on these issues, with several milestones happening soon, so we would appreciate your help in getting feedback as soon as possible. Important milestones are: Next week: Receiver specification review. Any comments on receiver stability which you may be able to provide by next Wednesday (Nov-20) would be helpful. February 2003: Review of calibration requirements with science examples complete This review covers the calibration specifications for ALMA to achieve the science goals set for it by the community. Specific science goals will be used to illustrate the specifications. To be able to meet this Milestone, the collection of Science Examples should be complete by mid-December at the very latest. Given other commitments, I suggest a firm deadline of Dec 12 for sending your examples. An alternate possible deadline is Nov 28, which would result in an earlier delivery of the Summary. If most of you could meet this deadline, it would leave more time for any iterations. I will collect the answers and prepare a summary. The summary will then be discussed at our usual teleconf. Stephane The ALMA project is currently encountering some difficulties in developping and realizing devices to be used for the calibration of the various instrumental parameters. Among the possible devices which have not been sufficiently developped is an accurate coherent source (made of a phase-locked photonic emitter located in the subreflector) which, if sufficiently accurate (in amplitude, phase and polarisation as function time and frequency) could have been used to calibrate the instrumental parameters. Current projections from the engineers quote amplitude variations as function of frequency of several 10 %, unclear polarisation properties, and significant difficulties to achieve high amplitude stability versus time. In our present understanding, it is unlikely that such a device can be used successfully to achive high accuracy calibration of either the bandpass or the polarisation. To help investigating the problem, it is important to re-examine what are the REAL specifications for the calibration of ALMA, and to put also in perspective what would be goals to be achieved if possible. Specifications are the performance ALMA should deliver in all (average) circumstances. In terms of instrumentation, instrumentation out of specification should be refused. Thus, we cannot impose unachievable specifications. On the other hand, "Goals" indicate what level of performance might be reached on special circumstances, involving either dedicated observing modes, or fortunate coincidences (such as exceptionally good weather, or strong astronomical calibration source available). The "old" ALMA project Book mentionned the following for Calibration Table 3.1 ALMA Calibration Requirements. Pointing 0.6" absolute Primary Beam 2-3% Baseline Determination 0.1 mm Flux Calibration 1% absolute flux accuracy goal Phase Calibration 0.15 radian at 230 GHz Bandpass Calibration 10000:1 to 100000:1 Polarization Calibration 10000:1 Single Antenna Calibration Employed This is a mixture of Scientific specifications (e.g. 0.15 radian at 230 GHz), goals (e.g. 1%), Engineering specifications (e.g. 0.6") and "out of the blue" numbers (e.g. Bandpass Calibration 100 000:1) We now have to set Engineering specifications on some key items in ALMA. For example, the receiver stability has to be specified. It can be derived from several of the above numbers. Single-Dish observations require short term (1 second) stability of order 10^-4, but this number is Frequency dependent. 1 % amplitude calibration accuracy requires the receivers to be significantly more stable than 1 % (say 0.1-0.2 %) but on timescales much longer (say 10 minutes). The most difficult issues to be translated in Engineering specifications are the Bandpass and the Polarisation Calibration. To help us in doing so, we would like to re-assess the actual astronomer needs. To do so, we ask you to send us a few Science Examples (2 or 3 would be sufficient) which would represent - a "normal" experiment, which would be used to derive ALMA Specifications - a "demanding" experiment, which would be used to derive ALMA Goals focussed on Bandpass and Polarisation issues. The Science Example should be short (1 to 2 lines of description will in general be sufficient). It should contain the expected observing frequency, the typical linewidth, and the expected signal intensity (in Flux density, Brightness or Line-to-continuum ratio). For Polarisation, the expected percentage of polarisation and requested precision on the position angle of linear polarisation should be mentionned. Also, it may turn out to be difficult to equip ALMA with adequate devices (most likely quarter wave plates) for polarisation measurements at all frequencies. Your help is requested to define which frequency(ies) is(are) the most important. ----------------------------------------------------------------------------------------- Here is an example (for Bandpass). - "normal" experiment: Detection of narrow absorption lines in front of a quasar. Observing Frequency: any from 90 GHz to 700 GHz Linewidth: 1 km/s Line-to-continuum ratio: 0.01 Implications on Bandpass accuracy 1000:1 on a 1 to 3 MHz window - "demanding" experiment: Observation of a narrow absorption lines in front of a quasar to measure accurate isotopic rations. Observing Frequency: 300 GHz Linewidth: 1 km/s Line-to-continuum ratio: 0.01 Implications on Bandpass accuracy 10000:1 on a 3 MHz window (to get a few % accuracy on the line intensity) - "demanding" experiment: Detection of a broad, weak emission line from CO around a bright quasar Linewidth: 600 km/s Line-to-continuum ratio: 0.01 Implications on Bandpass accuracy 1000:1 on a 600 MHz scale