DRAFT*DRAFT*DRAFT*DRAFT*DRAFT*DRAFT*DRAFT*DRAFT*DRAFT*DRAFT*DRAFT*DRAFT* MEMORANDUM Date: 28-June-2000 To: Wolfgang Wild & John Payne, ALMA JRDG From: ALMA Scientific Advisory Committee (ASAC) Topic: Comments on ALMA Receiver Subsystem Top-level Requirements and Specifications (specifically Draft Version 1.3, 19-May-2000, http://www.cv.nrao.edu/~awootten/mmaimcal/receiverspecs.html) ------------------------------------------------------------------------ This document summarizes the ASAC's commnents on the ALMA Receiver Subsystem Top-level Requirements and Specifications document. ------ * Section 2.2 REFERENCE DOCUMENTS ASAC: Add CDR and PDR reports to be found at http://www.alma.nrao.edu/administration/index.html --- * Section 3.1 FREQUENCY COVERAGE (in particular Table 1): Issue: First sentence ("The ALMA receiver subsystem will cover all the available atmospheric frequency windows between 30 GHz and 950 GHz.") ASAC: Since the definition of "atmospheric frequency windows" is, e.g. opacity dependent, we recommend to change this to "The ALMA receiver subsystem will cover frequencies between 30 GHz and 950 GHz as given in Table 1." Issue: lower frequency range of Band 3 (now 86 GHz) ASAC: The ASAC actually wrote (http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node3.html): "We strongly urge that the JRDG study the possibility of extending the lower frequency range of Band 3 to include the SiO maser transition near 86 GHz. If this is possible, Band 2 would drop to third priority." Since the VLBA 3 mm receivers will go down to 84 GHz, perhaps some attention should be paid to the actual number of the lower limit. We recommend that 84 GHz is adopted. --- * Section 3.2 POLARIZATION ASAC: The ASAC report at http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node12.html needs to be made more specific. Larry D'Addario and Steve Myers have put work into this, and their draft recommendations are located at: http://www.aoc.nrao.edu/~smyers/alma/polspecs-imcal.txt We invite comments on this document. The polarisation purity requirement mostly influences the optics and horn/orthomode transducer. One would assume that the 1% goal in calibration requires a cross-polarisation better than 20 dB, and that all antennas should also be co-aligned in polarisation to the same level. Polarisation experts should comment on this! Note that it may be difficult to reach this level with an orthomode transducer. * Section 3.3 OPTICAL COUPLING TO THE TELESCOPE ASAC: The ASAC urges that coupling efficieny specs are defined and that a measurement scheme for various efficiencies are discussed. --- * Section 3.4 RECEIVER NOISE PERFORMANCE Issue: Receiver noise requirements ASAC: These should be stated in a more specific way. Our recommendation: The noise temperature measured at the dewar input window to the cartridge shall not exceed a value of TrxSSB for SSB response and 0.5*TrxSSB for DSB response. TrxSSB will in general be a function of the frequency nu and is given by the following formula TrxSSB= A * (h*nu/k) + 4 K where h and k are the usual physical constants. The frequency dependent quantity A has the following specification and goal values: Bands 1-6 (below 275 GHz) Spec: A = 6 / 10 Goal: A = 3 / 5 Bands 7-8 (275-500 GHz) Spec: A = 8 / 12 Goal: A = 4 / 8 Band 9 (602-720 GHz) Spec: A = 10 / 15 Goal: A = 6 / 9 Band 10 (787-950 GHz) Spec: A = 10 / 15 Goal: A = 8 / 12 For both, the specification and the goal values, two numbers are given. The first one of these refers to the value that A must not exceed over the 80% range of the nominal bandwidth that has the best performance, whereas the second value may not be exceeded at any frequency within the nominal bandwidth. Furthermore, the receiver temperatures should be measured in a reference plane outside the dewar, which involves the following technical issue. Ideally, one would like to measure at the secondary focus, including all receiver related optics, i.e. with the full final cryostat and optics. However, since receiver cartridges will be developed and tested separately, this cannot be performed on the development site. We propose that the receiver group equips the test cryostats for the cartridge testing in such a way as to provide a comparable receiver temperature reference plane. --- * Sections 3.5 SIDEBANDS, 3.6 IF BANDWIDTHS, and 3.7 SIMULTANEOUS OPERATION OF BANDS ASAC: Specs made here are rather vague. The goal is: SSB 2 x SB 2 x Pol = 4 x 8 GHz (sideband separating) The Munich PDR (http://www.cv.nrao.edu/~awootten/mmaimcal/ALMA-DesRevRec4.html) said: "In addition to the baseline IF bands of 8 GHz Upper and Lower Sideband, receiver designers are free to select any of the following alternatives: 8 GHz Single-Side-Band, Upper or Lower, 8 GHz Double-Side-Band or 4 GHz Upper and Lower Sideband. In all cases, dual polarization for a total of 16 GHz IF band width. Sideband separation in DSB-mode will be possible for integration times in multiples of 1 sec. Depending on the choice, and maintaining the currently proposed LO coverage, this might lead to some loss of frequency coverage. The impact of this should be evaluated by the Science Group." This is only a recommendation, but it might be a starting point for more stringent specs. --- * Section 3.6 IF BANDWIDTH Issue: 4 GHz IF bandwidth as a fall back position ASAC: The 8 GHz IF bandwidth is a very strong science requirement and every effort should be made to keep the IF bandwidth at 8 GHz at the lower frequencies, where continuum sensitivity and broad bandwidth for high-z CO line searches are most crucial. At higher frequencies, this could perhaps be somewhat relaxed, but only as a last resort. Officially declaring 4 GHz IF bandwidth as a fall back position even for only the initial bands is a significant deviation from the original specs. Any specific proposal for a design with less than 8 GHz needs more discussion. What would be the cost of upgrading receivers with 4 GHz IF bandwidth to 8 GHz bandwidth? As a basis for discussions of the tradeoffs between bandwidth and receiver temperature, Neal Evans has made the following proposal, which will be discussed by the ASAC. We include it here to indicate in which direction these discussions may lead. "I don't like the idea of relaxing the bandwidth spec so easily, but I recognize that there are tradeoffs. It would be better to formulate a figure of merit that balances T_rx with BW. If one sacrifices too much on T_rx to meet a spec on BW, one may quickly lose more than one gains. If we adopt Al's formulation, with T_rx = A hnu/k + 4 K and IF bandwidth symbolized with BW, the figure of merit for line observations is (I leave out the constant 4 K for clarity, but it could be worked in) F_line = T_rx/A For continuum, it is F_cont = (T_rx/sqrt(BW))/(A/sqrt(8GHz)) Note that this formulation relates to what you can achieve in a fixed integration time, which is a more realistic approach in my view than asking what integration time is required for a given sensitivity level, which would lead to squaring these figures. The overall figure of merit depends on your evaluation of the overall importance of line and continuum observations. Call the overall figure of merit F_total. Then F_total = X F_line + Y F_cont ; X + Y = 1 The ASAC could vote on the values of X and Y. My vote would be X= 0.6, Y = 0.4. The reason for this choice is not that I think line observations are more important than continuum, but because they are harder-- the continuum sensitivity of ALMA will be very good, but for some line problems, the sensitivity will be more marginal. One could make X and Y a function of band, but I think there won't be large differences. The idea is that one would relax the BW spec, replacing it with the spec that F_total be less than or equal to unity." --- * Section 3.7 SIMULTANEOUS OPERATION OF BANDS Issue: It is stated tha "The water-vapor monitoring radiometer shall operate simultaneously with bands 2 to 10, but not with band 1 which can operate without the water-vapor monitoring radiometer. ASAC: The ASAC actually said: "The ASAC does note that the simultaneous operation at 183 GHz and Band 1 receivers is not a scientific requirement, so it is straightforward to locate these systems in the same Dewar if that makes sense." In the best of all possible worlds, the WVR would work with band 1 as well as the other bands--this will be necessary under a wide range of conditions at Chajnantor for which ALMA may be operable at band 1 with WVR but ALMA would be shut down otherwise. However, if the cost of operating both simultaneously is very high, one possible sacrifice might be simultaneous operation of WVR and band 1. This is very different from designing a receiver system which will not accommodate simultaneous operation of both from the outset. --- * Section 3.8 RECEIVER STABILITY Issue: It is stated that "A preliminary suggestion for a gain fluctuation limit is: 1e-4 rms over a 1 sec interval." ASAC: This was indeed suggested in the ASAC report. A memo from Wright (http://www.alma.nrao.edu/memos/html-memos/abstracts/abs289.html) suggests 1e-4 over 0.1s, which begins to define the spectrum of stability. --- * 4.3 SELECTION OF A NEW OBSERVING BAND Issue: The statement "Selecting and tuning a new band shall require no more than 15 min." caused quite some confusion. ASAC: This issue was clarified by Wolfgang Wild, who states that "What we meant with the 15 min is not the tuning time, it means that you need to switch on a particular band 15 min before using it. This is to reach thermal equilibrium. In practice, this time could probably be shorter, but nobody has measured it so far. Switching from on band to another would be done in 1.5 sec (if the band is switched on). Since we cannot have all bands switched on at all times due to cryogenic limitations we introduced this spec to minimize the impact on scheduling." We recommend that this explanation should be incorporated into the document.