1) What do we know? Review of current level of attack on radiometry

Series of 10-15 minute discussions--

Staguhn, Woody, Welch -- MDC BIMA/OVRO efforts -- 22 GHz

Butler -- VLA efforts -- 22 GHz

Min, Wiedner -- 183 GHz on Mauna Kea

Guilloteau -- 22 GHz at PdBI

Richer -- 183 GHz at Chajnantor--European plans

2) General discussion -- identify missing elements of an overall implementation strategy for ALMA.

Theory. How well do we understand the atmospheric frequency structure and the spatial structure. At OVRO, experiments have been done with measuring the height of the fluctuating layer through pointing two telescopes at a point at 2km elevation and correlating the fluctuations observed. At Chajnantor, there are some radiosonde launches. With a multichannel spectrometer, there is a hope of modeling the location of this layer. Advantages and disadvantages of WVR approached through the 22 GHz and 183 GHz lines

22 GHz		183 GHz
Advantages	Disadvantages	Advantages	Disadvantages
Always optically thin	Weak line	Optically thin ??% of the time	Can become optically thick (??% of the time)
Small width (8 GHz )	Unmatched optics	Matches optics	Broad width (to 30 GHz )
			Measurement far in line wings inherently uncertain
			Variable brightness temperature - to - microns of water ratio
			Accurate broadband receiver measurement required

Instrumentation

What, if anything, should be done with the ALMA prototype antenna/interferometer?

Can 183 GHz work on the VLA site? How much of the time? Should we build an evalutation or prototype receiver? Combined 22 GHz/183GHz system? Comparison with VLA at 22 GHz. How does the prototype location compare with the location of VLA antennas?

What sort of backend? (VLA is 3 channels)

Derivation of the phase correction, and application to the data

Algorithms, data tracking philosophy

Implementation

2) our best estimate of the site elevations, etc. Simon can you bring the best digital survey information we have on the site? I understand that the newest bestest models will not be available.

3) we need to simulate observations of an MMA-like array at some point.

4) Following up on Min's suggestion, Jean Turner will canvass the MAC for sources appropriate to our image library, which currently only contains Tam's BIMASONG image of M51, I believe.

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System Engineering Questions on Science -- Emerson,Wootten

Darrel posed several interesting questions for us:

(1) What tuning step is acceptable for the first LO of the array? Some engineers would like this to be as coarse as 1600 MHz. Making the step as fine as 400 MHz would be no problem, but insisting on something as fine as a few MHz would might change the some of the proposed frequency synthesis schemes. The overall tuning step size is of course a combination of first LO and 2nd or later LO tuning.

The tunable first LO will be used to set the location of the IF within the overall system bandwidth. The correlator's 2 GHz channels may not be exactly aligned with the 1.6 GHz comb provided by the tunability of the first IF. Finer tuning will be available in the 2nd LO. Perhaps this will have a 500 MHz tunability. The FIR filter will also provide some tunability. Its 2 GHz band can be degraded in powers of two, so it will be possible to have a 1 GHz band from 0-1 GHz or from 1-2 GHz, or 4 500 MHz bands located at 0 - 500 MHz, 500-1000 MHz etc. etc. This provides a fair amount of flexibility but is it enough?

(2) [Not unrelated to the above question.] What is the finest tuning step acceptable for the 2nd or later LO? Double sideband systems might be an issue here - the main difference between tuning the first and later LO is that when you tune the first, signals from opposite sidebands move in different directions. The answer to this question is linked to the operation of the FIR filter, which has some constraints on its tunability.

Remember that it is possible that the MMA will be a double sideband system, as is BIMA, if the SIS plans don't proceed in the most optimistically contemplated manner.

A related issue: will diurnal variation in Doppler tracking be followed by one or other LO, or can we rely on software to interpolate the spectra? Again, remember there's a difference for DSB and SSB schemes in which way the signal(s) move.

I had thought that we would probably have to rely on software Doppler tracking since the frequency range covered by the broad band cannot all be tracked correctly at the same rate.

(3) From the astronomical point of view, what level of amplitude stability is needed for the entire system? I presume this limit will be set by the need for single dish continuum observations (true?) There is no point in doing better than the atmospheric limitation, and the backend stability only needs to be a little better than the frontend stability. From the astronomical point of view, what are the requirements, in terms of allowable gain change as a function of time?

(Related to this: this is really a pure engineering question, but does anyone have any good numbers on HEMT and SIS mixer gain instabilities?)

These questions fall somewhat under the realm of the Holdaway memo which Bryan has been going over. Mark did set specs on the correlated noise performance of the receivers, based on that not dominating other sources of noise, from the sky or thermal, in his proto-memo. I reminded Darrel of these numbers in the project book.

Also, Darrel relayed John Webber's response when he asked how we were planning on measuring total power in the presence of an FIR filter. John replied: We have two kinds of needs for total power, as you point out. We have discussed this at some length (Dick Thompson leading) and decided that for the signal going into the sampler (and at the earlier stage of input to the laser if we use analog transmission) we need a set-and-forget amplitude control which remains constant during an observation.

For proper astronomical calibration, we must measure total power within the correlated band.

To get the first requirement, we will do two things: first, we will need a power detector at the sampler input; second, Ray will provide a state counter by means of which we can check the sampler input threshold calibration. The 16 states will all be available, but sequential rather than simultaneous (this check is for maintenance day).

For the second requirement, a combination of state counters on the 2-bit output of the FIR filter (which might be on the memory card rather than the FIR card) and the value of the 0 lag of the autocorrelation will give the total power reading. Somebody should check us on whether this is sufficient information.

(4) Will we ever want to observe simultaneously in total power mode and interferometric mode? e.g. if we're doing an OTF interferometric observation, do we really want to record total power OTF data at the same time. (The optimum scan rates will be quite different.) The relevance of this is that some schemes for stabilizing the IF gain may introduce some variable phase errors. Since high gain stability is only really required for single dish operation, do we care of the interferometric phase is less stable during single dish operation? Conversely, do we require single-dish levels of gain stability during interferometric observations?

I could not think of why one would observe in this mode, except perhaps for very time variable events, such as a solar flare perhaps, but I wouldn't think that the single dish flux in such a case would differ by much from the interferometric flux. Perhaps a cometary observation, Bryan? I suspect we could just say we do not require this mode and not upset many scientists.

(5) From the astronomical point of view, what level of rejection of unwanted signals is acceptable. For example, in some of the filtering downstream in the IF strip, there will be filters used to narrow down the passband before sampling, and to reject in-band opposite sideband signals in downconversion mixers. 40 dB of out-of-filtered-band rejection is probably adequate.(?) Would 20 dB also be adequate? What engineering spec should we put on rejection of spurious signals at different mixers in the IF chain?

Related question: other engineering factors, such as intermodulation products, can produce spurious signals. What level of rejection is acceptable? -40 dB? Is this different for single-dish and interferometric operation? Is this just too hard a question to answer now?

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System Engineering Questions on Science -- D'Addario, Wootten

Larry D'Addario wrote: A great deal of the detailed design of the MMA electronics is driven by the choice of the first IF passband, now specified as 4 to 12 GHz. In spite of this, we seem to have very little in our documentation that justifies or explains this choice. In order to ensure that we achieve the optimum cost-performance tradeoff, we would like to re-visit the question of what the passband edge frequencies should be, and we ask that this be done system-wide, so that the full implications will be understood.

Let us assume that several other parameters remain unchanged from present values: the first IF bandwidth remains 8.0 GHz; the number of IF channels per antenna remains 2 or 4, depending on receiver; and the number and bandwidth of the final channels to be accepted by the correlator remain 8 (maximum) and 2.0 GHz, respectively.

Under these conditions, we ask each engineer whose design might be affected to describe the implications for his portion of the system of each of the following options for the first IF band:

Whereas some urgent design work depends on this choice, responses are requested by 1999-Jun-01. Division heads are asked to ensure that all appropriate people in their divisions are consulted.

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Action Items 24May99

UPCOMING REVIEW: On 7 June 1999 we review results from the OVRO and BIMA phase correction systems.

DECISION: Configurations--where are we? 9 June assessment MAC meeting: 23 June DECISION: 183 GHz or 22 GHz phase correction?

DECISION: Is a nutating secondary necessary?

DECISION: What is the effect of 1/f noise in the HEMT amplifiers of SIS receivers upon our ability to combine total power and interferometric images into a faithful representation of the sky?

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Travel

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T. Helfer: 14 May - 3 Jun

A. Wootten: 28 May - 3 Jun AAS 5-9 Jun I99 9 - 18 Jun CSO 19 - 30 Jun Japan.

J. Mangum:

M. Yun:

B. Butler:

S. Radford: