[mmaimcal]Subarrays
Bryan Butler
bbutler at aoc.nrao.edu
Mon Sep 16 19:19:58 EDT 2002
all,
i agree with al here - it has nothing to do with the VLA (and, FWIW, we
didn't use 5 subarrays when filming 'Contact' anyway - we used only 3 -
i know because i set up the files and still have them...).
we had a very long email discussion of this in september of 1999, which
you will find attached (and might find mildly interesting [or amusing :]).
-bryan
On 2002.09.12 12:16 Al Wootten wrote:
> > Richard Simon wrote to Peter Gray:
> >
> > > About 2 years ago, it was "decided" that ALMA would support only 4
> > > subarrays, and not 5. The reasoning was based on VLA experience,
> > > where 2 subarrays is routine, 3 is often used, 4 has been rarely used,
> > > and 5 subarrays were used only for the filming of Contact (!).
> > > Accordingly, the relevant budget planning numbers were adjusted.
> > > However, the change was never officially made (to our unofficial
> > > documents).
> > >
> > > I would like to request that the formal process be completed, so that
> > > our budget planning (presumably for 4 subarrays) is in agreement with
> > > the technical specifications. If if turns out that 5 subarrays are
> > > required, I will adjust the budget numbers
> > >
> > > While it might be fun if I were to initiate a Change Request, it is
> > > probably most appropriate if one of you initiates the process.
> > >
> Peter responded:
> ---------------------------------------------------------------------------
> Richard,
> I have checked (with SEARCH) through the various project documents that
> we have collected
> in the almaedm archive and found amongst others the following references
> to the number
> of sub-arrays.
>
> - specification ALMA-90.00.00.00-001-A-SPE, old ProjBook Chpter 2,
> current title
> " Science - Specifications and Requirements"
> p.16 says in system flexibility spec "4 sub-arrays"
> p.19 Table 1 says Number of sub-arrays >3 TBD, goal 5
> see in almaedm:
> http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/showFolder/100023/d20020807173924/No/t100023.htm
>
> - spec ALMA-70.00.00.00-001-A-SPE which is the Computing spec
> http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100019/d20020807165242/No/t100019.htm
> says up to 6 as per the correlator spec
>
> - spec ALMA-60.00.00.00-001-A-SPE Correlator spec
> http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100018/d20020807164617/No/t100018.htm
> p.10 says "full support for up to 6 independent correlator sub-arrays"
>
> - spec ALMA-50.03.00.00-001-A-SPE Local Oscillator
> http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100017/d20020807191726/No/t100017.htm
> in Table 1 has the same as ALMA-90.00.00.00-001-A-SPE that is >3 and goal 5.
>
> - in the report from the Systems PDR 200-02-28
> http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/systemspdr/docProfile/100004/d20020604122843/No/sysPDRreport.pdf
> "The operation of ALMA with maximally 4 Subarrays is recommended."
>
> - in a joint ALMA DH meeting dated 2000-May-02 it says:
> "Engineers and scientists have agreed on subarray requirements. Four
> simultaneous LOs will be required. Also all antennas can be controlled
> individually. A report defining what type of sub-arrays will be
> available and how many will be issued soon and added to the
> recommendations of the System Review."
>
> - in the system block diagram which has just been submitted for approval
> http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100067/d20020909202652/No/t100067.htm
>
> it is careful not to specify the number of sub-arrays but uses a number N.
>
> I conclude from all the above that the number has been generally agreed to
> be 4 sub-arrays, but that this specification has not been clearly defined
> and presented in an approved project level specification.
> I am not sure of the consequences of the choice of 6 sub-arrays for
> correlator and computing, but perhaps this does not have a big cost
> impact.
>
> I propose to do the following:
>
> 1/ split out the Engineering requirements section of Chapter 2 into a
> seperate document to be called something like
> "Top-Level System Engineering Reqirements" and to make
> it more visible. This will be for now a simple cut-paste as for the
> other chapters. Then
> get it project approved.
>
> 2/ after approval of the above, ask you to submit a change request CR to
> change in the
> above doc the ">3 and goal 5 " reference to a fixed 4 spec
> sub-arrays, and to modify any other references in this document to suit.
> This CR can then be reviewed
> and commented and can be used as a way of informing the project properly
> of this fixed requirement.
>
> 3/ start the process of getting this Enginering Requirements document
> revised and reformatted to make it
> clearer and referenceable (ie number and clearly define each requirement
> in the tables). Then make sure
> cross-referencing inside this and other documents specifies the
> requirement reference, not the requirement.
>
> Peter
>
> PS A reminder for those on the CC list for this email, it would help the
> above process if you could
> also complete asap the review of the above documents so that we can
> complete the approval process
> and move on to try to organise ourselves to avoid future confusions like
> this "number of sub-array" question.
> ---------------------------------------------------------------------
> I responded:
>
> Dear Peter and Richard
>
> This decision had nothing whatever to do with Contact nor with the VLA;
> I don't know where this silly impression arose. In my recollection, this was
> discussed at the System PDR and revisited many times since for not very
> good reason. A subarray is a slippery beast, and the first thing that
> must be understood is that it means different things to different people.
> In its loosest sense, it is a collection of antennas whose signal may be
> correlated independently of other collections of antennas. This is a
> correlator subarray, and I understand that the current correlator supports
> 16 of this variety of subarray. This sort of subarray should make little
> difference to costing.
>
> Another variety of subarray (which I think you consider) consists of 'groups
> of antennas with independently tuned LOs', which is a different item.
> Yet another variety mixes these two; during Early Science for example,
> I envision an Early Science array operating at 3mm and 1.3mm while the
> Commissioning Array operates at perhaps 3mm and Band 7 or 9 while a total
> power antenna operates in yet another 'array' at one of these frequencies.
> This might constitute four LO subarrays, but five correlator subarrays for
> example.
>
> This is in accord with the System Review, endorsed by the Division Heads on
> 2000-May-02 as stated by Peter.
>
> The Science Group has a number of similar examples which may easily be found
> by using the search engine at:
> http://www.cv.nrao.edu/~awootten/mmaimcal/
>
> Steve Myers wrote a report proposing eight subarrays 29 Feb 2000, available as
> http://www.aoc.nrao.edu/~smyers/alma/subarrays.txt
>
> In the report
> on the first meeting of the ASAC, held in Leiden, The Netherlands, on
> March 10-11, 2000, the ASAC said:
>
> System
>
> The ALMA system deals with many aspects of ALMA. We expect to revisit many
> of these areas in the future. We summarize below our recommendations on
> the issues addressed at this meeting.
>
> 1. The main array should consist of a number of 4 to 6 sub-arrays, but the
> number of frequencies operating simultaneously will not exceed 3 or 4. At
> present we could envision 4+1 subarrays. Namely:
> (a) The main interferometric subarray
> (b) Antennas for reconfiguration and baseline determination
> (c) Two subarrays to simultaneously carry out two of the following
> functions:
> Secondary subarray at second frequency band
> Transient event monitoring
> mm-wave VLBI
> Testing, repair, receiver warm-up or cool-down, etc.
> (d) The single-dish subarray or an ultra-compact array (if included
> in the final project).
> 2. The prototype antennas should be equipped with nutators and stable
> receivers. The number of ALMA antennas equipped for total power measurements
> (nutators) should be 4, but this number will be reconsidered
> after the tests with the prototype antennas. If feasible, the rest
> of the array antennas should be equipped with receivers of good gain
> stability ( in 1 second). (See also section 3 and 6).
> ...
>
> The ALMA Correlator handles 16 correlator subarrays; the group working on the
> next generation correlator asked the ASAC how many subarrays should be
> functional in that instrument. At their meeting in Santiago, they replied:
> * Capability for being a spectrometer for up to 5 subarrays (the ALMA
> Correlator handles 16).
>
> I confirmed with Ray Escoffier that the first quarter of the ALMA correlator
> offers the full capabilities of the final correlator, though with some
> wiring and 'personality' differences to achieve this. It will have full
> subarray capability.
>
> In my opinion no change request is needed, only a clarification to the
> unclear documentation.
>
> Clear skies,
> Al
> -----------------------------------------------------------------
> However, since the documents are now controlled and
> are internally inconsistent, a change request does seem mandated.
>
> Al
>
>
> _______________________________________________
> mmaimcal mailing list
> mmaimcal at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/mmaimcal
>
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>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 08:21 MDT 1999
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Date: Fri, 10 Sep 1999 10:20:48 -0400 (EDT)
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From: "Larry D'Addario" <ldaddari at cv3.cv.nrao.edu>
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To: Simon Radford <sradford at cv3.cv.nrao.edu>
Cc: "Larry D'Addario" <ldaddari at cv3.cv.nrao.edu>,
Darrel Emerson <demerson at cv3.cv.nrao.edu>,
Image Calibration <mmaimcal at polaris.cv.nrao.edu>
Subject: [mmaimcal] Re: subarrays
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Simon,
If subarrays were free, or if they could be created with "only
software," perhaps the I&C group's idea would be feasible. This is
not the case. Lots of expensive hardware must be multiplied by K for
K subarrays.
Like all other proposals, this one does not deserve to "get into the
specs" without thorough justification. To be seriously considered, we
need a document showing why 10 subarrays are *needed*, and considering
the cost tradeoffs. If, as I suspect, there is no such need but the
number is just the largest one for which anyone could imagine a use,
then we need not consider it at all.
I believe we should support two subarrays. This provides hardware
redundancy when only one is needed, and it supports testing and
calibration on some antennas simultaneously with science on the
others. I would not be surprised if justification is found for
extending this to three subarrays, but I think it will be hard to make
a case for going beyond that. Nevertheless, I'd be happy to hear any
such arguments.
--Larry
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 08:36 MDT 1999
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From: Bryan Butler <bbutler at aoc.nrao.edu>
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To: sradford at cv3.cv.nrao.edu, ldaddari at cv3.cv.nrao.edu
Subject: Re: [mmaimcal] Re: subarrays
Cc: demerson at cv3.cv.nrao.edu, mmaimcal at polaris.cv.nrao.edu
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larry,
three is incredibly easy to make a case for - just add a VLBI
experiment.
we will have to think about the justification of more.
in order to do a cost-benefit analysis we need to know the cost.
can you provide this?
-bryan
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 08:40 MDT 1999
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From: Michael Rupen <mrupen at aoc.nrao.edu>
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To: sradford at cv3.cv.nrao.edu, ldaddari at cv3.cv.nrao.edu
Subject: Re: [mmaimcal] Re: subarrays
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Just a quick response on the subarrays question...I think there's quite
a good case for having many more subarrays on ALMA than we're used to thinking
about. I agree with Larry that we need to weigh those scientific/practical
benefits against the real engineering costs, but I don't think two or three
is anywhere near the number such a careful analysis would come up with. We
summarized some of the reasons for subarraying in the infamous correlator
"white paper", _Astronomical Requirements for the MMA Corraltor_, Rupen,
Shepherd, & Wright 1998, section 3.7 . Very briefly, one wants subarrays
for a wide variety of purposes:
* Simultaneous multi-freq. capability, useful for
- solar flares
- atmospheric studies
- cases (as at the VLA currently for 7mm) where only some of the antennas
are outfitted with certain types of receivers or bands. I suspect this
will be much more common for an array as huge as ALMA.
* Calibration. If we're at all thinking about simultaneous calibration using
part of the array, to monitor opacity and/or gain on short timescales
through tipping scans or other measurements at one or more wavelengths, that
suggests subarrays to do so.
* Mapping large areas. I know, I know, no-one but me & some other crazy
Americans will ever want to do this :) but with M subarrays you do shorten
the time to map a given area by a factor M, if sensitivity is not an issue.
This is useful e.g. for finding interesting "weather" on the Sun and other
solar system objects (this can change VERY fast), or simply for doing big
surveys...though as Larry will point out the latter does not absolutely
_require_ subarraying.
* Fast-response observations. If something nifty goes off you may want to
quickly dash over to have a look, without stealing every antenna in the
array. Similarly ALMA will be so absurdly sensitive that monitoring obs.
even of faint sources might easily be done with a subset of the antennas.
* Lower data rates. Experiments which require lots of channels or very high
time resolution will probably have to cut down on the number of baselines;
this will undoubtedly be true initially.
* VLBI. I don't imagine people will want to give up all or even half of ALMA
on a regular basis for VLBI, but it will certainly be in enormous demand
for high-freq. experiments.
* Inhomogeneous arrays. If ALMA does go this route (cf. Wright's memos),
there are probably reasons to split into homog. subarrays every now and then.
* Single-dish observations. ALMA will probably do its own single dish work,
but it's not clear whether one wants (or will be able) to use the entire
array for this all at once. So we'll want to split off bunches of single
dishes, which you may or may not want to call subarrays.
* Some folks have suggested faking big dish diameters by phasing up lots
of little (well, moderate-sized) antennas (cf. MMA Memo 165).
So I think there are some good reasons for a fair number of subarrays.
Exactly how many depends on how you think the instrument will be used, and
how much it costs in $$ and complexity to build them in. I'd be very surprised
if we wanted fewer than 6 or so myself.
-- Michael
-
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 08:41 MDT 1999
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From: Bryan Butler <bbutler at aoc.nrao.edu>
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To: ldaddari at cv3.cv.nrao.edu
Subject: Re: [mmaimcal] Re: subarrays
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in fact, i can pretty easily justify 4.
2 for "science" (here, we might want more than 2, but 2 is relatively
easy to justify).
1 for "testing and calibration" - e.g., if we're going to be moving
antennas around all the time, they will constantly need pointing,
baselines, delays, etc...
1 for VLBI.
-bryan
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 08:45 MDT 1999
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To: sradford at cv3.cv.nrao.edu, ldaddari at cv3.cv.nrao.edu
Subject: Re: [mmaimcal] Re: subarrays
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Larry,
The C&I group was asked on this issue purely from the scientific
point of view. Most of us could very easily come up with situations
where we could be using several subarrays at once -- two subarrays
operating simultaneously may be a daily event if reconfiguration is
done in a more or less continuous fashion. Even a good fraction of
all VLA observations are done in two or more subarrays these days,
and I personally feel the subarray limitation of the current VLA
is nearly as bad as the limitations of the correlator.
So, it comes down to cost. I have heard of "we cannot have too
many of these because they cost money" too often these days.
It would help to know how much an additiona subarray will cost, and we
will come up with a detailed case for the different number of
subarrays. I am sure this is what you had in mind afterall.
-- Min
p.s. I just got Michael and Bryan's e-mails. I guess these are
good beginning points for us.
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 09:18 MDT 1999
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Subject: [mmaimcal] Re: subarrays
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The recent emails from Brian and Michael seem to confirm my suspicion,
where I said:
> ... If, as I suspect, there is no such need but the
> number is just the largest one for which anyone could imagine a use,
> then we need not consider it at all.
The mere fact that you can think of a way to use some capability does
not mean that we should build it (except in the unusual case that it's
truely free). Let me remind you of a general principle on which there
seemed to be wide agreement when it was discussed in other contexts:
If there is some significant science that cannot be done without some
specific feature of the instrument, then that feature must be
seriously considered. (Even then, the cost must be traded off against
benefits, including whether the science could be done by some other
instrument.) But if the feature would merely *speed up* the
observations or make them more convenient, so that the same science
could nevertheless be done without it, then including the feature
should have very low priority.
Can you suggest some valuable science that cannot be done at all if we
have only 2 subarrays?
--Larry
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 09:29 MDT 1999
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From: Bryan Butler <bbutler at aoc.nrao.edu>
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To: ldaddari at cv3.cv.nrao.edu
Subject: Re: [mmaimcal] Re: subarrays
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the argument that the only drivers for designing the array are
whether "significant science that cannot be done without some
specific feature of the instrument" is a specious one. this
is not the way we should design this instrument. sure, we
can get by with only 2 subarrays (in fact, we could get by with
only 1 - just demand that the entire array shuts down whenever
we need to redetermine baselines, pointing, delays, etc...),
but if this is going to mean that it will take us significantly
longer to do some (or all) kinds of things, then we need to
weigh that against the cost of having more subarrays.
since you have provided us with no cost estimate, we cannot have
a serious discussion or study of the tradeoff.
-bryan
>From ldaddari at polaris.cv.nrao.edu Fri Sep 10 09:34 MDT 1999
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Bryan Butler writes:
> since you have provided us with no cost estimate, we cannot have
> a serious discussion or study of the tradeoff.
Since you have provided us with no benefit estimate, we cannot have a
serious discussion of the tradeoff.
--Larry
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 09:37 MDT 1999
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From: Michael Rupen <mrupen at aoc.nrao.edu>
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Solar flares. They don't last long enough to wait around while changing freqs.
VLBI whilst using the array for any other purposes (I assume maintanence+regular
usage+VLBI).
Simultaneous SD & array observations to maintain identical atmospheric
conditions & calibration.
But basically I totally disagree with the attitude that convenience or
efficiency is not an argument when put up against inconvenience or cost of
design. Making a telescope more efficient is equivalent to enlarging the
collecting area, even more so when working at wavelengths where the atmosphere
changes rapidly & frequently prohibits work at the highest observing
frequencies. A $400M (or is it $600M now?) telescope should not be provided
with only the barest minimum of capabilities. My car does have air
conditioning, and I suspect yours does as well. It can even carry more than
one passenger, although I could always drive back to pick #2-4 up later.
We should not have to scrape by on a day-to-day basis, for normal observations,
at the very limit of what we can do. If half the antennas are not outfitted
with receiver X, and only 3 antennas have single-dish capability, and the
engineers/software types want to futz with 4 more, we should be able to use
*all the antennas* despite this complexity: that means
subarray 1 freq. X (equipped antennas)
subarray 2 freq. Y
subarray 3 single dish (equipped antennas)
subarray 4 maintenance/improvement work
and I haven't even invoked VLBI yet. I admit that given infinite observing
time we can just use one or two subarrays, but if we had infinite time
we could just bang away with PdB or SMA. More importantly, you haven't
mentioned any strong, *quantitative* driver for the truly minimal number
of subarrays you suggest. How painful would it really be to allow 6 or so
subarrays? How much would it cost, in $$$, in design time, and in maintenance?
Productivity and efficiency are important, as well as uniqueness.
-- Michael
>From ldaddari at polaris.cv.nrao.edu Fri Sep 10 09:48 MDT 1999
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Bryan Butler writes:
> i thought michael just provided us with a list of benefits? i think
> we could most likely expand it as well.
No, he just provided a list of possible uses. It was not at all
quantitative, yet you demand a quantitative cost. What fraction of
the time would >2 arrays be useful? Which of the observations could
be done in another way with no loss of efficiency? Are *any* of the
observations impossible without more subarrays?
>From your previous email: "sure, we can get by with only 2 subarrays,"
I take it that you are answering my earlier question in the negative:
you cannot think of any science that is impossible with only 2
subarrays. When you say, "this is going to mean that it will take us
significantly longer to do some ... things," you should realize that
that might be perfectly OK. If it takes twice as long to do something
that is only done 0.1% of the time, then the efficiency is reduced by
only 0.1%. And some things on Michael's list are needed 0% of the
time.
--Larry
>From mrupen at zia.aoc.NRAO.EDU Fri Sep 10 10:02 MDT 1999
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From: Michael Rupen <mrupen at aoc.nrao.edu>
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To: bbutler at zia.aoc.NRAO.EDU, ldaddari at cv3.cv.nrao.edu
Subject: Re: [mmaimcal] Re: subarrays
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A couple points:
* splitting according to antenna capability seems pretty fundamental
to me. Am I missing something here?
* I agree that several of the points I raised were not very quantitative --
I haven't seen any real discussion yet of the way calibration or the
finding of phase calibrators in real time might be done, although several
suggestions involving subarrays have been made. In print even :)
* I've been thinking of subarrays as all or nothing -- either you have the
capability to use 6 independent subarrays, or you don't. If it matters
how often you use it (apart from the basic question of whether it should
be made available at all) it would be good to know why, & what tradeoffs
are useful to think about.
* Has there been anything, even at the level of my bare-bones rambling,
addressing the question of subarray cost? Maybe I've been out of the loop
too long & missed it. I've chatted with a number of different correlator
types about this, not to mention Barry, and no-one seemed to think it
was a big deal, so long as it wasn't really ridiculous (e.g. 1 per antenna).
So I didn't realize this was a huge cost driver -- is it?
-- Michael
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 10:02 MDT 1999
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From: Bryan Butler <bbutler at aoc.nrao.edu>
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To: ldaddari at cv3.cv.nrao.edu
Subject: Re: [mmaimcal] Re: subarrays
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i wrote:
>> i thought michael just provided us with a list of benefits? i think
>> we could most likely expand it as well.
to which larry replied:
>No, he just provided a list of possible uses. It was not at all
>quantitative, yet you demand a quantitative cost. What fraction of
>the time would >2 arrays be useful? Which of the observations could
>be done in another way with no loss of efficiency? Are *any* of the
>observations impossible without more subarrays?
"uses" vs "benefits"? sounds like a semantical argument to me. i
agree that these are not quantative, but at least we have presented
a framework. all we have hear from you is that "it will cost, therefore
it is bad". we have no even rough idea of what the cost is. is it
$1000 per antenna? $10000 per antenna? $100000 per antenna? $1M per
antenna? does it even scale with number of antennas? does it scale
linearly with number of subarrays (and why)? how is the cost affected
by the decision on a photonic system? we have heard none of this.
and, i wrote:
>>From your previous email: "sure, we can get by with only 2 subarrays,"
to which larry replied:
>I take it that you are answering my earlier question in the negative:
>you cannot think of any science that is impossible with only 2
>subarrays.
no this is _not_ what i meant. i meant that we could "get by" with
only 2 with some sacrifice in the science. there are certainly bits
of science that can only be done with more than 2 subarrays - time
variable sources are the ones that come immediately to mind.
and, finally, larry also wrote:
>And some things on Michael's list are needed 0% of the time.
according to whom?
-bryan
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 10:10 MDT 1999
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From: Harvey Liszt <hliszt at cv3.cv.nrao.edu>
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Subject: Re: [mmaimcal] Re: subarrays
To: mmaimcal at polaris.cv.nrao.edu
Date: Fri, 10 Sep 1999 12:10:17 -0400 (EDT)
Cc: hliszt at polaris.cv.nrao.edu (Harvey Liszt)
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At what point in the installation are the hardware costs of subarraying
located?
Am I wrong in assuming the costs are mostly concentrated at the
central location where the telescopes are commanded? Or does the
issue of subarraying affect how we distribute the computing power
and other hardware in the array, i.e. making individual antenna
installations more/less intelligent and less/more dependent on
the central command and control facility?
How does the complexity of k subarrays scale with k? Does most
of it accrue just from the difference between k = 2 and k = 1
(the latter case being "all Alma all the time").
Will it be possible to avoid k > 2 subarraying if the Japanese
contribution is inhomogeneous (or if ALMA is inhomogeneous?)?
regards, Harvey
>From owner-mmaimcal at kochab.cv.nrao.edu Fri Sep 10 10:11 MDT 1999
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Michael Rupen writes:
> ...* Single-dish observations. ALMA will probably do its own single dish work,
> but it's not clear whether one wants (or will be able) to use the entire
> array for this all at once. So we'll want to split off bunches of single
> dishes, which you may or may not want to call subarrays....
Each element of the array will operate in a total power mode. I suppose we
call one of these elements a separate subarray when it is doing something
entirely different (frequency wise) from the rest of the array. Having
one telescope which operates this way has been part of the baseline plan
from the beginning of the project. This is significant science which cannot
be done unless we allow it; particularly there will be an element which may
be used for single element single receiver sorts of experiments (the [N II]
line at 205 microns) on occasion. That element would be operating
independently of the array during its specialized experiment. I fully expect
that some element of the array will operate in this mode or a similar one for
much of the time. There isn't a proposal yet for a bolometric camera on
such an element but there will be; such an instrument would be very powerful
and in great demand. Is such an antenna a subarray?
If so, three subarrays is an absolute minimum, in my opinion.
I'd imagine that the cost of subarrays isn't really a dollar quantifiable
item; part of the problem must be the complexity, which I would imagine could
grow to unmanageable proportions.
Al
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