[mmaimcal] forwarded message from Stephane Guilloteau

[Jeff Mangum]

Hi Al,

Mark Holdaway, Simon Radford, and myself discussed Stephane's points
and conclude that his arguments are a bit weak on a couple of
points...

"Al" == Al Wootten <awootten at nrao.edu> writes:

Al> Stephane disagrees, believing that one of the 12m will suffice even
Al> at THz frequencies.  I just don't see that.  Comments?
Al> Al

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Al> From: "Stephane Guilloteau" <guillote at iram.fr>
Al> To: "Al Wootten" <awootten at NRAO.EDU>, "Jacob Baars" <jbaars at eso.org>
Al> Cc: <wwelch at astron.Berkeley.EDU>, <rbrown.kmenten at mpifr-bonn.mpg.de>,
Al>         <rkurz at eso.org>, <R.Wade at rl.ac.uk>, <mrafal at NRAO.EDU>,
Al>         <demerson at NRAO.EDU>
Al> Subject: Re: Nutators for ACA?
Al> Date: Thu, 28 Sep 2000 12:55:18 +0200

Al> Dear Colleagues,

Al>     I disagree with Al's statement on the need of wobbling subreflector for
Al> ACA.  Let us consider several cases

Al> 1) ACA is only used at the ALMA frequencies.
Al>     - then the total power is provided by the ALMA 12-m antennas, either in
Al> total power (e.g. OTF spectral line)
Al>       or using the few antennas equipped with a subreflector (4 is enough).
Al>     - The ACA antenna would NOT need wobbling secondaries

Probably not true that the total power provided by the 12m antennas
will be as "pure" as that provided by the smaller ACA antennas.  The
ACA antennas will likely have better and more stable pointing
performance and surface accuracy, which will produce a better total
power measurement.

Al> 2) ACA covers all ALMA frequencies, but ALMA not (for cost reasons,
Al> schedule, etc...)
Al>     - the good way to solve that problem is to equip 1 or all of the 4 ALMA
Al> 12-m antennas which have the wobbler
Al>       with the same receivers as ACA.

To equip all 12m antennas would probably be the safest thing to do.

Al> 3) ACA goes above one TeraHertz, where ALMA does no longer operate
Al>     - given the small number of ACA antennas (< 16), a SINGLE  antenna
Al> equipped with a wobbler is sufficient
Al>       (4 makes it for 64...).

...only if you believe that 4 really does make it for 64.

Al>     - that can either be an ACA antenna, or the best 12-m antenna...
Al>       A larger antenna offers some advantage, since it provides better UV
Al> coverage overlap with the ACA array.
Al>       The phase errors introduced by pointing errors at the overlap point in
Al> the UV plane are actually 25 % smaller
Al>       for a 12-m/8-m combination than for m 8-m-8m overlap when the pointing
Al> error is proportional to the beamwidth,
Al>       and 20 % larger if the pointing error is constant.
Al>       On the other hand, 12-m antennas will have somewhat poorer surface
Al> accuracy (to be quantified, using only
Al>       night time conditions).
Al>     - Arguments of high dynamic range and/or high fidelity above 1 THz seems
Al> quite weak to me, and would need
Al>       to be quantified. The fidelity estimate should take into account the
Al> considerable difficulty of amplitude
Al>       calibration at these wavelengths, which may be the dominant problem.

Al> 4) In comparing wobblers for ACA and ALMA antennas, remember that the
Al> wobbling frequency should be inversely proportional to the antenna diameter.

I think that this is just a statement regarding the crossing time
across the telescope aperture.

Al> As a conclusion, I believe that
Al>     - it would unwise to let the close packing characteristics (which are
Al> the prime concern for
Al>     ACA antennas) be driven by the wobbler problem.
Al>     - it may be wise to have one ACA antenna with a special subreflector
Al> package, of different size, with a wobbler
Al>     in it.
Al>     - the real good solution would be to get ONE good 12-m antenna equipped
Al> with a wobbler and the same
Al>     receiver package as the ACA antennas.

Al> Whether the close packing limit is actually affected by a wobbler or not is
Al> an engineering problem.

Al>         Stephane

...but it would probably be safest to equip *all* ACA antennas with
wobblers.  I guess it depends how much we want to roll the dice on
this issue...

Cheers,

Jeff