[mmaimcal]forwarded message from Stephane Guilloteau

[Jeff Mangum]

Hi Folks,

"Bryan" == Bryan Butler <bbutler at aoc.nrao.edu> writes:

...(snip)...
Bryan> as to the amplitude calibration systems, stephane asks:

>> Al, would you take in charge on the US side a computation of the
>> expected level of standing waves for 3 cases
>> 1) optimal subreflector shape, with a tangent cone matching the
>> primary central obscuration
>> 2) subreflector equipped with a lens/horn for the photonic
>> calibration system. The diameter of the lens/horn device should be estimated
>> by the
>> 3)  subreflectir equipped with a dual-load of appropriate size, i.e.
>> matching the central obscuration of the primary.
>> Two sub-cases must be considered: load in place, or backside conical mirror
>> in place. A range of distances between the subreflector edge and the conical
>> mirror should be considered, unless there is a specific design already
>> (which I have not seen).

Bryan> to do this properly would require a full 3-D e-m treatment of the antenna
Bryan> structure, something which i'm pretty sure nobody in the imcal group has
Bryan> at hand, or can obtain.  i think it would be extremely difficult for us
Bryan> to address this question/problem analytically.  our approach has always
Bryan> been to see how well it works on the BIMA antennas, and then to perhaps
Bryan> attempt to put one on one of the ALMA prototype antennas.  i think this is
Bryan> a sound approach, and still advocate it.  the problem is manpower, as
Bryan> usual.  who will work on it?

Bryan> i see no reason to prefer getting rid of the dual-load system over the
Bryan> semitransparent vane system.  neither has ever been shown to work on a
Bryan> functioning antenna, and until this is the case, i think it would be
Bryan> premature to cut one or the other of them.  in a budgetary sense, it might
Bryan> be OK to say that in the end we only need 1 of them, and budget it that
Bryan> way, but to stop development on one or the other at this point is not
Bryan> the right decision, i think.

I second Bryan's suggestion.  The BIMA folks have been making good
progress (on their own budget) on their prototype, and it still holds
the most promise for meeting the needs of the ALMA amplitude
calibration requirements.  Fundamentally, a dual-calibrated-load
system is the only device which can allow for accurate calibration in
the submillimeter.  The added risk of not meeting the amplitude
calibration specification that ALMA would assume is far larger than
the 800 k$ budgeted for this device.

Cheers,

Jeff