[mmaimcal]forwarded message from Stephane Guilloteau

[Bryan Butler]

on the subreflector photonic bandpass (and other, possibly) calibration
system:

i second mark's opinion on this.  i was just trying to look up some
numbers before composing my reply.

the current spec on bandpass stability is somewhat waffly, but in
the project book engineering specs section and calibration chapter
it is listed as "10000:1 to 100000:1" (table 3.1.1).

stephane thinks that astronomical bandpass cal will get you to
0.1%.  this *may* be true, but that does not reach the spec.
we must either argue for a relaxation of the spec, or for a
system other than looking at astronomical sources to calibrate
the bandpass.  the only such proposed system we have is the
subreflector photonic transmitter.  perhaps we should investigate
alternatives, but that's what we have for now.



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).

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

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


	-bryan


On 2002.08.13 08:47 Mark Holdaway wrote:
> 
> 
> My impression is that a bandpass accuray of only 0.1% is
> unacceptably poor.
> 
> But presumably there are other ways to improve this (and the polarization)
> besides the photonic option?
> 
> 	-Mark
> 
> 
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