[mmaimcal]FE Stability

[Al Wootten]

Folks,

Agenda for the meeting tomorrow is in the usual place.

As you know, a workshop to establish front end specifications beyond
those in FE Specs v2.0
http://www.cv.nrao.edu/~awootten/mmaimcal/asac/ALMA_FE_specs_V20.pdf
will be held in Charlottesville 20-21 November.  One specification which
has garnered concern on the part of the FE group has been the receiver
stability specification, based on memos by Wright and by Welch and
endorsed by the ASAC.  Charles posed several questions which he sent to
me, which I'd like to discuss at Tuesday's meeting.

He sent me the first point in the receiver PDR (see 5 Nov 02 mmaimcal agenda)
As you will note, I feel that we may need
to understand how the stability will affect the science more deeply than
we currently do, perhaps establishing a specification and a goal.  On the
other hand, I have yet to meet a single scientist who disagrees with us on
the spec for stability. OTOH ALMA needs a specfication which isn't so rigid as to
scare off any FE bidders; but we need to be sure we can image as we have proposed.
Welch's solution, to temperature stabilize receiver components, is expensive
in terms of costs and also in terms of the heat budget of a dewar which many
believe might not even comfortably accommodate the ten cartridges in it during
cooldown.  I understand that this argument has gone back a long way--Kellerman
was just telling me they had had the same discussions for centimeter receivers
in the 60's, but of course they were not cooling to 4K.  It may be easier to
thermally stabilize HEMT receivers at a warmer stage; we might consider
whether to have different specs for them.

  Front End Issues for FrontEnd Review in Charlottesville (from Charles) with
some instant thoughts for discussion (not yet forwarded to him):

 FE stability for the total power antennas?
AW>For now, the specification is "a fractional receiver gain stability of about
AW>$1\times 10^{-4}$ in a one second time interval for all the antennas.
AW>There should be no difficulty in achieving this."  on which I think we 
AW>scientists agree, for  stability (which I take as Antenna/FE/BE).
AW>This is from Appendix D of the March 2000 ASAC Report.

 FE stability for the rest?
AW>An issue to be discussed.  In my opinion, all of the antennas are total
AW>power antennas and will be used as such particularly at frequencies below
AW>300 GHz.  It is at submillimeter frequencies that I believe the total power
AW>role may be taken over, perhaps entirely, by the antennas with nutators.
AW>I worry about how the deconvolution will affect those images.

 How many TP antennas are there and who builds them?
AW>I think that there are 64, built by EU and NA in a fashion to be determined.
AW>Japan may at some stage provide 'four high performance' (term TBD) total power
AW>antennas; Ishiguro told me that one of these is to be the JP prototype so
AW>we will shortly know what this term means.
AW>This was stated by the ASAC in their first report (see
AW>http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node6.html
AW>I endorse their view.

 What's the FE stability spec and what's the goal?
AW>The specification is now one part in 10^4 in one second and one part in
AW>10^4 in 0.1 second for total power stability.
AW>

 How do we divide up the stability budget antenna/FE/BE etc
AW> A question for discussion since the Antenna/BE contributions will constrain
AW>the FE stability; all will contribute to the overall total power stability.

  I guess polarization and calibration are other areas of doubt and uncertainty?
AW>...

Thoughts??

Clear skies,
Al