[mmaimcal]ALMA in Total Power mode

[Al Wootten]

Folks,

       There has been some confusion over total power operation of ALMA.  
As stated in Project Book Chapter 2 Table 2.1 'Science Flowdown to Technical
Specifications' Item 6 'Wide Field Imaging' total power and interferometric
capability are required of all ALMA antennas; this is elaborated in 2.1.2.5.

	   However, at a front end meeting in November of 2002 in Charlottesville
it was noted that some specific cases of wide field imaging, in particular
mosaicked continuum imaging, place difficult-to achieve requirements on the
front ends.  The scientific basis for these cases is detailed in several
documents.  The Total Power 
Stability specification arose from the March 2000 ASAC Report, with details 
in Appendix D of that Report which is also available as a postscript standalone 
document. Also relevant is ALMA Memo 289.1 (rev) by Mel Wright. Accurate
polarization imaging also requires stable receivers, as described in the
ASAC Report of Feb 2001.  Accordingly, it was determined at the front end
meeting that a subset of front ends which met this challenging total power
specification could be placed on the four antennas equipped with
nutating subreflectors in order to achieve these specific demanding scientific
goals.  To my knowledge, this is not written down now, which apparently has led
to some confusion.  A new version of front end specifications is, I understand,
imminent.

       Many, if not most, wide field imaging problems addressed by the array
will not fall into these two specific categories--in particular most spectral line
experiments are less demanding for total power than the continuum analog.  For these 
experiments, the entire array may be operated in total power mode.

       This science requirement demands a clean frontend bandpass which remains
stable over time.  The definition of 'clean' and 'time period' need to be 
specified, but a variation of not more than 10% of Tsys in a 10 kHz spectral
chunk has been suggested over one spectral line switching period.

      We'll work on clarifying this in the specifications.

      Clear skies,
Al