Memo Review

Memo: 372 - An Amplitude Calibration Strategy for ALMA
      Moreno & Guilloteau, 2002May10

Reviewer: Jeff Mangum

Date Received: 2002Sep10



I enjoyed the breadth of this memo, but thought that it diverged
extensively from its title.  The point of these diversions is to show
how they affect calibration, which makes them valuable additions to
this memo.  

In a number of places in this memo, conclusions are drawn based on
incorrect or unsubstantiated facts.  A good example is the dismissal
of asteroids as absolute amplitude calibrators.  In many of these
instances the conclusion drawn by the authors is in fact incorrect.

In the end, though, I think that this memo will serve as a very good
starting point for discussion on the calibration strategy to be used
for ALMA.  The basic conclusion that flux calibration is a research
project for ALMA is absolutely correct.

Individual Comments:

Section 3.3: Asteroids
-- The emission from asteroids is better modeled than the authors
   indicate.  The series of papers by Lagerros, Muller, and
   collaborators have shown that the emission from many asteroids
   can be modeled to better than 5% accuracy over the wavelength
   range from 5 to 200 micron.  In the millimeter/submillimeter range,
   the modeling accuracy is a bit worse, in the range 5-10%.  This
   incorrect analysis also invalidates some of the conclusions in
   Sections 12.4 and 13.

Section 6.1.1:

-- It appears that Jbg has also been ignored in Equation 17.

-- The partial derivatives listed are only correct for the low-opacity
   limit.  Since this assumption is not made later, in section 6.2.1,
   where the dual-load uncertainties are considered, the comparison
   between the two chopper calibration systems is uneven.

Section 6.2.1:

-- "Accordingly, the absolute calibration with the dual-load method is
   intrinsically less accurate than the vane calibration technique."
   This statement is not correct, as it is based on an uneven analysis
   of the uncertainties inherent in the two load calibration systems
   considered.  Other uncertainties in the semi-transparent vane
   calibration technique, most notably the uncertainty in the mean
   atmospheric temperature, dominate.  A complete analysis of these
   uncertainties leads to the opposite conclusion, that the dual-load
   method is intrinsically *more* accurate than the single-load (of
   which the semi-transparent vane is one type) method.

   This incorrect analysis invalidates a number of statements in this
   regard in the Abstract and in Section 13

Section 11: Flux Scale

   I do not see the distinction between the "a posteriori" and
   "ab-initio" methods.  If the instrumental gain is stable, then the
   standard bootstrapping method for flux scaling will be more stable
   with time.