Memo Review

Memo: 422 - The Dual-Load Calibration device revisited
      Guilloteau, 2002May23

Reviewer: Jeff Mangum

Date Received: 2002Aug27


The analysis of the dual-load calibration system in this memo has been
thrown into question by David Woody's review.  The difficulties with
the dual-load calibration system have been well documented by the BIMA
team prototyping this system.  None of the information regarding this
system indicates that its development should be abandoned.  Given also
that it is the only amplitude calibration system that has been
prototyped, development of this system should most certainly be
continued within the ALMA project.

Specific comments on each section follow...

Section 2.1:  I don't believe that the dual-load subreflector
calibration system was developed to mitigate the adverse affects due
to saturation of receivers by ambient loads.  The dual-load
calibration system allows for the separation of what Larry D'Addario
correctly refers to as "sounding" and "calibration" measurements.  As
the dual-load system is in effect a built-in signal source, it can be
used to characterize the instrumental calibration as separate from
measurements of the atmospheric parameters (what Larry calls
"sounding" parameters).

Section 3:  The dual-load calibration system operates simultaneously
with astronomical observations.  Therefore, the fact that sufficient
signal-to-noise requires 6-8 seconds of integration time at
submillimeter wavelengths should not be considered a drawback of the
dual-load calibration device.

David Woody has suggested that the chopping frequency analysis in this
section is too simplified and leads to an incorrect result.  This
discrepancy should be considered by the author.

Section 4:  The statement regarding knowledge of the exact value for
the coupling factor (f) is not really correct.  In fact, one really only
needs to know that the coupling factor is constant to the required
precision.  Any absolute value differences between the true coupling
factor and the assumed coupling factor can be accounted for in the
absolute amplitude calibration step (the conversion of Ta* to Tr).

Table 2: A number of the "pros" and "cons" listed for the vane and
dual-load systems are not really correct:

-- Stating that accessibility to the "widget space" above the receiver
   is easier than the accessibility of the apex is open to
   interpretation.  Access to the apex requires the use of a lift,
   while access to the widget space requires climbing into the invar
   cone above the receiver cabin (for the Vertex antenna design, at

-- There are no facts to base an assessment regarding reliability or
   maintenance of either of these devices.

-- The facts regarding the influence of standing waves for both of
   these devices is not yet determined.