[mmaimcal] Calibration message from Stephane Guilloteau

[Al Wootten]

(I have not seen Dick Plambeck's document; I'll try to get a copy and
circulate it on the agenda URL  so this isn't entirely out of context).

From: "Stephane Guilloteau" <guillote at iram.fr>
To: <alma-jrdg at eso.org>
Cc: "Richard Kurz" <rkurz at eso.org>, "Robert Brown" <rbrown at NRAO.EDU>,
        "Al Wootten" <awootten at NRAO.EDU>, "Darrel Emerson" <demerson at NRAO.EDU>,
        "Gie Han Tan" <ghtan at eso.org>, <jbaars at eso.org>,
        "Stephane Guilloteau" <guillote at iram.fr>
Subject: Calibration system
Date: Wed, 26 Jul 2000 16:54:22 +0200

Dear Colleagues,

    Following discussions with Matt Carter, Bernard Lazareff, Doris Maier
(on the
receiver side) and Rafael Moreno (on the astronomy side), I would like to
emphasize a few points in the decision to be made on the receiver design
concerning the calibration system.

    The calibration system should be derived from the ultimate goal of
providing
1 % ABSOLUTE calibration accuracy. Whether this is realistic or not will
come out
the on-going studies, but this goal should not be compromised too early by
unjustified choices.

    I have read Dick Plambeck's document in which he concludes that cold
load
may not be needed, but would like to challenge his conclusion. Though I
follow
perfectly Dick's approach, I believe one cannot conclude from this approach
that
cold loads should be discarded.

    My conclusions come from 5 facts. A more thorough analysis would be
required to go further

1) In Dick's approach, the goal of 1 % accuracy can never be reached. So we
cannot
base definite conclusions on this document alone.

2) In Dick's document, cold loads still give the best results at mm
wavelength,
while "hot" loads work equally well or better at sub-mm wavelengths. This is
indeed
a fairly general conclusion, which holds because calibration works well when
the
calibration levels are "as close as possible" to the working level, yet "as
widely
separated as possible".

3) The assumption of 1 % precision on the receiver gain ratio is unfounded.
Better
    accuracy can be obtained.

4) Similarly, the assumption of  1 % error on the forward efficiency is
unfounded.
    To quantify the precision of the determination of this parameter is
difficult, and
    requires more work, with different approaches. In some cases, this
assumption
    (with the assumption in 3)) is the dominant cause of error.

5) Dick's analysis in fact points out that, besides the two parameters
mentionned
    in 3) and 4) which can NOT be calibrated at the receiver level anyhow
(but can
    be calibrated by other techniques), the key parameter is the saturation
curve
    of the  receiver.


With Rafael Moreno, I have been working on a global calibration scheme for
ALMA
which includes primary and secondary calibrators, and various techniques to
measure
the atmospheric transparency.  We cannot yet conclude
on what level of accuracy is required in the "initial" Ta*-like temperature
scale from
the receiver to fulfill the final goal. There is still quite some work to do
for that,
but in our proposed method, what is important is the differential opacity
between source and primary/secondary calibrators.



>From these premises, I conclude that
A) The receiver group should find a method to measure with "sufficient
accuracy"
    the saturation curve of the receivers (i.e. the conversion "Input Power"
to output
    voltage). The "sufficient accuracy" has to be quantified too, however,
starting
    from the final goal of 1 % accuracy, I would suggest not to allow more
than 0.3 %
    of error for this sole cause.

B) Because of point 2) , it would be highly premature to propose a front-end
design
    which does not include a cold load for the millimeter domain at least.

C) The selection of which calibration scheme is actually included in the
front-end
    design cannot be made by the JRDG alone. It has consequences for the
    back-end group (input power range), as well as for the "Calibration and
Imaging"
    group (atmospheric transparency calibration), and should be made under
control
    from the Systems group.

Since I understood that this topic is now becoming a bottleneck in the
receiver design,
I suggest we discuss that at the next telecon, and decide on strategies to
get as fast
as possible to a decision.

        Best regards,

            Stephane


Dr. Stephane GUILLOTEAU
ALMA European Project Scientist                Phone: (33) 476 82 49 43
(IRAM)
IRAM                                                            FAX:
(33) 476 51 59 38  (IRAM)
300 Rue de la Piscine                                  Phone: (49)  893 200
6589 (ESO)
F-38406 Saint Martin d'Heres
        France                                                  E-Mail:
guillote at iram.fr    (IRAM)

sguillot at eso.org  (ESO)





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