Notes/MillimetreObservations?

Add notes here about things in miriad that may have to change in relation to calibration and processing of data from the mm-wave receivers.

12mm

7mmm

2007-09-04 Discussion with Maxim and Ilana

• Will the 2GHz bandwidth mean that we need to worry about opacity varying with frequency across the band?

The band covers about 32-42 GHz. It is defined by an Oxygen absorption at one end and water at the other. Probably there will not be any features to worry about, but some experiments should be done. One place to try is at Mopra which has 8GHz of bandwidth and can do dips or paddle calibration (a good summer student project, perhaps?)

The task to examine is opcor. It's unclear how complex the frequency dependency is.

Subject: RE: opacity @7mm
Date: Thu, 19 Apr 2007 15:06:23 +1000
From: Bob Sault <rsault@physics.unimelb.edu.au>
Reply-To: <rsault@physics.unimelb.edu.au>
To: <Ilana.Feain@CSIRO.AU>

Well ... my guess (without having done any analysis to
half convince myself!) is that 7mm opacity could
be handled the same way as 12mm opacity.

At the risk of repeating material you will know
already - but just to make sure we are talking
the same language ...

For the 12mm system, you have a system temperature
measurement from the noise diode. This is a system
temperature at the feed horn, rather than an above
atmosphere system temperature. As such, it corrects
for the effects *except* for opacity. In Miriad
ATLOD, the opacity is estimated using a model
atmosphere. The inputs to the model are temperature,
humidity and pressure. It includes propagation
effect models which are a function of frequency and
elevation. It assumes clear skies (no clouds). The model
will be better than nothing, but it is still just
a model, and it is trying to simplify a possibly
complex atmosphere to a few parameters.

However the calibrator and the program source will
be in the same part of the sky: the opacity difference
between program source and calibrator should be small
to start with, and the residual difference should be
smaller still after the opacity model correction. A
bigger deal is the difference in the opacity from
observations of the flux calibrator and the
secondary calibrator.

My guess is that you can do the same at 7mm as 12mm.
I assume 7mm has a noise diode??

One problem with sky dips (which I took too long to
appreciate) is that the antennas are only ~60% efficient.
That means that effectively 40% of the collecting area is
looking at other parts of the sky. Because the lower
elevations in the beam are asymmetrically "hotter" than
closer to the zenith, this biases sky dips, and they
do not give the right results. I found, for example,
that the inferred T_receiver was always poorer when
the weather was poorer - which implies the deduced
opacities were on the optimistic side (note T_receiver
and T_sky should be independent).

--

3mm

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