Talk at NRAO/Charlottesville: First Tests of Atmospheric Phase Correction for ALMA with 183 GHz Water Vapour Radiometers

You can get PDF of the talk at:

http://www.mrao.cam.ac.uk/~bn204/publications/2010/2010JanNRAO.pdf

ALMA Memo 590: Atmospheric dispersion and the implications for phase calibration

This memo was posted in December.

Abstract:

The success of any ALMA phase-calibration strategy, which incorporates
phase transfer, depends on a good understanding of how the atmospheric
path delay changes with frequency (e.g. Holdaway & Pardo 2001). We
explore how the wet dispersive path delay varies for realistic
atmospheric conditions at the ALMA site using the ATM transmission
code. We find the wet dispersive path delay becomes a significant
fraction (5 per cent) of the non-dispersive delay for the high-frequency
ALMA bands (160 GHz, Bands 5 to 10). Additionally, the variation in
dispersive path delay across ALMA's 4-GHz contiguous band- width is not
significant except in Bands 9 and 10. The ratio of dispersive path delay
to total column of water vapour does not vary significantly for typical
amounts of water vapour, water vapour scale heights and ground pressures
above Chajnantor. However, the temperature profile and particularly the
ground-level temperature are more important. Given the likely
constraints from ALMA's ancillary calibration devices, the uncertainty
on the dispersive-path scaling will be around 2 per cent in the worst
case and should contribute about 1 per cent overall to the wet path
fluctuations at the highest frequencies.

Full text:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/ALMAMemo590.pdf

Fitting of models of radio spectra -- talk at NRAO/Green Bank

I gave a talk today at NRAO/Green Bank on the fitting of models of radio
spectra. The PDF of the talk is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/specbasic.pdf

While the full detail on the method is available in this paper:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/specanalysis.pdf

Fitting and Comparison of Models of Radio Spectra

I've put a paper on this topic on ArXiv, here is the abstract:

I describe an approach to fitting and comparison of radio spectra based
on Bayesian analysis and realised using a new implementation of the
nested sampling algorithm. Such an approach improves on the commonly
used maximum-likelihood fitting of radio spectra by allowing objective
model selection, calculation of the full probability distributions of
the model parameters and provides a natural mechanism for including
information other than the measured spectra through priors. I this
paper I cover the theoretical background, the algorithms used and the
implementation details of the computer code. I also briefly illustrate
the method with some previously published data for three near-by
galaxies. In forthcoming papers we will present the results of applying
this analysis larger data sets, including some new observations, and the
physical conclusions that can be made. The computer code as well as the
overall approach described here may also be useful for analysis of other
multi-chromatic broad-band observations and possibly also photometric
redshift estimation. All of the code is publicly available, licensed
under the GNU General Public License, at
http://www.mrao.cam.ac.uk/~bn204/galevol/speca/index.html.

As always, the paper is available from the publications list at:

http://www.mrao.cam.ac.uk/~bn204/publications/publicationlist.html

Command line interface for retrieval of phase-correction coefficients

I've made a little command-line program that calls the routines for
retrieval of water vapour and phase correction coefficients that I
described in memos 587 and 588.

So far it only uses the very simplest model and the only input are the
four channel temperatures, but it may be useful for quick checks of
observed data and also of course as an illustration of how the libraries
work. There is more information at:

http://www.mrao.cam.ac.uk/~bn204/alma/libaircmdline.html

and how to build & install at:

http://www.mrao.cam.ac.uk/~bn204/alma/sweng/libairbuild.html

One possible use is using the "evidence" value to check for unphysical
readings from the WVRs.

Let me know if you find any problems or unexpected results.

Best,
Bojan

--
Bojan Nikolic Tel: +44 1223 746432
Astrophysics Group Mob: +44 7894 223621
Cavendish Laboratory Fax: +44 1223 354599
Cambridge CB3 0HE

Re-modelling of the web pages

I have re-modelled my web pages at:

http://www.mrao.cam.ac.uk/~bn204/index.html

Let me know if you think something got lost along the way....

Building CASA using SCons

I've put together a prototype SCons build framework for CASA, here is a brief description:

http://www.mrao.cam.ac.uk/~bn204/alma/sweng/casabuild.html

The advantages are that it will be hopefully clearer and easier to extend the build with SCons, and also the parallel build functionality can accelerate the process significantly. On the computer I've been using the whole build from scratch takes about 18 minutes