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

CASA -- some opinions and suggestions on software engineering aspects

I've posted a very short document with some suggestions for further development of the CASA (http://casa.nrao.edu/) package here:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/CASARev2009.pdf

Any comments & opinions on my opinions very welcome!

First WVR and interferometric observations at the OSF

The first simultaneous WVR and interferometric observations at the OSF were taken on the night of 4th October at 230 GHz. There are some quick plots linked from this ALMA Science IPT meeting page:

https://safe.nrao.edu/wiki/bin/view/ALMA/21Oct09Agenda

They look encouraging...

Science at Q-band meeting, Manchester

I am at the science at Q-band meeting for the first part of this week:

http://www.alma.ac.uk/documents/science-at-q-band-early-summer-2009

See you maybe here...

Radio interferometry at millimetre and sub-millimetre wavelengths -- lecture at ERIS

This is the lecture I gave at the European Radio Interferometry School:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/NikolicERIS2009.pdf

European Radio Interferometry School 2009

I will be at the European Radio Interferometry School in Oxford for a couple of days next week:

http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome

See you maybe there...

New version of AATM

I've released a new version of the ALMA atmospheric modelling library.

You can view what is new and grab the source code at:

http://www.mrao.cam.ac.uk/~bn204/alma/atmomodel.html#Current+version%3A+V0.08

As a side note, we are experimenting with the use of the collaborative platform launchpad. The project page is:

https://launchpad.net/aatm

The talk at JAO/Santiago

Here is a link to the talk I presented here in Santiago:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/ALMAAugust2009.pdf

Memo on inference of phase correction coefficients

This is the second memo in the series.

Abstract:

By observing bright and compact astronomical sources while also taking data with the 183 GHz Water Vapour Radiometers, ALMA will be able to measure the `empirical' relationship between fluctuations in the phase of the astronomical signal and the fluctuations of sky brightness around 183 GHz. Simulations of such measurements assuming only thermal noise in the astronomical and WVR receivers are presented and it is shown that accurate determination of the empirical relationship should be possible in a relatively short time. It is then proposed that the best way of using these empirical coefficients is to include them as a constraint on a physical model of the atmosphere -- this allows them to be used for longer period of time, increasing the efficiency of observing. This approach fits naturally into the analysis framework presented in the previous memo, which has now been extended to implement it. The technique is illustrated via simulations and on a short data set collected at
the SMA.


Full text:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/ALMAMemo588.pdf

Visiting ALMA Santiago office and the ALMA Site

For the next two weeks I'm in Chile visiting the ALMA offices in Santiago as well as the ALMA site itself where we will be doing some experiments with the ALMA Water Vapour Radiometers.

My talk at JAO/ALMA FP6 Enhancement meeting

Is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/ESOJuly2009.pdf

JAO/EU FP6 ALMA Enhancement meeting

I will be at this meeting in Munich on 24th July... See you maybe there.

European Union FP6 ALMA Enhancement at Cambridge

We have put together a small number web-pages that summarise the work that we are doing on advanced phase correction for ALMA through a grant by the European Union Framework Programme 6. The address is:

http://www.mrao.cam.ac.uk/projects/alma/fp6/

Talk on OOF holography at Cambridge

I gave a talk here in Cambridge this week on the OOF holography technique, link and screen shot below

Title: The OOF Holography Technique: Correcting the Effects of Gravity and Thermal Gradients on Large Filled-Aperture Telescopes

http://www.mrao.cam.ac.uk/~bn204/publications/2009/MRAOApril09.pdf

Abstract:

The maximum practical size of large single-dish telescopes is mainly defined by the effects three forces: gravity, thermal gradients, and wind. All of these can in principle be corrected by the new generation of active surfaces on telescopes if we knew the actual deformations that are caused by them. At current cm, mm and sub-mm telescopes, measuring these deformations corresponds to measurement of one part in a million which is challenging for conventional survey techniques. In this talk I will introduce the phase-retrieval holography technique based on Out-Of-Focus (OOF) beam maps that is particularly suited to measuring these deformations. I will also present the application of this technique to the 100m-diameter Green Bank Telescope where we have been able to eliminate residual gain variation due to gravity and open up the daytime for efficient observations at mm-wavelengths.

Early high-z Science with ALMA

Next Monday I will be at the "Early high-z Science with ALMA" in
Oxford:

http://www-astro.physics.ox.ac.uk/~ianh/alma/

Maybe see you there....

New Memo on inference of phase coefficients

I've just posted the completed version of the first memo on the
inference of phase correction coefficients. This the link to the memo:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/ALMAMemo587.pdf

All of the software and data is available for download from here:

http://www.mrao.cam.ac.uk/~bn204/alma/memo-infer.html

And all the other publications are as usual available from:

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

A note on using 183 GHz Water Vapour Radiometers to correct total power observations

I've posted a short note on using 183 GHz Radiometers to correct for
the atmospheric emission fluctuations that can limit total power
observations. The note is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/tpower-v01.pdf

The attached graph is a summary of how well the correction would is
expected to work -- see the article for the detail.

OOF software

I've released a new version of the OOF analysis software which you can
get from:

http://www.mrao.cam.ac.uk/~bn204/oof/software.html#Current+version%3A+V1.3

The main improvements have been in the configuration and building of
the packages which should be more adaptable now. For details see the
NEWS files in each of the constituent packages.

Turbulent 3d fields and bnlib-1.3

I've uploaded a new version the "bnlib" library that amongst other items contains the code to generate very large simulations of turbulent fields. The library can be downloaded at:

http://www.mrao.cam.ac.uk/~bn204/alma/memo-turb.html#Current+version+of+library%3A+V1.3

An illustration of the sorts of effects you can investigate with such
fields here is a slide from a presentation I gave a while back. You
can get the whole presentation at
http://www.mrao.cam.ac.uk/~bn204/publications/2008/ALMASim08.pdf

Bayesian retrieval of water vapour column

This week I've been in part working on Bayesian retrieval of water vapour column from prototype WVR data.

The attached plot shows a typical result: It shows the retrieved *zenith* water vapour column as a function of time while the telescope was tracking a source for about an hour (it was setting at the time). The colour scale represents the posterior distribution and is hence an indication of the confidence in the retrieved values.

You can find out more detail on the retrieval method in my most recent talk at these web pages:

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

casa-core

I've recently tried to build casa-core (http://casacore.googlecode.com) and found that it is surprisingly easy to build on my laptop system (Ubuntu/8.10 32bit). In fact, the only configuration item that required any fiddling was the build of the WCS library. The main gripe is excessively long compilation time.

In order to make management of the casa-core source more flexible, I used the bazaar version control system (http://bazaar-vcs.org/) and the bzr-svn plugin to check-out the casa-core source code. This allows some significant advantages compared to plain SVN :

* Access to full history information without connection to the
internet

* Ability to make local branches

* Ability to layer local changes on top of the revisions contained in
the SVN

If you want to know more, feel free to write to me...

We will be using casa-core to, at the very least, access Measurement Set data produced by ALMA.

Talk at Charlottesville

The talk I gave in Charlottesville is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/NRAOJan09followup.pdf

Visiting NRAO/Charlottesville

For the first part of this week I'm visiting NRAO Charlottesville offices: http://www.nrao.edu/

Talk at the URSI NA meeting in Boulder

I gave a talk about ALMA phase correction and calibration at the URSI
meeting in Boulder. The PDF is available from the usual place:

http://www.mrao.cam.ac.uk/~bn204/publications/2009/NikolicURSI09.pdf