This blog is moving

This blog is now being integrated into my main web-pages and is
therefore moving to the following address

http://www.mrao.cam.ac.uk/~bn204/blog/

All of the articles have been transferred there and all new posts will
go there only. Thanks for reading and please update your Atom/RSS
readers!

Astrophysical techniques graduate course

I've just finished lecturing the astrophysical techniques course -- if
anybody is interested all of the slides are available here:

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

Atmospheric Phase Correction for ALMA & The WVRGCAL program

The slides of this talk that I gave at the NRAO ALMA Software
Development Workshop are available here:

http://www.mrao.cam.ac.uk/~bn204/publications/2011/2011-October-Charlottesville.pdf

A Better Interface Between Scientists and Data Reduction Software

This is a talk that I have at the NRAO ALMA Software Development
workshop
(https://science.nrao.edu/facilities/alma/naasc-workshops/almasoft2011/index). The
slides are here:

http://www.mrao.cam.ac.uk/~bn204/publications/2011/2011-10-cv-workshop-slides.pdf

And this is the abstract

I this talk I will argue that, although it has much improved in recent
years, the typical way we interact with data reduction software (that
is, the software that turns the bulky observed measurements into a
smaller data set of images, spectra, etc, that is then further
interpreted) is still not ideal. Some particularly important
shortcomings are:

- Operations on data are performed only ever strictly in the sequence
supplied by the user (either interactively or through
functions/scripts)

- Commands available to the user often combine instructions about what
needs to be done with the details of how this is done

- Information on (potential) problems in the data is difficult to
share across different observations

The consequences of these apparently simple shortcomings are that data
reduction is much more labour and computer intensive than it needs to
be. Since with ALMA, EVLA and forthcoming SKA precursors we are
likely to be short both of available labour and computing power, I
suggest that investing in better interfaces is essential.

In the second part of this talk I will present a set of straw-man
requirements for a better interface between scientists and data
reduction software and show how these requirements would resolve the
shortcomings listed above. I will discuss a possible way of
implementing these and show that such a system can easily build on top
of existing data reduction systems such as CASA. I will argue that the
three benefits of such a system would be:

- Increasing the quantity of science from each scientist and kWh of
compute power by improving their efficiency

- Improving the quality of science by easing repeatability and
reducing scope for error

- Making it easier for user scientists to send instructions to the
observatory on how their data needs to be reduced -- an essential
feature for forthcoming telescopes with extremely high data rates

Atmospheric Phase Correction for ALMA with 183 GHz Water Vapour Radiometers -- URSI GA 2011 paper

The PDF of the paper is available here:

http://www.mrao.cam.ac.uk/~bn204/publications/2011/nikolicga11.pdf

The PDF of the slides of the presentation at the conference:

http://www.mrao.cam.ac.uk/~bn204/publications/2011/nikolicwvr11.pdf

Abstract:

One of the great challenges for ALMA is overcoming the natural limits set by the turbulence in the atmosphere to achieve resolutions as fine as ten milli-arcseconds. A critical component in the strategy to achieve this are mm-wave radiometers on each of the 12m diameter telescopes that observe the emission from the atmospheric water vapour line at 183GHz. The information from these radiometers can be used to compute the fluctuations in total water vapour along the line of sight of each telescope, and from this, the fluctuation in effective path to each antenna. The estimates of path fluctuations are then used to phase-rotate recorded visibilities leading to much increased coherence. In this paper we briefly review the design of the radiometers, describe the software processing steps to derive phase corrections and show some of the first results from the ALMA site in Chile.

ESO Messenger article on tests of radiometric phase correction at ALMA

The PDF is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2011/messenger-no143-11-15.pdf

Talk at the University of Cambridge

I gave the following talk here at the University of Cambridge:

Abstract:

Observed broad-band radio spectra of both galactic and extra-galactic
objects contain valuable information about their energetics, dynamics
and the physical processes involved in the radio emission. Extracting
this information is however complicated by the fact that there are often
several alternative models for the radio emission; and, by the many
parameters in these models which are only approximately a-priori
constrained.

I will describe a computer program for straightforward Bayesian analysis
of radio spectra based on the nested sampling algorithm. Such an
analysis provides for objective model selection, easy handling of
nuisance parameters, and of course a natural way to handle a-priori
constraints on parameters. I will show some example analyses: a
supernova remnant, a star-forming region in a near-by spiral, and a
number of nearby (U)LIRGs.

PDF: http://www.mrao.cam.ac.uk/~bn204/publications/2010/2010-11-02-Cambridge.pdf

Talk at the ALMA EU FP6 Enhancement meeting at RAL, UK

This talk reviews some of the progress in the last year in our work on
advanced techniques for radiometric phase correction for ALMA (i.e., WP5
of the ALMA Enhancement programme). The PDF is available here:

http://www.mrao.cam.ac.uk/~bn204/publications/2010/2010-10-14-Annual.pdf

Talks at the Observatory of Cagliari

Recently I gave three talks at the observatory of Cagliari:

* Fitting and Comparison of Models of Radio Spectra, with updated slides
on the modelling of spinning dust emission:

http://www.mrao.cam.ac.uk/~bn204/publications/2010/2010-09-Sardinia.pdf

* 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/2010/Sardinia10.pdf

* OOF Software: Its Structure and How to Extend It

http://www.mrao.cam.ac.uk/~bn204/publications/2010/2010-09-OOFSoftware.pdf

Talk on the Oxygen sounder thermal profiler for ALMA

The PDF is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2010/2010-08-General.pdf

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

Anomalous dust emission in NGC 6946

New paper out today...

The abstract:

We report new cm-wave measurements at five frequencies between 15 and
18 GHz of the continuum emission from the reportedly anomalous
"region 4" of the nearby galaxy NGC 6946. We find that the emission
in this frequency range is significantly in excess of that measured at
8.5 GHz, but has a spectrum from 15-18 GHz consistent with optically
thin free-free emission from a compact H II region. In combination
with previously published data we fit four emission models
containing different continuum components. These fits show that, in
combination with data at other frequencies, a model with a spinning
dust component is slightly preferred above those which possess more
well established emission mechanisms.

PDF:

http://www.mrao.cam.ac.uk/~bn204/publications/2010/N6946.pdf

Software:

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

Talk on fitting of radio spectra at the JAO, Santiago, Chile

The PDF is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2010/2010-JAO-intro.pdf

Introductory talk at the Kavli Institute for Cosmology, Cambridge

This is the brief introductory talk that I gave at the Kavli Institute for Cosmology, Cambridge:

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

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