B. Nikolic Research

Atmospheric Phase Correction for ALMA & The WVRGCAL program

2011-10-17T01:55:00.001-07:00

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

2011-10-17T01:54:00.001-07:00

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

2011-08-19T08:43:00.001-07:00

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

2011-03-29T00:18:00.001-07:00

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

2010-11-03T01:34:00.001-07:00

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

2010-10-15T01:15:00.001-07:00

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

2010-09-14T01:59:00.001-07:00

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

2010-08-19T17:41:00.000-07:00

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

2010-04-29T07:19:00.000-07:00

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

2010-04-01T11:39:00.001-07:00

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

2010-03-22T12:21:00.001-07:00

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

2010-01-14T01:29:00.000-08:00

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

2010-01-04T12:28:00.001-08:00

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

2009-12-31T11:44:00.001-08:00

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

2009-12-14T00:12:00.001-08:00

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

2009-11-28T09:15:00.001-08:00

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

2009-11-25T14:46:00.001-08:00

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

2009-11-06T02:17:00.001-08:00

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

2009-10-26T08:14:00.001-07:00

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

2009-10-22T07:14:00.001-07:00

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

2009-09-15T01:21:00.001-07:00

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

2009-09-09T02:18:00.001-07:00

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

2009-09-06T03:08:00.001-07:00

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

2009-08-18T04:21:00.001-07:00

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

2009-08-08T07:35:00.001-07:00

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

2009-08-05T06:17:00.001-07:00

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

2009-07-27T08:57:00.001-07:00

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

2009-07-24T02:06:00.001-07:00

Is available at:

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

JAO/EU FP6 ALMA Enhancement meeting

2009-07-23T13:18:00.001-07:00

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

European Union FP6 ALMA Enhancement at Cambridge

2009-06-23T08:13:00.001-07:00

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

2009-04-30T03:42:00.001-07:00

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

2009-03-24T06:35:00.001-07:00

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

2009-03-20T09:57:00.001-07:00

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

2009-03-13T03:38:00.000-07:00

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

2009-02-19T06:37:00.001-08:00

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

2009-02-06T05:25:00.000-08:00

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

2009-01-29T09:14:00.001-08:00

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

2009-01-22T10:03:00.000-08:00

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

2009-01-14T06:58:00.001-08:00

The talk I gave in Charlottesville is available at:

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

Visiting NRAO/Charlottesville

2009-01-12T15:39:00.001-08:00

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

2009-01-07T08:10:00.001-08:00

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

Calculating dispersive path delay due to the atmosphere

2008-12-10T06:50:00.001-08:00

I've posted a simple command-line program to compute the dispersive path delay due to the atmosphere using ATM (more precisely my re-packaging AATM). The program is available at:

http://www.mrao.cam.ac.uk/~bn204/alma/aatm-dispersive.html

I had not appreciated that even at 330GHz, the dispersive wet delay is about 10% of the non-dispersive wet delay.

Out-Of-Focus holography with mm-wave cameras

2008-12-10T01:24:00.001-08:00

Recently, we (meaning the groups at NRAO, at University of
Pennsylvania and myself) have made progress on enabling the OOF
holography technique (see http://www.mrao.cam.ac.uk/~bn204/oof/) on
large-format radio wavelength cameras, in particular the MUSTANG array
at the GBT.

This work is being documented on the following wiki:

http://wiki.gb.nrao.edu/bin/view/PTCS/OOFHolography

The oof software is available for download at:

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

Finally, as an illustration, below is a map of the GBT surface
re-created from out-of-focus MUSTANG images. Although this particular
observation took of the order of 15 minutes, it is likely we will be
able to reduce this to the order of a couple of minutes, allowing
effective and rapid correction of all gravitation and thermal effects
on the telescope.

My talk at the Oxford Algorithms Workshop

2008-12-02T01:24:00.001-08:00

The talk I gave at the Oxford Algorithms workshop is available at:

http://www.mrao.cam.ac.uk/~bn204/publications/2008/AlgorithmsOxford09.pdf

Almost all of the talks I've given recently are available too:

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

Algorithms meeting in Oxford

2008-12-01T03:19:00.001-08:00

For a part of this week, I'm at the Oxford Algorithms workshop:

http://astrowiki.physics.ox.ac.uk/cgi-bin/twiki/view/Algorithms2008/WebHome