Radcomms 2012Radio Astronomy

John Dickey, HoS Maths & PhysicsUniversity of Tasmania

6 June 2012

Hobart 26m telescope

Ceduna 30m telescope

Outline:

1. A plea to the industry:

reduce interference!

2. The SKA: Australia's role now and in the future

3. What do we do at UTas? VLBI for astronomy and geodesy

1. Interference is a terrible problem in Radio Astronomy

Data taken 31/5/12 at 1.43 MHz, UTas Mt. Pleasant radio observatory

Interference for us is like clouds for an optical astronomer. Here is the spectrum at L-band, as observed in a little test last Thursday evening.

Internationally protected bandfor passive use only (21-cm line)

In this test, I am looking for the 21-cm line from atomic hydrogen in the interstellar medium – the gas between the stars.

The direction is toward the Large Magellanic Cloud, the nearest galaxy to the Milky Way.

The line rest frequency is 1420.4 MHz, shifted by the Doppler shift to about 1419 MHz.

The radio frequency band in the Hobart area is very crowded!

Here we are zooming-in on just the frequency of the spectral line from hydrogen.

As we zoom in closer, we can see the hydrogen line. But the spectrum is polluted by baseline ripples.

A standard technique to deal with these is to point the telescope off-source, measure a reference spectrum, and divide.

Here is the difference spectrum, (source – reference) / reference

The ripples come from the interference spikes at other frequencies, some aliasing into the band from outside.

Now we can't see the line at all.

The same data, after a whole lot of work:

Finally, we can see the sky:

21-cm line from the Milky Way

But not the galaxy we were looking for:

21-cm line from the Large Magellanic Cloud ?

This is how it looked 20 years ago at Narrabri:

from Dickey et al. 1994, Astron. Astrophys. 289, 357.

Fair disclosure: this spectrum had a longer integration (5 hours vs. 5 minutes), with a more powerful telescope, in a more remote site (Australia Telescope Compact Array – near Narrabri, NSW, in 1992 July).

This is what it looks like in the optical, from the Hubble Space Telescope:

"In practice, man-made in-band signals that exceed the levels of detrimental interference given in this Recommendation cannot be easily removed from the data... These thresholds ... are ... the values above which radio astronomical data are degraded or completely obliterated." (13 July 2011)

Australia doesn't have high mountains, for siting new world-class optical telescopes, but we do have some of the best locations in the world for radio astronomy. And we have the most advanced equipment and the world leaders in radio astronomy research.

So let's protect our scientific resource: The Radio Frequency Spectrum.

The Square Kilometre Array (SKA)(artist's conception of the South African design)

ASKAP = the Australian Square Kilometre Array Pathfinder36 dishes of 12m diameter

The Australian Square Kilometre Array Pathfinder (ASKAP) under construction by CSIRO Astrophysics and Space Science

The Phased Array Feed receiver gives ASKAP a huge effective primary beam (30 sq deg).Note: a single 12m diameter dish has primary beam area = 0.89 sq deg.The point source sensitivity is the same as Parkes and the ATCA (total collecting area).

AIP = Advanced Instrumentation ProgramSKA1 = Phase 1 SKA (first 5 to 10 years)

SKA2 = Phase 2 SKA (post 2020, JD guess)

(below 500 MHz)

Alternate idea: replace mechanical pointing, beam forming by electronic means

Phased ArrayxNTD LFD

SKA_Low will probably look like the Murchison Widefield Array, an international collaboration managed by the International Centre for Radio Astronomy Research (ICRAR) at the Curtin University of Technology and the University of Western Australia. A 32-tile precursor has been in use for 2+ years. Now the build-out to 128 tiles is underway (completion end 2012).

http://www.phys.utas.edu.au/physics/index.html

The UTas radio astronomy research group has 6 PhD students, 4 postdoctoral fellows, 3 senior scientists and 4 professional staff.

Research topics include active galaxy nuclei, the interstellar medium, star formation, astrophysical masers, and geodetic VLBI.

I will mention some interesting VLBI projects, because they make good pictures.

Many smaller spacecraft have provided unique astronomical data. Typically these have specific objectives, lower cost, faster development time, and smaller communities.

The Japanese VSOP (VLBI Space Observatory Program) 1996-2005 with heavy UTas collaboration.

VLBA +VSOP

UTas observations 10 May and 18 May, 2012

Geodetic VLBIVLBI is used for astronomy, and also for geodesy. The most important geodetic application is to determine the Earth rotation parameters, to support GPS and other global navigational satellite systems (GNSS). The goal is to establish and maintain the international celestial reference system.

Yarragadee

Katherine

Ceduna

Hobart

ASKAP

http://auscope.phys.utas.edu.au/webcams.html

The AuScope VLBI array was constructed with funds from the National Collaborative Research Infrastructure Strategy through AuScope, Ltd. It is currently operated for geodetic VLBI with funding from AuScope and Geoscience Australia.see www.auscope.org.au and www.ga.gov.au

http://lupus.gsfc.nasa.gov/vlbigallery.htm

VLBI measurements of continental drift:

UTas is a member of theInternational VLBI Service

The IVS is a collaboration of 29 radio telescopes thatkeep track of the Earth Orientation Parameters, numbersthat tell how the earth is rotating and which way the axisis pointing. These are used to calibrate GPS and all otherlocation-finding equipment.

Stations of the International VLBI Service

VLBI telescopesused for geodesyin the southernhemisphere -today

VLBI telescopesused for geodesyin the southernhemisphere -after AuScope

Motion of theearth’s polaraxis (north)measured onthe sky, relativeto extragalacticradio sources.

1 mas ~ 10-9 ofa circle (one4-millionth ofa degree)~ 3 cm on the earth’s surface

The length of a day varies by a few hundred micro-secondseach day. This is partly predictable, partly unpredictable.

get this data from http://hpiers.obspm.fr/eop-pc/

IVS measured position of TIGO (Conception, Chile) vs. Hobart

Conclusions:

- We will work together to protect radio astronomy bands from interference!

- Australia is the leader in Southern Hemisphere radio astronomy. We will work with other nations to build the SKA.

- UTas is training the next generation of hands-on, astro- and radio-physicists.

http://users.on.net/~cdadsl/livepages/webcams.html