Outline:1. A plea to the industry: reduce interference!2. The SKA: Australias role now and in the future3. What do we do at UTas? VLBI for astronomy and geodesy
1. Interference is a problem in Radio Astronomy 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.Data taken 31/5/12 at 1.43 MHz, UTas Mt. Pleasant radio observatory
In this test, I am looking for the21-cm line from atomichydrogen in the interstellarmedium – the gas between thestars.The direction is toward theLarge Magellanic Cloud, thenearest galaxy to the Milky Way.The line rest frequency is1420.4 MHz, shifted by theDoppler shift to about 1419MHz.
The radio frequency band in theHobart area is very crowded!Here we are zooming-in on justthe frequency of the spectralline from hydrogen.
As we zoom in closer, we cansee the hydrogen line. But thespectrum is polluted by baselineripples.A standard technique to dealwith these is to point thetelescope off-source, measure areference spectrum, and divide.
Here is the differencespectrum, (source – reference) /referenceThe ripples come from theinterference spikes at otherfrequencies, some aliasing intothe band from outside.Now we cant see the line at all.
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. 5minutes), 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 theoptical, from the Hubble SpaceTelescope:
"In practice, man-made in-band signals that exceed the levelsof detrimental interference given in this Recommendationcannot be easily removed from the data... These thresholds ...are ... the values above which radio astronomical data aredegraded or completely obliterated." (13 July 2011)
Australia doesnt have highmountains, for siting new world-classoptical telescopes, but we do have someof the best locations in the world forradio astronomy. And we have the mostadvanced equipment and the worldleaders in radio astronomy research.So lets protect our scientific resource:The Radio Frequency Spectrum.
The Square Kilometre Array (SKA)(artists 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 ASKAPa 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).
(below 500 MHz)AIP = Advanced Instrumentation Program SKA2 = Phase 2 SKA (post 2020, JD guess)SKA1 = Phase 1 SKA (first 5 to 10 years)
Phased Array LFD xNTDAlternate idea: replace mechanicalpointing, beam forming by electronic means
SKA_Low will probably look like the Murchison Widefield Array, aninternational collaboration managed by the International Centre forRadio Astronomy Research (ICRAR) at the Curtin University ofTechnology and the University of Western Australia. A 32-tileprecursor has been in use for 2+ years. Now the build-out to 128tiles is underway (completion end 2012).
The UTas radio astronomy research group has 6PhD students, 4 postdoctoral fellows, 3 seniorscientists and 4 professional staff.Research topics include active galaxynuclei, the interstellar medium, starformation, astrophysical masers, and geodeticVLBI.I will mention some interesting VLBIprojects, because they make good pictures.
The Japanese VSOP (VLBI Space Observatory Program) 1996-2005 with heavy UTas collaboration. VLBA +VSOPMany smaller spacecraft have providedunique astronomical data. Typically thesehave specific objectives, lower cost, fasterdevelopment time, and smaller communities.
Geodetic VLBIVLBI is used for astronomy, and also forgeodesy. The most important geodeticapplication is to determine the Earthrotation parameters, to support GPSand other global navigational satellitesystems (GNSS). The goal is to establishand maintain the international celestialreference system.
http://auscope.phys.utas.edu.au/webcams.htmlThe AuScope VLBI array was constructed with funds from the National CollaborativeResearch Infrastructure Strategy through AuScope, Ltd. It is currently operated forgeodetic VLBI with funding from AuScope and Geoscience Australia.see www.auscope.org.au and www.ga.gov.au
VLBI measurements of continental drift:http://lupus.gsfc.nasa.gov/vlbigallery.htm
UTas is a member of the International VLBI ServiceThe 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.
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 theearth’s surface
get this data from http://hpiers.obspm.fr/eop-pc/The length of a day varies by a few hundred micro-secondseach day. This is partly predictable, partly unpredictable.
IVS measured position of TIGO (Conception, Chile) vs. Hobart
Conclusions:- We will work together to protect radioastronomy bands from interference!- Australia is the leader in SouthernHemisphere radio astronomy. We willwork with other nations to build the SKA.- UTas is training the next generation ofhands-on, astro- and radio-physicists.