June 28, 2010

Wider Bandwidths and a New 12m Antenna will keep 
the Arecibo Radiotelescope at the cutting edge of
science By: Chris Salter / Tapasi Ghosh


12m reference antenna                                                                                    Photos by: Tony Acevedo

Very Long Baseline Interferometry (VLBI) is the technique used to combine signals from radio telescopes separated by distances that can span different continents, or even reach between the Earth and an orbiting
space craft. The signals are recorded simultaneously at the various antennas via special computer disc packs. These are then shipped to dedicated centers, where the signals are combined in a "VLBI correlator", and the results from this sent to the principal investigator for processing into images intelligible to humans. These VLBI images represent the highest resolution pictures of the heavens yet produced, typically revealing milliarcsecond-sized detail in celestial objects; (a milliarcsecond is the angle a US quarter would subtend at a distance of about 5200 kilometers!)  Recently, a new version of VLBI, named eVLBI, has been developed. This dispenses with the "disc recording and shipping" stages by sending the signals direct to the VLBI correlator via the internet. The
Arecibo 305-m telescope is an active participant in both of these forms of VLBI.

To assist Arecibo VLBI operations, a new 12-m antenna from Patriot Ltd. has recently been installed on a hill directly overlooking the observatory gatehouse.  From this hill top, some 800 meters from the center of the 305-m telescope, this 12-m dish will have a clear view of almost the whole "sky" down to its 5 degree elevation limit. Its surface is capable of supporting operations up to the 10-GHz limit of the 305-m dish -- and then some! However, for the foreseeable future the plan is for it to operate between 1 and 10 GHz, starting with receivers at 2.5 and
8.5 GHz.  Soon, a single wide-band receiver system will replace these, borrowing on the design efforts underway for the VLBI2010 geodesy project, and the Square Kilometer Array (SKA) and its precursors.

The prime motivation for investing in this small telescope lies in the area of "phase-referenced VLBI" for very high resolution astronomical imaging of faint sources, in which the 305-m telescope is expected to slew between a given astronomical target and an angularly nearby reference source.  The 305-m telescope has, due to its size, a slow slew speed, meaning that some of the great sensitivity provided by its huge diameter is lost due to the time taken for it to move between target and reference sources. In future, the small dish will track the reference source, meaning that its "big brother" need only observe this occasionally. Calibration values derived from combining signals between the 12-m antenna and other telescopes in a given VLBI array will then be used to correct the 305-m's signal combinations wth the other dishes,
as if it, itself, had observed the reference source.

As well as its usage as a "reference antenna" along with the 305-m, the 12-m antenna can itself stand in for the 305-m telescope in VLBI work on the stronger radio sources.  Especially, the  design of this fast-slewing 12-m dish makes it highly attractive for geodetic VLBI observations used to measure the rotation parameters of the Earth, continental drift, etc.  It will also be possible to connect the signals from this small telescope to Arecibo's existing signal processors, opening up its use as a stand-alone telescope for education, outreach and a range of single-dish research projects.

In addition to the acquisition of the 12-m antenna, and to keep in step with the rest of "World VLBI", Arecibo is presently upgrading its VLBI instrumentation. Recent developments in digital data acquisition systems and disk recorders will allow the 305-m dish to contribute its full receiving band of 1 GHz to VLBI endeavors in the near future.