Tuesday, February 8, 2011

LOFAR - the Largest Telescope in the World....?

The International LOFAR Telescope (ILT) claims to be the largest telescope in the world..... why?

Obviously radio telescope are much larger than telescopes in any other wavelength. The largest optical telescopes at the moment have mirrors up to about 10 metres across - an extremely impressive feat considering the small wavelength of the light they observe (which means the mirrors have to be extremely smooth).  But radio waves are much larger (metres in size instead of nanometres) so radio telescopes have to be proportionally larger.

Mist filling the Arecibo Radio Telescope Dish. Credit: Karen L. Masters (June 2000). 

 Single dish radio telescopes are in operation which are up to 30 times larger than the largest optical telescope (the giant Arecibo radio telescope  shown above is 305 metres across). Dwarfing even those though are the interferometers. These are networks of radio telescopes or antennas which use computers to combine the signals they collect and make virtual telescopes with massive sizes. As computers have got more and more powerful, it is this kind of radio telescope which makes the most sense to build, and at the moment the ILT is the largest of of them all.

The LOFAR Superterp (part of the core) in the Netherlands. Credit: ASTRON

Once you move to "virtual telescopes" like this though, there are several different ways to describe the size of the telescope. The most obvious is the width of the array (the largest distance between two antennas in it). This sets the resolution of the images your array will take (the sharpness, or zoom level if you like). Then there's the surface area - basically how much of the area inside your array is covered by radio antennas or dish. This helps to set the sensitivity of the telescope - the more of the radio signal you can collect the fainter the objects you'll be able to see. And finally there's how many different antennas you have. Some arrays are made of a small number of fairly large radio telescopes, others (like LOFAR) have a larger number of smaller antennas arranged in stations.

So how does LOFAR compare with some of the other radio interferometers out there. Well I have collected some numbers in the below table. As you can see LOFAR (or the ILT to use the proper name) is one of the largest in terms of maximum dimension - however both the US and European VLBI (Very Long Baseline Array) facilities have larger sizes by that metric. But LOFAR easilly beats everything at the moment in terms of the number of antennas, number of stations, and (effective) collecting area. So it can really claim to be the largest telescope in the world.

Of course larger facilities are being planned. LOFAR in many ways is a pathfinder for the SKA (Square kilometre array - the name of which refers to the goal of having 1km^2 collecting area (1 million m^2). The details of exactly how this will be achieved are still being worked out, but it's likely to involve very large numbers of antennas. Also in the construction/design phase are many other similar facilities like ASKAP (The Australian SKA pathfinder), MWA (The Murchison Wide Field Array) and LWA (the Long Wavelength Array) all consisting of large numbers of dipoles arranged in stations (edit: thanks to John Swinback for pointing out ASKAP is actually an array of 36 12m dishes - not dipoles).

So LOFAR's claim to be the largest telescope in the world likely won't be true for long. We live in exciting times for radio astronomy.

Table comparing properties of LOFAR (ILT) with other currently running large radio telescope arrays (note that that other facilities listed below also work at shorter radio wavelengths than LOFAR).
NameLocationNumber antennasNumber stations(Effective) collecting areaMax dimension
LOFAREurope2976 LBA and 1680 HBA tiles (a)31 (b)Upper limits range between 396800 m^2 (LBAs at 15 MHz), 14000 m^2 (HBAs at 240 MHz) for 31 stations (c) 920 km (d)
EVLAUSA27 dishes2727*25m dishes = 13250 m^236 km
European VLBI NetworkEurope16 dishes16 (e)11000 km
VLBAUSA10 dishes1010*25m dishes = 4900 m^28600 km
eMERLINUK7 dishes76*25m dishes, plus Lovell. (76.2m) = 7500 m^2217 km

(a) Dutch stations have 96 LBA and 48 HBA tiles each, International stations have 96 LBAs and 96 HBA tiles each. Currently 4 international stations (384 LBA and 384 HBA times), and 27 Dutch (2592 LBAs and  1296 HBA tiles), although each HBA tile actually has 16 antennas in it - so could count this as 20736 Dutch HBA antennas and 6144 international HBA antennas.

(b) 31 stations "completed" as of Feb 2011, 9 more under construction. Current status: http://www.astron.nl/~heald/lofarStatusMap.html

(c) This is wavelength dependent for dipoles. Numbers based on Discussion of LOFAR effective area on ASTRON website.

(d) Largest dimension at present is Tautenburg-Chilbolton which is 920 km.

(e) Still to work out. Need to find specs for each dish.


  1. Great comparision!

    What about MeerKAT - 64 x 13.5m dishes?

  2. Fred - thanks. You're quite right, MeerKAT is another example of a large interferometer currently in the design/installation phase. I'm sure I missed out more than that one!