Friday, January 23, 2009

The dish

Today was the last day of the meeting and involved a trip to the National Astronomy and Ionosphere Center (aka the Arecibo Observatory - or the big dish where Bond fights Trevelyan at the end of GoldenEye). Travelling there, through the rather hilly and forested terrain, we first got a glimpse of the three ~100m towers that support the receiver instruments from a few miles out. They got more and more impressive as we approach and could really start to appreciate the size of them. When there we got to walk down underneath the dish, which is a mesh of aluminium panels formed into a spherical surface and kept to an (rms) accuracy of 2mm using many metal cables under tension. I'd always been under the false impression that the dish was situated in a crater caused by a meteor strike, or an extinct volcano, but it (and the formation of the generally hilly terrian) is actually a sink hole formed from a collapsed cave system. Rather than resting on the ground the dish actually is supported around the edge and sits quite far above you - the plant and animal life still lives quite happily under there and there's plenty of room for people to wander about. After seeing the underside of the dish we went into the control room, which was quite interesting (for us astronomy types) and consisted of banks of computers and electronic equipment, but wasn't as exciting an experience as looking at the main dish (I've seen computers before). We got into the control room just before a torrential downpour, but other groups touring the telescope weren't quite as lucky and they ended up getting thouroughly soaked. After that we headed up to the main visitors centre from which you got a great view of the platform on which the antenna feeds are placed - there are two main feeds: the older line feed [the thing pointing down in the right of this picture], which sits along the focal point of the spherical dish (waves reflected from the edge of the dish get focused lower down than waves reflected from near the middle, so to get them all your antenna feed has to cover many focal distances); and the newer Gregorian dome [the domey thing in the previously linked to picture], which focuses the waves to a single point. [I may have to incorporate this, and my own photos, into my next lecture]. Unfortunately most of us didn't get to go our to the antenna array platform, which can be reached by a cable car or catwalk, due to restrictions on the number of people who can go at one time - so we didn't get to reinact any GoldenEye fight scenes. It really was a cool place to go and very impressive, so I'd recommend it to anyone visiting Puerto Rico.

I should just note that the next GWDAW meeting (GWDAW 14) is going to be hosted by the University of Rome and will take place around (I think) the 25th Jan 2010 [let's see if this post becomes the number one google hit on searches for GWDAW 14!].