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April 2003 Sky from the Keeble Observatory
The "satellite dish" is a ubiquitous sign of space technology. Communication satellites, in particular the "geosynchronous" satellites orbiting above Earth's equator, have permitted nearly instantaneous global communication. That can, of course, be a mixed blessing - bringing both vitally important real news from Iraq, and inane "reality programs" on myriad channels. Whether one has a large, conventional dish or one of the new mini receivers, one thing you've probably noticed is that they all point roughly south, aimed at points about 40 degrees off the horizon.
Why, then, is there a 10 foot dish next to the Keeble Observatory which typically points straight up? That is the antenna for the "Center of the Universe Radio Telescope" - CURT for short. CURT was begun about three years ago by R-MC physics student Scott Lansdale, who graduated in 2002. Scott began the radio telescope project as a one-semester research course, which expanded into a summer research fellowship, and eventually into a two-semester senior project.
Recall our previous description of a typical reflecting telescope. A "primary mirror" collects light and brings it to a focus. For an optical telescope, we often reflect the light from the primary back to the "Cassegrain focus" - behind the primary mirror. This is the design of the 12" telescope in the Keeble Observatory. Indeed, it is the design of virtually every large telescope, including the Hubble Space Telescope. In the design of a radio telescope, the "dish" takes the role of the primary mirror. Rather than attempt to reflect the radio signals back to a receiver at the Cassegrain focus, we collect those signals at the "prime focus." Look at the CURT antenna (or any satellite dish!) and you'll see wires running from a small nose cone held in front of the dish by a "spider" or other structure. This is the location of the feed horn - the actual antenna which converts the radio waves into an electrical signal that can be processed and analyzed.
CURT is designed to receive radio signals at a wavelength of 21 cm - the ubiquitous signature of cold, neutral hydrogen atoms. Technically, this is a "ground state forbidden transition" caused by flipping the spin orientation of the electron in the atom relative to that of the lone proton it the atom's nucleus. Here, "forbidden" means rare - but hydrogen is ubiquitous in the universe, so there's plenty of signal! The mapping of 21 cm radiation from interstellar hydrogen allows us to study the structure of the galaxy. The maps we produce will be at very low resolution, but the ongoing project will allow students first-hand experience observing with an instrument which would have been at the cutting edge of astronomy a mere 50 years ago.
For more information about CURT, go to the web site directly.
Lunar phases for April: New on the 1st, at 2:20 pm; First Quarter on the 9th, at 6:41 pm; Full on the 16th, at 2:37 pm; Last Quarter on the 23rd, at 7:19 am.
Morning planet watchers will note that Venus rises only an hour before the Sun, just about due east all month. Mars is to the southeast at sunrise.
Evening planet watchers will have a richer selection. Jupiter is almost directly overhead at sunset, with Saturn high to the southeast. Jupiter will move through the "Beehive Cluster" early in the month - it's worth a look with binoculars. Mercury is low to the west as the sun dips below the horizon, reaching its maximum eastward elongation on the 16th - look for it just below the Pleiades as twilight begins.
An overhead view after sunset finds Jupiter brighter than all the real stars, midway between the Twins, Castor and Pollux in Gemini and bright Regulus in Leo. Below Gemini is the familiar shape of Orion, which will disappear into the glare of the Sun as summer approaches. To the south-southwest you'll see the only star to rival Jupiter - brilliant Sirius in Canis Major. To the north, Ursa Major's familiar dipper shape is inverted. Find the middle star in the "handle" and see if your eyes can resolve it as two, known as Mizar and Alcor. Now, with binoculars or a small telescope, notice that it's actually four stars! The Milky way runs from due north to due south, but arches toward the western horizon. Notice the rich star fields running through the plane of the Galaxy. Now look to the east - you'll see fewer bright stars, and fewer faint ones, too. Here you're looking out of the plane of the Milky Way. Wait until midnight, and the Milky Way circles the horizon, and zenith marks the direction toward galactic north.
For your own monthly star chart, you can direct your web browser to http://www.skymaps.com. You will find extensive descriptions of what's worth looking for, and you can download and print a single copy for your personal use.