Arecibo Diaries: Time to Move Up

Nov. 21, 2003
by Peter Backus - Observing Programs Manager

For most people, the term "Microwave Window" brings to mind a bag of popcorn inflating on a rotating tray. For SETI astronomers, its the portion of the radio spectrum where we can best listen for the faint whisper of a distant civilization. Below a frequency of 1000 MHz the radio hiss of the galaxys electrons rises to a dull roar. Above 10,000 MHz, the atmosphere and other effects make detection difficult. The quiet zone of the cosmic radio dial is in the range from 1,200 to 3,000 MHz. Project Phoenix explores this entire rangewhen possible.

Frequently, observatories have receivers that cover only the lower portion of the range, from 1,200 to 1,750 MHz. Those frequencies include the radio signatures of Hydrogen atoms (H) and Hydroxyl radicals (OH) and define the so called "Water Hole" (H + OH -> H2O). Many SETI projects limit their observing to what some people call the "magic frequencies" associated with H and OH themselves or the Water Hole they define. Project Phoenix takes a more comprehensive approach towards the 700-plus stars on its target list, with an ultimate goal of covering the entire quiet zone, including the range from 1,750 to 3,000 MHz.

Since September 1998, we have visited Arecibo nine times, each visit observing for about 200 hours, always in the lower frequency range. To observe the entire range, its time to move up, a step we are taking that brings with it advantages and disadvantages.

As followers of Project Phoenix know, weve conducted prior observations using a second instrument, the Lovell Telescope in England, which served as an immediate confirmation site for candidate ET signals. Because the Lovell Telescope does not have a receiver for the upper part of the frequency band, our Arecibo observations were all in the lower frequency range.

To attain our goal of observing in the entire "quiet zone," we have consolidated our New Search System equipment in Arecibo, where we will now observe 56 MHz at a time. It will take more time to check on candidate signals without a second telescope, but we should have just enough time to cover stars weve observed previously only at the lower frequencies.

The last time we were able to observe our target stars at the complete range of relevant frequencies was in 1998, at the NRAO in Green Bank, West Virginia. The NRAO was in a national radio quiet zone, and we saw very little interference. Much has happened in this part of the spectrum since then. We now have to contend with satellite radio and new cell phone frequencies. With our sensitive new equipment and a bit of luck, we just might hear a distant hail from a world far away, despite our own cacophony.

In the meantime, that microwave popcorn will have to wait until I return to California. Radio astronomers frown upon microwave ovens cluttering their window with spurious signals!