Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 May 28 
Posted on 05/27/2002 9:37:39 PM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 May 28
Explanation: The most photogenic array of radio telescopes in the world has also been one of the most productive. Each of the 27 radio telescopes in the Very Large Array (VLA) is the size of a house and can be moved on train tracks. The above pictured VLA, celebrating its twenty-second year of operation, is situated in New Mexico, USA. The VLA has been used to discover water on planet Mercury, radio-bright coronae around ordinary stars, micro-quasars in our Galaxy, gravitationally-induced Einstein rings around distant galaxies, and radio counterparts to cosmologically distant gamma-ray bursts. The vast size of the VLA has allowed astronomers to study the details of super-fast cosmic jets, and even map the center of our Galaxy. An upgrade of the VLA is being planned.
Get on the APOD PING list! 
I was going to ping you to this but I see that petuniasevan beat me to the punch ...... :-)
Hi! thanks for the ping. Indeed the choice of frequencies is rather an interesting challenge:
There are two real sources of noise that limits the radio astronomer's ability to search for very weak signals. 1) The Galactic noise halo interferes with us below 1Ghz and noise due to earth's atmosphere interferes with us above about 10Ghz. This pretty much keeps all SETI searches (at least radio ones) between 1 and 10Ghz. Between the two, the noise is around the 2.7K background from 1.4 to 7Ghz. This is why most of the SETI searches are around the frequencies that the OH (hydroxyl) and H (hydrogen) molecules masers emit. This is the so-called water hole.
Most of the SETI searches are near the 21cm band. Since this is one of the coveted frequencies of radio astronomers, we have international treaties to not broadcast at this frequency at all. So here we are looking for signs of a narrowband signal heralding the fact that intelligent life is not wholly constrained to this little planet at this frequency. Now if any intelligent race develops radio and radio astronomy, they too will recognize the importance of this 21cm band. And they also may instigate a SETI search using this frequency. So here is the question. Would they hear us at that frequency? It is the one that we are not transmitting on at all. I could just see 500 races all looking for each other at the very frequency none of them are transmitting on due to the very nature of the importance of that frequency to the exploration of the universe
Thus our search is approximately 3 times that frequency and we are looking at over 3 million channels simultaneously.
I believe the 21 cm band is the "hydrogen" band...My problem is that I've never understood why that section is quiet? Does a Maser Absorb hydrogen frequency broadcasts? Filtering noise of natural origin? I am totally ignorant of what a maser is and how it "clears" the water hole...If you're ever in Seattle, I will be your Paspartu!!! Thanks in Advance....and Good Morning!
I have now entered the realm of far out ideas...We're not listening in the above 10GHZ range because Earth's atmosphere fuzzes up reception...We're blind to any "tool users" that operate outside of a gaseous "noiseball" planet. The mind reels! I can see I'm going to have to go over your answers to grok all the implications...This is so like Kennedy's moon speech..."We choose to do it because its HARD"...\:-D tip of the lid...
The Neutral hydrogen (H1) in the interstellar regions (mostly contained in the plane of the galaxies) emit at the 21 centimeter band due to its microwave transition. We then are able to us radio telescopes to detect this emitted frequency and map the H1 regions. This 21 cm band fall within the quiet band from about 1 to 10Ghz.
This emission process by the Hydrogen is a Maser (Microwave Amplification by Stimulation Emission of Radiation).
This is why it would be marvelous to have a radio telescope array on the far side of the moon. Not only would there be no atmosphere to contend with, but the moon would block all earth generated interference as well.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.