Expert: Jayendra Upadhye - 7/20/2008

Question
hello,

What is the difference between large and small radio telescopes? do big telescopes detect weaker signals than small ones? can both sizes detect man made signals?

Thankyou for your time and help,

Alice


Answer
Hi Alice,
One must understand that in the end both radio and optical telescopes are doing the same thing, concentrating weak electromagnetic signals into stronger ones, by "focusing the beams to a given area".

Just like in optical telescopes, smaller radio telescopes collect "less radio waves" and thus the "image" they form is fainter, and thus one gets "less detail".

The larger the parabolic antenna (Arecibo is so large you know about it), the more "radio signal" it catches, and the stronger is the ultimate "image".

Thus with very large radio telescopes, one can "listen" (in the radio range), or "look", whichever term one prefers to use, and gather muchj more information, than optical telescopes.

Mount palomar and the 200 inch Hale telescopes were the world's biggest optical ones for a long time, followed by the Keck telescope.

Jodrell Bank in the UK, and the VERY LARGE ARRAY telescopes in south america were the high end radio telescopes in size. The very large array is a combination of many radio telescopes synchronously watching the same spot.
The wider baseline gives a sharper image.

Both can detect man made signals, but electromagnetic radiation in the optical region, (shorter wavelength compared to radio) tends to be dispersed and absorbed by the intervening gas and dust. That puts a limit on optical observation.

Radio waves however can pass thru dust clouds easily.

That is why the  "look at the galactic center", perinnially obscured by gas and dust clouds within the spiral arms, had to await the launch of radio telescopes into space, to "observe" it.
That are of the sky (Sagittarius), where the galactic center resides, is more easily observed in radio range than in optical range.

"[Because of cool interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet or soft X-ray wavelengths. The available information about the Galactic Center comes from observations at gamma ray, hard X-ray, infrared, sub-millimetre and radio wavelengths.]"

refer:- http://en.wikipedia.org/wiki/Galactic_Center

For this same reason, the study for signals from distant civilisations has to rely on radi signals in a particularly narrow radio band that passes thru intervening dust etc over large distances.
[example SETI].
Look for it on wikipedia.

hope that suffcies.

Please do rate the answer if you find it helpful.
Jayen