# Stargazing Telescope size and wavelengths

1. May 14, 2010

### Zman

I have been reading an article that refers to telescopes but I don’t really understand telescope sizes;

http://www.eso.org/public/news/eso0226/

For instance take the 3.5-m New Technology Telescope.
I assume that the 3.5m refers to the diameter of the parabolic mirror.

Does the 3.5m give any indication of the range of wavelengths that can be received?

There is also the 10m Keck telescope. Presumably this can detect much fainter objects than the 3.5m but once again does the size of the mirror give more information than the cross sectional area of photons it can make use of?

Both the above telescopes were used to examine the centre of our galaxy.

But radio emmissions are long wavelength and x-ray are very short wavelength.

Can the above telescopes really cope with such a range of wavelengths?

2. May 14, 2010

### tiny-tim

Hi Zman!

No, the telescopes with 3.5m and 10m diameters can only receive visible wavelengths.

The visible photos of the centre of the galaxy must have been compared with radio and x-ray pictures from radio and x-ray telescopes, to find out exactly where the radio waves and x-rays were coming from.

Radio telescopes need much larger mirrors, while x-ray telescopes can't be made in the usual way because x-rays go through nearly everything, so they're difficult to focus!

3. May 15, 2010

### Mu naught

Why should the diameter of a telescope determine the wavelength of light it can receive? Radio telescopes can vary from a few meters to over 300 meters in diameter...

The only thing that should determine what wavelength a telescope can see is the material the reflector is made from, be it glass, metal screen, concrete etc... and the optical system which is actually recording the photons.

4. May 15, 2010

### mgb_phys

Tiny-tim meant the the two telescopes the OP linked to are both visible/near IR.

5. May 15, 2010

### Creator

Hi Zman;
You have apparently mistakenly read this paragraph from the article:
[/ "Using the motions of these stars to probe the gravitational field, observations with the 3.5-m New Technology Telescope (NTT) at the ESO La Silla Observatory (Chile) (and subsequently at the 10-m Keck telescope, Hawaii, USA) over the last decade have shown that a mass of about 3 million times that of the Sun is concentrated within a radius of only 10 light-days [5] of the compact radio and X-ray source SgrA* ("Sagittarius A") at the center of the star cluster."

The article may seem to imply that these two telescopes observed the radio and x-ray source, but in reality they only observed the motions of several stars (optically) that are very rapidly orbiting about the area of an already known radio and x-ray source....an area which was previously determined by radio and x-rays telescopes.

..