Exercise:
Draw a picture of a telescope

Find pictures of the following (I suggest using hotbot with "image required" checked). You may have to check several sites to get a satisfactory picture of the telescope. The brief descriptions may help you pick the most satisfactory.
        Refracting telescope
        Subaru telescope
        Keck telescope(use Keck telescope HQ site)
        Radio telescope
        X-ray grazing incidence telescope
        HESSI

How do these telescopes differ?
Why do they differ? (See the explanation after you have thought about this.) Which one looks like the one you drew?

You probably drew one that looks like the refracting telescope. The refracting telescope was very  popular up until the middle of this century and is still used for many small telescopes. However, the largest ever built has a diameter of only one meter and it is probably impossible to build a larger one. Now we build optical telescopes eight or ten meters in diameter.

The sharpness of the image (called resolution) depends on the size of the telescope in units of the wavelength of the radiation. Hence to get the same resolution at 0.0012 mm (in the near infrared) as at 0.0004 mm (blue) requires a telescope three times as large. How large must a radio telescope be if it observes radiation with a length of 20 cm in order to get the same resolution as a 10 cm optical telescope observing visible light at 0.0005 mm? (A 10 cm telescope is a  small amateur telescope.)