Do telescopes harm while observing sun through them? I don't know whether we can observe a sun through a telescope. But if we can then will it harm our eyes and if such telescopes exists what is their configuration?
Observing the Sun through a telescope is very dangerous, whatever the telescope you use, if you don't use the appropriate tools.

A telescope a basically a light collector: its purpose is to collect all the light that is arriving on his primary mirror and focus it on a point.

You may have already tried to make the Sun light converge through a little magnifying glass (which works quite similarly to a telescope). If you make it converge on something like paper, it will simply burn it. This can eventually be the cause for big fires (example, a glass bottle in a dry forest, concentrating the Sun's light on leaves). Here is an example to convince you: http://www.youtube.com/watch?v=N7nJ3wIxt3o

A telescope works the same, except that the collecting surface is usually bigger, so it will be more effective at burning ... your eye for example (astronomers make this joke that you can observe the Sun with a telescope twice in your life: once with your left eye, once with your right eye). Or, for bigger telescope, it could even start a fire, or burn instruments...

To observe the Sun, you will need a proper filter that you will put usually "before" the telescope. You may also observe it indirectly on small telescope by projecting the image of the Sun.You can see examples here: http://www.skyandtelescope.com/observing/objects/sun/Observing_the_Sun.html.