Without filters, looking directly at the Sun through a telescope has the potential to damage your eyes. This is due to the high intensity of direct sunlight. The problem with solar rays is that they contain damaging UV radiation.
UV rays are high-energy electromagnetic waves emitted by energetic particles like those found in the sun. These wavelengths can cause eye injuries. An entire family of diseases called "photoreceptor degeneration" are caused by exposure to harmful radiation from solar flares or other sources such as x-rays and gamma rays.
One such experiment was carried out to show the damaging effect staring directly at the Sun through a telescope can have on an eye.
Using an eye from a dead pig, it was placed under the viewing section of the telescope. After about 20 seconds, smoke started to appear in the eye as it began to burn through the socket.
After having a look at the extent of the damage, a hole was burned through to the retina.
Now whilst most people wouldn't sit looking directly at the Sun for 20 seconds, it still shows the negative consequences of direct eye contact with the Sun.
This is because a telescope will act as a magnifying glass, causing all of the energy to concentrate on a small point; in this instance the eye socket.