The Sun is very bright and by focussing the light onto the back of your
eye (the retina) with or without a telescope, you are putting a lot of
energy (both optical light and infra-red) onto a tiny area. At some
point in your life you may have tried to set paper on fire using a
magnifying glass. so just think about that being done to the back of
your eye. It isn't nice. Even more scarily is the fact that the retina
of your eye does not have pain receptors, so you will not even feel the
damage being done. It may not even become apparent until later.