When viewing the Sun, you have to be careful, even more so when using a telescope as the focused light when viewed directly can cause you to go blind or will severely damage your vision. But if used carefully telescopes can still be used in a way that means there is little danger to the user but still allow the Sun to be viewed.
When viewing a solar eclipse the use of a telescope is rarely needed and solar viewers made from cardboard will suffice. But when there is an event such as a Mercury or Venus Transition, they appear so small against the Sun that you would be hard-pressed to see it without using a telescope.
One method you should never use is eyepiece solar filters. These can crack easily and will leave you blind or with extreme eye damage.
To view solar events using a Newtonian telescope you can use a herschel wedge (Herschel Wedge Example) . When this is placed on the eye piece it dissipates 99% of the light allowing you to view the remaining 1% safely. Below is an example of how the sun may look when using a Herschel wedge.
If you have a refractor you could use a Herschel wedge in the same way as Newtonian Telescopes to safely view the sun. The picture above was taken using a refractor telescope. Another way to use a Refractor to view solar events would be the projection method which involves putting a piece of white card behind the eyepiece so that the sun projects an image onto it. We do not recommend this method though as many telescopes have plastic parts inside and these can melt very easily when exposed to the temperature the telescope is exposed to. You can also use white light filters on the end of the telescope, you can even buy kits to make your own filter, such as this.
Catadioptric Telescopes come mainly in 2 varieties Schmidt-Cassegrain telescope and the Maksutov telescope. These telescopes must always be used with a filter when solar viewing as they contain delicate internal components that the heat can damage as there is no way to cool these telescopes down as it is an enclosed design. Herschel wedges will not work with these telescopes as the mounting for them isn’t available. One of the only ways to use these for solar viewing is by using a full aperture filter that fits on one end of the telescope such as these.
For more information on viewing the sun, please read here