Can any telescope be used for solar observing?

To view the Sun in white light, you need a dedicated solar filter. Several types are available, but the best are made from AstroSolar film made by Baader. This is a polymer (not Mylar) with extremely good optical properties, coated with metal to reflect 99.99% of the light hitting it. It blocks most of beamthe Sun's light and heat before it even enters the telescope. Filters made from Baader material are available in high quality cells from Kendrick. I have several Kendrick filters and they are excellent:

http://www.kendrickastro.com/astro/solarfilters.html

To see flares and prominences, you need a much more specialized filter which only passes hydrogen alpha light. These can be used in front of refractors, but most observers prefer a dedicated solar telescope like the Coronado PST (Personal Solar Telescope). I use one of these every clear day to view the Sun. Small flares and prominences are visible nearly every day.

Although I have used projection to view the Sun, I generally recommend against it for safety reasons. It puts all the concentrated energy of the Sun into the focal plane of the telescope, and can very easily damage or destroy eyepieces. The projection beam leaves the eyepiece in a very concentrated form, hot enough to ignite paper placed in its path. I simply feel much safer with a professionally made filter blocking the light and heat before it enters the telescope.

There are solar filters that you can buy for most telescopes. I have a 6" reflector and recently bought a solar filter for it and love it.

It's important to note that you really want a filter that will cover the full aperture of your scope (as compared to an eye piece filter) so that you don't focus the sun's intensity on to an eye piece.

What Water and Peter have mentioned is correct -- and the Balmer-alpha line (aka H-alpha) is the main spectral line that ground-based solar observatories observe, if they're not using a broad filter.

... but you're not going to see a lot of prominences, as they're more commonly associated with the transition region ... and for that, you're going to need EUV, which is blocked by the earth's atmosphere.

And for flares ... I don't know if you'd want to be looking at the sun during a flare, but although there are flares in the visible spectrum (and in radio, but then they call 'em 'radio bursts') whenever people talk about 'M class' and 'X class' flares, they're talking about X-ray flares, which again, is going to be blocked by the atmosphere.

Now, if you wanted to look for events from space-based telescopes, there's a number of resources for space weather observing, where you can get data and images from NOAA and NASA satellites in 'Near Real Time':

• SolarMonitor.org
• SpaceWeather.com
• Solar Dynamics Observatory
• STEREO beacon images
• TheSunToday.org's The Sun Now
• The Sun Today (at LMSAL)
• integrated Space Weather Analysis System
• Helioviewer

(disclaimer : I'm involved with a couple of those projects, but I'm in IT, not a solar physicist, so I likely need to get one of my co-workers to verify what I said at the top, once this site is out of private beta)