How do solar astronomers get good pictures of the sun? I’m guessing they use pretty strong filters to keep most of the light out, and that they have to use different filters for different wavelengths of light to get a complete picture.
The filters are a sheet of foil-- They look exactly like mirrors. Just an ordinary, achromatic filter will cost in the neighborhood of a hundred bucks, which is not all that much, compared to how much amateur astronomers usually pump into their hobby. If you really want to make out all of the cool features, though, you need a Hydrogen Alpha filter, which filters out all of the light except that produced by the alpha transition of hydrogen, a rather strong line in the Sun. Those can cost in the thousands, and are usually only used by professionals. There’s other parts of the spectrum which can be of interest, but most of them aren’t in the visible range: You can see all sorts of neat stuff in X-rays, for instance.
Yes, you can use a plain dark filter (often just a piece of plastic or glass coated with aluminum) on a plain old telescope and see the sunspots. These are called “white light” images. A better way is to pick out an emission line of a particular ion, such as H-alpha or Calcium K lines. This way you can pick out the areas with a narrower range of temperatures and densities, providing more contrast and more scientific information. This is usually done with interference filters - a film with a semi-reflective coating on both sides. Some light goes straight through, and some reflect twice (off front surface, then off back surface) before going through. Depending on the wavelength, these may reinforce each other, or cancel each other. Thus you can pick out a narrow range of wavelengths. Examples of H-alpha and white light images can be seen at the Big Bear Solar Observatory page.
Even more interesting are coronagraphs. The corona is a very high-temperature (millions of degrees, compared to the surface of the sun which is only 6000 degrees), low-density plasma normally visible only during a total solar eclipse. Rest of the time, trying to see the corona is like trying to see a firefly perched next to a car headlight - even if you shade your eye from the headlight, there is a lot of scattered light which washes out the faint firefly. Coronagraphs are usually built on tall mountains, use the purest, cleanest lenses possible, and have a special design which almost completely eliminates diffracted light from the lens.
One of the recent developments is to put a coronagraph in space, such as the SOHO LASCO instrument. You should be able to find some nice images on that site.
Those are just for visible light observations. Radio telescopes are often used for observing the sun, and there are even dedicated solar radio telescopes such as the Nobeyama Radioheliograph, a quarter-mile long array of 84 dishes. Extreme-ultraviolet (XUV) and X-ray observations are especially important, since the million-degree corona emits a lot of those. The SXT instrument on the Yohkoh satellite has been observing the sun in X-rays for over 8 years now. The more recent TRACE satellite has been taking XUV images which I think are as pretty as any Hubble or Chandra images.
OK, I’ll stop now. By the way, I don’t know why but the preferred term is “solar physics” not “solar astronomy”.
By the way, if anyone is tempted to put a filter on their home telescope and look at the sun, DO NOT use a filter that screws into the eyepiece! Those are dangerous!! The telescope concentrates the light and heat onto the filter, so the risk of the filter breaking is very high. It is much safer to buy a filter that covers the front of the telescope. An even safer method is to place a white piece of paper behind the eyepiece, so the telescope projects an image onto the paper. No filters necessary, though be very very careful that nobody looks into the telescope. An added bonus of this method is that you can sketch the sunspots by just tracing the projected image with a pencil.
Depends on what you’re doing. If you’re just counting sunspots, for instance, that’s solar astronomy. If you’re trying to predict sunspots, that’s solar physics. In general, astronomy deals with taking the observations, and physics tries to explain them. Usually, explaining is a lot more work than observing, so most folks who study the Sun call themselves physicists, not astronomers.
One other safety note about solar filters: Do not try to improvise one. Unless you’re buying something intended as a solar filter from an astronomy store or catalog, you’re very likely to make it either too opaque, and not see anything, or not opaque enough, and seriously mess up your eyes.