Projection with binoculars or a telescope.
Sky & Telescope illustration
You can form a much sharper and bigger Sun image by projection through a small telescope or binoculars (left)
. This is best done outdoors to avoid the distorting effect of a windowpane. To aim the instrument safely, look at its shadow on a white card as you swing the tube around. (Don't use your finderscope make sure it's capped at the front end or removed completely.) When the scope's shadow nears its minimum size, a brilliant beam of sunlight will burst out of the eyepiece and fall onto the card. Turn the focus knob and experiment with the card's distance behind the eyepiece until the Sun's disk is sharp and as big as you want. Look for sunspots!
If you prefer to look directly at the Sun, you can use a square or rectangular arc-welder's glass of shade #13 or #14, available for a few dollars from local welding-supply stores. (Don't get a lower-numbered shade; the Sun will be too bright to look at safely.) Alternatively, special, cheap "eclipse glasses" (right)
are widely made from safe solar filter materials.
A solar filter that's designed to be used with a telescope is also safe for viewing with the otherwise unaided eye.
Filters that are not necessarily safe, though sometimes recommended in old books, include smoked glass, stacked sunglasses, crossed polarizing shades, photographic neutral-density filters, or a filter intended to block visible light for infrared photography. While these may greatly dim the Sun's glare, thus appearing to do the job, invisible ultraviolet or infrared radiation may be getting through to damage your eyes. (See the article "Solar Filter Safety" for more details.)
Richard Tresch Fienberg.
The clearest and best views of the Sun are had through a properly filtered telescope. Sky & Telescope
reviewed commercial solar filters designed for telescopes in the July 1999 issue, page 63 ("Solar Filters: Which is Best?")
and September 2000, page 63 ("Baader AstroSolar Safety Film: A New Standard in Solar Filters")
The filter must be secured over the telescope's front to keep most of the Sun's light and heat out of the instrument (left). Never use a Sun filter at the eye end, where it could crack or melt in the concentrated heat.
Direct viewing with a telescope and proper solar filter gives the best views of sunspots and the complex details within them, as well as the progress of the Moon's jagged, mountainous edge making its way across the solar disk.
Remember, safety is paramount. Never look directly at the Sun without using a safe solar filter. If you don't have one, see our list of suppliers.