Might as well get the obligatory warning out of the
way. The sun generates more energy in one second than all the power plants
on Earth could create in a million years. It's really, really, really bright and
without a proper solar filter it will do unpleasant things like melt your
eyeballs, so be careful.
Types of Solar Filters
There are two types of solar filters: white light and hydrogen-alpha.
White light solar filters are simply very dark neutral density filters (see the
Planetary Filter section). These allow you to see sunspots on the surface
of the sun and are ideal for viewing solar eclipses and transits of Mercury or
filters are narrowband filters. These allow you to view solar flares,
prominences, and other associated activity.
Unlike other types of filters for astronomical observing, solar filters fit
over the front of the telescope, completely covering the
aperture. This is
to protect not only the observer's eyes but also the optics themselves.
The telescope's optics would become very rapidly heated and potentially damaged
without the filter in place.
White Light Solar Filters
White light filters transmit the entire visible spectrum of light but
attenuate it to a level that makes observing safe. A filter designed for
visual observation transmits only 0.001% of the sun's light. Solar filters
look like mirrors because so little light is passing through them.
White light filters allow the photosphere of the sun to be observed.
The photosphere is the "surface" of the sun (inasmuch as a ball of gas has a
surface). It is the layer of the sun that generates most of the light and
is therefore easiest to see. This is also where sunspots occur, so these
features are easily observed with white light filters. Events such as
partial and total solar eclipses and planetary transits are most often observed
with white light filters. In addition to sunspots, granulation (texture)
on the surface of the sun can be seen. However, solar prominences may not
be observed with a white light filter and require the use of a hydrogen-alpha
White light filters are made either from glass coated with metallic layers,
or are made from thin sheets of film such as Mylar or similar material.
The glass filters are often considered the safest option for sharing views of
the sun with groups of people, since it is hard to damage the glass filter while
in use. It is possible to tear a film-type filter, although you would have
to try pretty hard to do it. On the other hand, dropping a glass filter
will likely shatter it, while a film filter would most likely be unharmed.
The best quality filter currently available (2006) is made from a film material
called Baader Astro Solar Film.
Most filters provide a white image of the sun, while some give a
yellow-orange color. Mylar filters give a slight bluish color, but the
Baader film (which is similar to Mylar in physical appearance) gives a pure white image.
Observers sometimes use an orange colored filter to make the sun look the color
people expect to see, but the image quality is fine no matter the color.
Above: An image of the sun through a white light solar filter
Above: Typical white light solar filters made from glass
(top) and film
Hydrogen-Alpha Solar Filters
Unlike white light filters, hydrogen-alpha (H-alpha) filters work by
transmitting only one specific wavelength of light. This wavelength is a
deep red color of light emitted by hydrogen atoms, which make up the bulk of the
sun (and the rest of the universe, for that matter). The wavelength is
656.3nm, which causes the red color seen through H-alpha solar filters.
Like white light filters, the overall light transmission is attenuated to 0.001%
for safe observing, and potentially harmful infrared light is blocked
Due to the very different nature of their design, H-alpha filters are
necessarily more expensive than white light filters. Most H-alpha filters
cost several thousand dollars, but new technology has allowed some filters to be
priced well below $1000. Coronado's PST (Personal Solar Telescope) is the
least expensive H-alpha filter yet, at just under $500. This is in
contrast to the $2000-3000 necessary to get into H-alpha observing just a few
Hydrogen-alpha filters are most often designed to be used on smaller
refracting telescopes. One reason for this has to do with the nature of
the filter design and the fact that it works best with a short
telescope. Another reason is that the expense of the filter is related
primarily to the size of the filter, which is determined by the size of the
telescope. Bigger telescopes require bigger filters which require bigger
wallets. The sun, being as bright as it is, does not require a large
aperture to observe it in great detail. Most observers use 3-4" diameter
refractors for solar viewing. Coronado in particular also manufactures
dedicated solar telescopes which are similar-sized refractors with the filter
systems already built in.
Above: An image of the sun through a hydrogen-alpha filter
Above: A typical hydrogen-alpha telescope, Coronado's