White-light solar filters let you see sunspots and a few other intriguing surface features. But the white light passed by these filters washes out much of the delicate detail from more interesting sights such as solar storms, gigantic solar flares, and violent mass ejections. To see these dynamic features, you need a telescope outfitted with a hydrogen-alpha filter.
The idea behind these filters is fairly simple.
The sun’s photosphere– the region of the sun we see with optical instruments— is made mostly of hot hydrogen gas. Hydrogen, like all gases, emits light at dozens of wavelengths set by principles of atomic physics. When the lone electron of a hydrogen atom falls from its second to first excited state, it causes the emission of reddish-orange light at 656.28 nanometers. This is the hydrogen-alpha wavelength.
An image of the sun through a hydrogen-alpha filter
A hydrogen alpha filter passes light in a narrow band around this wavelength, while blocking the overwhelming yellow-white light that comes through the photosphere from deeper layers within the sun. This filtering effect lets you see only glowing hydrogen gas, and gives you the contrast to see the sun for what it really is… a dynamic, seething, and occasionally violent place where immense amounts of energy and hot gas are flung into space over the course of a few minutes or hours.
A hydrogen alpha filter comes in a set with three parts. The main filter is made of two reflecting plates that pass a narrow band of light at the hydrogen-alpha wavelength, as well as a comb of other wavelengths that satisfy the passband of the filter. The second part is a mechanical cell that holds the filter over the objective of a telescope, usually a small refractor. The third part is a filtering diagonal which takes the place the star diagonal used for night viewing. This solar diagonal must be used to block all wavelengths of light that pass through the main filter, except the band around 656.28 nm. A regular eyepiece fits into the diagonal to magnify the image.
The main filter also has a mechanism for slightly de-tuning its passband to account for wavelength shifts caused by air pressure and altitude at your observing site, as well as the slight shift caused by solar hydrogen gas moving towards and away from your point of view.
The view through these filters are astounding. The image above shows you what you might see at high magnification.
A Coronado hydrogen-alpha filter
The downside of these filters is the cost. The smallest filters with a 40 mm diameter will run about $1,500. The 60 mm version, which gives a brighter image and higher resolution, goes for $2500. And a big 90 mm filter sells for an eye-popping $5000-$6000. Telescope not included!
So if you’re on a budget, is all hope lost for hydrogen-alpha observing? Not totally. Coronado, one of the first companies to make these filters, has built a complete telescope package… small refractor, filter, diagonal, and eyepiece. Called the Personal Solar Telescope, this little packages go for about $500 new, though you can often find second-hand scopes for less. Lunt also sells a similar telescope, along with excellent filters. If you have the means and the time to observe the sun, these little scopes, or the full filter sets, are well worth it.
In upcoming articles, we’ll describe a little more about the science of what you can see on the sun.