Solar Filters

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 . These allow you to see sunspots on the surface of the sun and are ideal for viewing solar eclipses and transits of Mercury or Venus. Hydrogen-alpha 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 filter.

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 completely.

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 years ago.

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 focal length 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 SolarMax 90