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Types of Solar Filters for Telescopes

Telescopes were traditionally used to view the stars and constellations at night. However, with the availability of solar filter holders, it is also possible to observe the Sun with a telescope without causing severe eye damage. There are different types of solar filters made from suitable materials that block harmful ultraviolet rays from penetrating the telescope lens, enabling the observer to look directly at the Sun, for example during a solar eclipse.

  1. White Light Filters

    • White light filters are made from two types of material: glass and mylar. Mylar is a light and inexpensive plastic film. The average Mylar filter produces a blue tint, but upgraded filters produce a more natural image. Glass filters are flat, polished and coated with nickel and chromium. It is more expensive, but also more durable and gives a better resolution.

    Hydrogen-Alpha Filters

    • Hydrogen-alpha filters are arranged in two filter sets, the energy rejection filter and the tuning filter. The energy rejection filter restricts all light apart from one narrow band of red light, while the tuning filter makes further adjustments to the red spectrum. Hydrogen-alpha filters are also called solar prominence and are mainly used to observe the edges of solar disks.

    Other Filters

    • Some filters are designed with a reflective metallic coating, such as aluminized polyester filters. The metal coat provides extra eye protection and limits exposure to unfiltered light through any holes in the polyester. Other filters are designed using a black polymer, which produces an orange-yellow tint compared to the blue tint observed with aluminized polyester filters. All solar filters are mounted over the top of the telescope in a metal cell.

    Safety

    • There are a few safety precautions to take when using solar filters. The filter must be inserted securely in the metal cell at the front of the telescope before viewing. It must also be the right size to fit the telescope tube to prevent any unfiltered light from penetrating. It is also not advised to use filters made from materials such as smoked glass, compact discs or polyester coated with metals untested for solar viewing.

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