Solar Telescope Filters
Directly observing the sun’s bright face poses grave risks for human eyes, but a quality solar filter can provide the protection you need to safely view and capture images of our closest star.
A solar filter can turn any telescope into a solar telescope. Filters are available for telescopes of all sizes and for other viewing devices such as binoculars and cameras. Correctly placed over the lens, they block the sun’s intense light on certain wavelengths, making it possible to observe solar features such as sunspots and filaments directly, just as you might view a star at night.
Types of Solar Filters
Solar filters are available in two types: white light and hydrogen-alpha, or H-alpha. White light filters can block 99.999% of the sun’s light, which allows viewers to see the sun’s visible surface, called the photosphere. White light filters can be made from polymer, like Mylar, or glass. White light filters range in price from under $25 for solar film filters to $100 or more for quality glass filters designed for large-aperture telescopes.
Hydrogen-alpha filters provide sharp imaging of the sun’s chromosphere — the layers directly above the photosphere, where much of the sun’s activity takes place. Once available only to professional astronomers, H-alpha filters are considerably more expensive than white light filters, at prices starting at $200 or more. But H-alpha filters can reveal more detail than white light filters since they admit light from hot hydrogen atoms in the sun’s upper layers.
It’s also possible to make a telescope sun filter at home using Mylar film. But a poorly made or inexpensive solar filter can put your eyesight at risk since even an accidental pinprick or incorrect measurement can let in sunlight that’s magnified by the power of the telescope. That’s why it’s important to make sure you’re using a well-made filter and that it’s placed correctly on your telescope.
How to Use a Solar Filter
For maximum safety and best viewing, you should always place a solar filter on the front end of the telescope, and it should fit snugly over the optical tube. Some low-cost telescopes include solar filters for the eyepiece at the viewing end, but that can put highly concentrated sunlight directly into your eye.