As noted in How to View a Solar Eclipse Safely, with one notable exception it is never safe to look directly at the Sun through a telescope, binoculars, or camera lens without a solar filter. That exception is during totality, the total phase of a total solar eclipse, when the dazzlingly bright solar surface is completely blocked by the Moon. But totality is fleeting. Most of the time during a solar eclipse you’ll be watching the partial phases, during which filters are always required.
Telescopes, binoculars, and cameras need solar filters for two reasons: to protect them from intense sunlight and to ensure that you don’t accidentally look at the Sun through an unfiltered instrument. In every case, the solar filter must be attached to the front of your telescope, binoculars, or camera lens. This ensures that the Sun’s light and heat are kept out of the optics.
Make sure the filter is attached securely so it won’t pop off if your instrument is bumped or the wind suddenly gusts — but not so securely that you can't remove it easily at the beginning of totality!
If your telescope has a small auxiliary finderscope or other aiming device, make sure that it is capped, removed, or safely filtered just like the main telescope.
Solar filters provided with inexpensive telescopes, usually designed to thread into an eyepiece at the back end of the telescope, are dangerous. If the filter is attached to the spot where you place your eye, sunlight concentrated by your optics will burn right though it. This is also why looking through unfiltered optics while wearing eclipse viewers is extremely dangerous and a recipe for serious eye injury. We'll say it again: a solar filter must be attached to the front of your telescope, binoculars, or camera lens.
You’ll generally encounter three types of solar filters: metal on glass (usually the most durable and expensive), aluminized polyester film (frequently but incorrectly referred to as aluminized Mylar), and black polymer (sometimes with a metal coating on at least one side). Some render the Sun white, while others impart a yellow, orange, or bluish tint. All are effective, so choose the type that best suits your preference and budget.
Thanks to the growing popularity of solar observing, some manufacturers now produce telescopes and/or binoculars already equipped with safe solar filters, sometimes built right into the optics. These are very nice for viewing the partial phases of any solar eclipse and the annular phase of an annular eclipse, but unless the filters are removable (most aren't), these devices are useless during totality and useless for observing celestial objects at night. See our Telescopes & Binoculars page to learn more.
More information about safe solar observing:
- "How to View a Solar Eclipse Safely" (American Astronomical Society)
- "Eye Safety During a Solar Eclipse" (NASA)
- "How to Safely See a Partial Solar Eclipse" (Sky & Telescope)
- "How to Look at the Sun Safely" (Sky & Telescope)
- "How to Safely View a Solar Eclipse" (Exploratorium)
Some of this information is adapted from material provided to its travelers by solar-eclipse-tour operator TravelQuest International.