Safe Solar Observing

An annular eclipse occurs when the Moon is too far from Earth to cover the entire Sun as it passes between Earth and the Sun. Credit: Stefan Seip

Amateur astronomy isn’t an inherently dangerous pursuit. While it’s much safer than, say, rock climbing or scuba diving, one aspect of the field merits precaution: solar observing. Widely observable eclipses, like the North American total eclipse in 2017, always bring discussion of solar observing safety back to the fore.

We touched on this very briefly in our recent post on solar filters. With the annular solar eclipse of June 10 treating millions across North America and Europe to a deep partial solar eclipse, eclipse safety is paramount once again.


Over the next several years, we have another annular solar eclipse spanning North and South America on Oct. 14, 2023, culminating in the next total solar eclipse across North America, crossing the continent from Mexico the United States and the Canadian Maritimes on April 8, 2024.   

Unlike during solar totality, proper safety precautions must be taken throughout all stages of an annular eclipse, even during the ring-of-fire phase, when the bright ‘annulus’ of the Sun is visible around the Moon. We’ve stood in the antumbral shadow of the Moon before during the May 10, 1994 annular eclipse and can attest that 1 percent of the Sun is still pretty bright, enough to warrant eye protection.      

. Even if the eclipse will be total, while any part of the Sun’s bright photosphere you must use safe solar filters. Credit: David Dickinson

Eclipse glasses are sure to be a hot commodity as we approach these upcoming events. Counterfeits of dubious quality always turn up online before eclipses, a problem that was particularly acute leading up to the 2017 eclipse. Certified eclipse glasses have the ISO 12312-2 stamp on one arm though that can be faked as well. The NASA logo and other imprints that inspire confidence also show up on fakes. It’s best to purchase eclipse glasses from vendors known to source them from trustworthy manufacturers, such as those listed on the  American Astronomical Society’s eclipse website. Also, inspect those old glasses prior to use to be sure there are no holes, no matter how small. 

There are multiple ways to use solar filters safely, but be sure they are properly made. Credit: David Dickinson

Optometrists have long documented eye damage from eclipses (known as solar retinopathy), as permanent crescent-shaped scars seen in the interior of the eye. Certainly, this is an eclipse souvenir no one wants to permanently have. While tales of Galileo going blind observing the Sun are almost certainly apocryphal (more likely, it was due to the onset of cataracts), one true tale on sun-blindness was that of 19th century psychologist and physicist Gustav Fechner, who was fascinated by optical afterimages generated by staring at the Sun.     

The long list of what not to use to observe the Sun includes smoked glass, double-stacked sunglasses, and stacks of exposed film. Many old telescopes used filters filled with oil, and some old 60mm department store refractors still turn up at yard sales today, complete with solar filters that screw on to the eyepiece. These are dangerous, as they can overheat and crack in a hurry due to concentrated sunlight.

Safe filters for optics will fit snugly over the front of the aperture of a telescope or camera lens, and lock in place so that wind (or curious, prying hands) cannot remove them. Thousand Oaks Optical glass filters or Baader Planetarium film filter masks are good choices, and the American Astronomical Society’s eclipse website lists more. Again, though, be sure to inspect them before use. 

A snug fit is a safe fit. Credit: Baader Planetarium

The safest option for general viewing is what’s known as the projection method. This means just what it says: projecting the solar disk through a telescope onto a white, high contrast surface, so that multiple observers can gather around for viewing. You can even build a Sun-gun or Sun funnel, which affixes to the eyepiece of the telescope, and projects the Sun onto a rear screen for safe viewing by a crowd.

When it comes to projection, one caveat is in order: reflectors (especially Schmidt-Cassegrain telescopes, with folded optical paths) can overheat quickly when used for projection without a filter. This can cause plastic parts to melt, coating to scorch, and expensive optical elements to come unglued. Refractors or Newtonian telescopes are better for projecting the Sun. If you have to use an SCT, be sure to only use it for a minute or so, with cool-down periods in between.

See the American Astronomical Society’s projection page for details on these methods and more, including pinhole projection without using any optics at all.

Pinhoed projection: During the partial phase of a solar eclipse, small apertures will project crescents. Leafy trees, interlaced fingers, even this pasta strainer will do the trick.

Also don’t forget to guard your gear, especially during public events. This doesn’t only pertain to your telescope, but anything you have that could potentially give an unexpected, dangerous view of the Sun. True story: I once grabbed a finder-scope from a student, just as he had fished it out of my gear bag and was looking to aim it skyward. Take the finder scope off your telescope or cover it to avoid focusing the Sun’s image on a bystander, or worse, on someone’s eye when they’re tempted to take a look.

Smartphone safety was a topic that also makes rounds before eclipses. My advice is to simply save that precious sensor and not even aim it skyward. Even during totality, the eclipsed Sun will look like a tiny dot. If you have a safely filtered telescope though, you can—with practice—get a decent capture simply by aiming your smartphone camera at the eyepiece.

Another danger during solar observing that’s often overlooked is heat stroke. Transits of Mercury or Venus often involve hours of standing in the Sun. Stay hydrated, take breaks, and bring your own shade if you can. I recently purchased an adjustable clamp-on sport umbrella, that attaches directly to the telescope tripod offering instant, portable shade.

Finally, don’t be daunted by the above precautions: using a safe approach, solar observing can be fun and rewarding… and hey, you don’t need pristine dark skies!



About David Dickinson

David is a freelance science writer, frequent contributor to Sky & Telescope and Universe Today, author of several astronomy books and long-time amateur astronomer. He lives with his wife Myscha in Norfolk, Virginia.

Related posts