How to Safely Observe the Sun & Solar Eclipses
by zane landers
Observing the Sun is one of few astronomy activities you can do in the daytime. It’s great if you’re interested in something that doesn’t involve long nights or want to do astronomy outreach for the public, family, or friends in daylight hours.
Observing the Sun is fun, but it’s the only area of astronomy that can actually result in bodily harm and permanent eye injury if done improperly. There have been numerous cases of fake eclipse glasses abound during solar eclipses. Additionally, many older telescopes come with “sun filters” which screw onto the eyepiece – where sunlight is focused and concentrated already – and should not be used. Never look at the Sun without a proper solar filter or other safe optical aid that blocks out harmful UV rays and bright light.
The only truly safe ways of observing the Sun are:
Some common unsafe methods of observing the Sun (which you should NOT use) include:
Don’t use an open-tubed or truss telescope for solar observing, as light leaks can cause instant blindness or start a fire – it’s not worth it. A large telescope above 8-10” of aperture is also of little practical benefit for solar observing and more likely to cause potential safety issues than provide higher resolution.
Herschel wedges divert most sunlight out of the telescope into an unfocused beam – but often require the use of additional dimming filters at the eyepiece to be safe, and can easily allow the insides of telescopes to get too hot and suffer damage – or plain melt catadioptric telescopes. Only use a Herschel wedge if you are an experienced astronomer with a refracting telescope you know will be unharmed.
A white-light solar filter affixed to the end of your telescope is an easy and inexpensive way to get started with solar observing. It rejects almost all of the light entering the telescope as well as harmful UV rays. They can use either safety film or be made of solid glass. With safety film filters, always be sure that the filter is free of pinholes or tears before using it. The film is, however, intentionally meant to be a little loose in its housing, and thus creases and ripples are normal. You want a filter that tightly fits on the end of your telescope and preferably clamps to it using some sort of fastener or screws so it can’t be blown off or accidentally removed by bystanders. Most white light filters produce a yellowish, orange, or white view of the Sun depending on the exact material used.
A white light solar filter will allow you to see sunspots – patches of cooler and thus dimmer (to our eyes) plasma on the Sun’s “surface” (the photosphere) which move, expand, and disappear over time. On a day with steady atmospheric conditions, you can also see solar granules, convective cells the size of the Earth which churn and renew themselves once every hour.
White light solar filters are also great for viewing eclipses. You can clearly resolve the outline of the Moon’s jagged and irregular surface as it passes in front of the Sun, as well as any present sunspots. Similarly, transits of Mercury and Venus are ideally viewed through white light filtered telescopes.
A hydrogen-alpha solar telescope is a specialty instrument, but well worth the money. H-alpha solar telescopes use a complicated device known as a Fabry-Perot etalon to filter sunlight through a complex series of channels, and then a blocking filter to remove remaining extra light. The result is a reddish view that shows the chromosphere or upper atmosphere of the Sun, allowing you to see solar flares – which are called prominences, spicules or plages depending on their angle. You can still see sunspots and granules, though not as clearly. The flares on the Sun change minute by minute despite their huge size, giving you an exciting, dynamic real-time viewing experience.
While most dedicated solar telescopes come with a special built-in Sun finder, a normal telescope with a filter on it can be a little tricky to aim at the Sun since you can’t use your finder safely – make sure to remove or cover your finder if it’s a magnifying finder scope to avoid burning the crosshairs or burning anything behind it. To aim at the Sun, the easiest way is to point the telescope so that its tube has as little of a shadow as possible, then look through the telescope while panning around until you’ve got it.
Solar eclipses can be viewed with white light solar filters or hydrogen-alpha solar telescopes perfectly safely. If you’re watching a total solar eclipse, it is safe to take the filter off your telescope during totality to view solar flares and prominences which are otherwise only visible with a hydrogen-alpha telescope. However, if even a tiny sliver of the Sun’s disk is visible, keep your solar filter on at all times.
Solar eclipses can also be viewed with inexpensive eclipse glasses or by projecting the eclipse with a pinhole poked into any sort of flat sheet of material, which will project an image of the partially-eclipsed Sun onto the ground.