Choosing and Using Solar Filters for Telescopes

Sunspots are temporary phenomena on the Sun’s photosphere that appear as dark spots with reduced surface temperature caused by concentrations of magnetic field flux. Credit: David Dickinson

One of the most fascinating astronomical subjects to observe requires no dark skies at all. The Sun is intriguing to follow from day-to-day, as dark sunspots and bright faculae form, evolve and creep slowly across its dazzling photosphere.

Unlike distant nebulae or galaxies, which will look pretty much the same tonight as on the last evening that you will ever observe, things are actually happening at a fast pace on the face of the Sun, changing sometimes from one hour to the next. With solar cycle No. 25 ramping up this year, and an annular solar eclipse gracing North America on June 10, interest in solar observing is sure to increase.

You can only mess up when solar observing twice. Once for each eyeball! Credit: David Dickinson

Safety is paramount during solar observing: it’s probably one of the most inherently dangerous things we do in astronomy. A proper solar filter is one designed specifically for solar observing, and fits snugly over the front aperture of the telescope. Inspect any filter for cracks by holding it briefly up to the Sun prior to every use. Be sure to cover or physically remove and stow any finder-scopes, especially during public observing sessions: you don’t won’t the curious onlookers during a public observing session trying and peek through an available finder, not even for a second.

Likewise, safe solar glasses for use during a partial solar eclipse will have an ISO rating of ISO 12312-2 (sometimes written as ISO 12312-2:2015) stamped on them. Beware of subpar counterfeits out there; several knockoff brands turned up on Amazon leading up to the 2017 total solar eclipse.

Plan to see a total solar eclipse at least once in your lifetime. It’s the ONLY time you can look at the Sun safely without solar glasses or filters. Credit: David Dickinson

Dangerous filters which should never be used include: smoked glass, double sunglasses (!) and old screw-on eyepiece filters, which were standard on department store refractors sold in the 1960s and 70s. These still sometimes turn up at yard sales, and can easily overheat and crack during use. Another dangerous solar observing method of yore included hollow glass filters filled with oil. I once got a chance to inspect a 19th century solar filter set in the Robert Ariail collection at the South Carolina State Museum… every last one of which was cracked.

Also, beware of another overlooked solar observing hazard, especially during long drawn out public events: overheating. We once courted heat stroke whilst showing off an hours long transit of Mercury to the public under the hot Arizona sun. Our answer: a clamp-on sport umbrella creates mobile shade, and makes a great portable solar observing station.

Safe glass filters on the market include those made by Orion and Spectrum Telescope. These block 99.999% of the incoming sunlight, allowing you to safely observe the Sun. Though these are fine for causal observing, they can also blur an image a bit during photography owing to localized heating along the surface of the glass, induced during solar observing.

For solar photography, Baader safety film is your best bet. You can buy Baader filters built for selected apertures courtesy of AstroZap, or you can buy 140mm by 155mm sheets of Baader film directly from the company for 12.50 Euros (about $15 USD) and construct a filter yourself.

DIY solar filter construction is easy, but good craftsmanship is essential for safety! Credit: David Dickinson

Over the years, I used a single sheet of Baader safety film to build filters for telescopes, cameras, binoculars and even eclipse glasses. For a large aperture telescope, it isn’t even necessary to cover the entire opening; just building a 60mm offset aperture will suffice. You can easily build a filter using cardboard, glue and safety film.

Exploring the Surface of the Sun

With a white light filter, you’ll see the dazzling photosphere of the Sun, which is the roiling gaseous surface that shines down on us on a clear day. Sunspots are the most readily apparent thing you’ll see. These are dark, cooler patches of magnetic flux along the surface. Sunspots usually appear in pairs, with an opposite magnetic polarity. sunspots appear at higher latitudes early on in the 11-year solar cycle, and move progressively towards the solar equator as the cycle continues.

Sunspot and prominence activity during solar maximum and minimum. Credit: NASA/Solar Dynamics Observatory (SDO)

The Sun rotates once every 25 days near the solar equator, and slower towards the poles. This is because the Sun is a ball composed mainly of hydrogen-helium gas, without a definite solid surface. Look closer, and you may notice blank bright spots known as facula (plural: faculae) which are linked to sunspots and caused by concentrated magnetic field lines. These regions are often surrounded by grainy-looking specks of burbling convection cells known as granulation.

It’s fascinating to watch the daily activity of our host star, and a white light solar filter will enable you to safely do so.

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.

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