How To Choose a Solar Filter
Posted by Brian Ventrudo
We’re thinking of the sun again today, perhaps because we’ve seen so little of it this summer. But clouds don’t last forever, and when they finally pass, it’ll be time for a little daytime observing of our favorite star…
In the our first article on solar observing, we urged you to use filters that block most of the sun’s rays from entering the telescope. And we reminded you NEVER to let all the light from the sun enter a telescope, and NEVER to rely on a filter at the eyepiece to block concentrated solar rays. It’s just asking for trouble.
But enough of what you can’t do. Here are your options for safe filters for observing the seething surface of our home star…
* A 2 or 3-inch square piece of #14 welding glass from a local welding supply store in makes a fine low-cost solar filter (get #14, not #12). Welding glass isn’t optically flat, so its best for naked-eye observing… it gives a poor image with a telescope or binoculars. NEVER use this glass at the eyepiece of an otherwise unfiltered telescope… it will crack. And make sure you put the glass up to your eye BEFORE you turn your head up to the sun. Welding glass gives the sun a greenish tint, but you’ll see larger sunspot groups if they’re out.
* Metalized glass solar filters mounted at the top of the telescope tube are a superb choice for solar observing. These filters use flat polished glass coated with nickel and chromium to attenuate the sun to 1/1000 of 1% of full intensity. Glass filters give the sun’s image a pleasing orange-yellow tint. And since you use this filter with your telescope or binoculars, you can crank up the magnification to see sunspots and granulation on the sun’s surface (we’ll cover these phenomena in later articles). Thousand Oaks perhaps the best-known manufacturer of glass solar filters. Cost is roughly $60-$200, depending on your setup.
A metalized glass solar filter
* Mylar solar filters cost a little less than glass filters. They’re mounted in a metal cell that fits over the top of your telescope to block most of the sun’s rays. They give the sun an odd blue tint, which some observers don’t like. Make sure you use mylar filters specifically designed for solar observing. As we mentioned in the previous article, don’t use low-grade Mylar.
* Baader Planetarium has moved beyond Mylar and designed a specialty “solar safety film”, which is a high-strength polymer metallized on both sides. Filters made with this material are durable, and they give you a natural white image of the sun. Don’t be surprised to see wrinkles in these thin-film filters… image quality isn’t affected. Baader filters are sold directly, or by third-party vendors such as Celestron. Cost… roughly the same as glass filters
If you get an objective-mounted filter, whether glass or thin film, you’ll want to get a full-aperture filter if your telescope’s objective is less than 5-8 inches or so. Above 10 inches, get an “off-axis” filter that has an off-center aperture that’s smaller than the size of your telescope’s objective. This makes the filter less expensive, and also reduces the effects of warm, unsteady daytime air that degrades your view in a large scope.
A few more safety tips for observing with solar filters…
* Except for welding glass, the above filters sit in a metal cell which fits securely at the front of your telescope. Make sure you double check to make sure the filter is securely fixed before you aim at the sun
* Make sure you get a filter that fits your the tube of your scope or binoculars. One size does not fit all.
* Cover your finder scope… and NEVER use it to find the sun. Instead, aim your telescope such that it casts a circular shadow on the ground. When it does, you’re pointing in the right direction. Use your lowest power eyepiece at first to get the widest field of view.
* Some may disagree, but forget about projecting the unfiltered image of the sun onto a white screen. Projection is only safe for small refractors (< 3 inches), and it shows far less detail than you’ll see with filters. It’s rarely worth the trouble or the risk…
Next time, you’ll learn about another type of solar filter that’s shockingly expensive, yet gives you spectacular views of the surface of the sun, as well as glimpses of giant flares on the sun’s edge. But that’s enough for today…