Solar Filters: How I Buy & Use Them Safely

This article will go into the details I know and have experienced in purchasing and using a white-light (where the entire visible light spectrum is transmitted) solar filter and a film for a custom-made solar filter.

The Checklists That I Follow While Buying/Using Solar Filters

It is important to be careful when purchasing a solar telescope or solar filter to ensure that you are getting a high-quality, safe product. Even a momentary flash of unfiltered sunlight will cause temporary or permanent eye damage! Unfiltered sunlight coming through a telescope, finder scope, or binoculars can also set people or objects on fire.

The Solar Filter Buying Checklist

• I always check that the filtering device is ISO and/or CE certified and that the filter has no pinholes, tears, or coating errors.
• I make sure that I can fit the solar filter snugly and that it has adhesive, clamps, set screws, or Velcro securing it to the telescope. Some cheap solar filters that I’ve tried have perfectly safe film but don’t have a secure attachment mechanism.
• I never buy or use a solar filter that does not go over the front end of the telescope. Not only is an eyepiece-end solar filter likely to be of low quality, but the nearly-focused sunlight entering it can easily crack the filter. This would allow direct, unfiltered sunlight into the eyepiece, and we also risk melting the innards of our telescope!

The Solar Filter Usage Checklist

• I never use a solar filter on a truss or collapsible telescope with an open tube. Sunlight entering at an angle, such as in a truss tube telescope, can still be focused enough to cause blindness, melt the telescope, or start a fire.
• I never leave my telescope unattended, as someone could accidentally remove the solar filter or otherwise cause damage.
• I always make sure that finder scopes, if any, are removed or have their caps kept on. If not, I’ll have them have their own solar filters installed.
• I find using a telescope with an aperture greater than 10” pointless. The resolution is limited due to the turbulent daytime sky, and the larger light-collecting surface serves as little more than an unnecessary fire risk.

Glass or Film: The Best Material Debate

White-light solar filters can be made of glass or a special reflective film similar to Mylar. The exact material used in the filter can affect the appearance of the Sun, causing it to appear white, yellow, orange, or with a slight greenish or blueish tone.

A variety of companies offer safe solar filters made of either film or glass, including Celestron, Seymour Solar, Bresser, Explore Scientific, Thousand Oaks Optical, Baader Planetarium, Orion Telescopes, and Spectrum Telescopes.

While some other brands make solar filters, the aforementioned companies provide the best optical quality out there. My personal favorite is Baader Planetarium, which also makes filters for pretty much every sized spotting scope, camera lens, or even binoculars out there.

• It is important to measure the inner and outer diameters of the front of your telescope tube or dew shield to ensure you get the right size filter. Some manufacturers make filters in fixed sizes, while others offer filters with some leeway for adjustment to fit different telescope tube diameters.
• You get what you pay for. Any reputable manufacturer is going to sell the safest material available, but if you want the sharpest and most pleasing view possible, the more expensive stuff is more likely to satisfy your demands.

The American Astronomical Society and others have a list of approved brands. Here is a condensed list of the brands that they and I would recommend for completely safe solar filters and eclipse shades:

Making Our Own Solar Film Mounting System

An alternative option to a ready-made solar filter is to purchase solar film from Baader Planetarium or Thousand Oaks Optical and create your own mounting system out of cardboard or wood. I’ve even come across telescopes where this was the only way to create a secure enough attachment mechanism.

This can be a cost-effective solution if you have the skills and confidence to do so, but it is important to be extra vigilant about checking for pinholes and light leaks to ensure safe operation.

You also have to balance securely mounting the film to its holder with avoiding pulling the material too taut, which will make it far more likely to tear or create pinholes.

If you make your own solar film holder, the responsibility is entirely on you to make sure that you and other people looking through your telescope are protected from exposure to direct sunlight, and you are liable if your homemade mounting somehow fails.

My Recommendations: Solar Film

I always buy sheets that are at least 1” bigger than the telescope and wrap the excess film around the lip of its holder like I’m covering a food storage container with foil.

• Thousand Oaks Optical Solar Film: This stuff is available in up to 12” x 12” sheets and produces a deep orange tint
• Baader AstroSolar Film: Available in a few different rectangular sizes that I cut/trim myself. It produces an off-white hue. Most people will want the 7.9” x 11.4” sheet for 7” and smaller telescopes or the larger 19.7” x 39.4” sheet if you use an 8-12” scope or want to make multiple filters.

My Recommendations: Film Filters

Baader AstroSolar Film is available from Baader themselves, AstroZap, and others in pre-made holders.

Thousand Oaks Optical also sells their film in pre-made holders alongside film sheets (see above).

Spectrum Telescope Solar Film filters are also a great choice, while Celestron EclipSmart filters are also available for select telescopes up to 8” but only the 5”, 6” and 8” SCT formats are full-aperture. Orion also sells a few film solar filters.

My Recommendations: Glass Filters

Glass solar filters are available from Spectrum Telescope in up to 14” sizes and Meade in up to 12” (aperture approximately; tube outer diameter is about 1-2” bigger)

A Note on Herschel Wedges: I’d Avoid Them

John Herschel created the Herschel Wedge, an antiquated design for a solar filter that takes the place of the star diagonal in a refracting telescope. It uses a prism to redirect almost all of the light out the back end of the telescope, then a few neutral density filters to filter down the remaining light to a safe level.

I do not recommend buying a Herschel wedge for the following reasons:

• They are expensive compared to solar filters and they didn’t give me significantly better images, despite what some people may claim.
• In the interest of keeping pricing low (or at least making it appear to be that way), I’ve seen some manufacturers make the consumer buy the filters separately. This can cause confusion and risk them not obtaining the right/enough/any filters to save money (or simply due to being unaware), which is, of course, a safety hazard.
• The beam of light out the back of the wedge is a fire and eye safety risk. Some wedges have a ceramic cooling system to disperse this beam but I’ve also seen some that don’t.
• Because UNFILTERED sunlight is entering your refracting telescope and being focused by the objective lens, the risk exists that you could accidentally burn the internal light baffling of the scope if you are pointed slightly away from the Sun.

My Other Related Articles That You Might Like

Hydrogen-alpha solar telescopes almost always have built-in filters and are far more expensive. My article on purchasing solar telescopes offers details and recommendations for buying these instruments, as well as a few telescopes that come bundled with white-light filters.

My article on observing the sun offers a further explanation of what you will see.