The Sun is an incredibly dynamic and active body that offers day-to-day changes that cannot be seen on any other celestial object. With a white-light filter attached to a conventional telescope, you can safely observe features such as sunspots and the Sun’s granulated surface. Sunspot groups appear, disappear, and move as the Sun rotates on its axis. Convective granules the size of the Earth come and go on the Sun’s surface within hours, though steady daytime seeing conditions are required to resolve them clearly. A white light solar filter is a great way to observe solar eclipses and transits of Mercury, too, allowing for a close-up view of the Moon’s jagged edges in silhouette and resolving Mercury’s disk as it moves along the face of the Sun.
This article will go into information on purchasing a filter or film for a white-light solar filter. Hydrogen-alpha solar telescopes almost always have built-in filters and are far more expensive. Our article on purchasing solar telescopes offers detail and recommendations for buying these instruments, as well as a few telescopes that come bundled with white-light filters. Our article on observing the Sun offers a further explanation of what you will see.
Safety While 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. Always check that the device is ISO and/or CE certified and that the filter has no pinholes, tears, or coating errors. Your filter should fit snugly and have adhesive, clamps, set screws, or Velcro securing it to the telescope. Some cheap solar filters use perfectly safe film but do not have a secure attachment mechanism. Even a momentary flash of unfiltered sunlight will cause temporary or permanent eye damage! Don’t leave your telescope unattended, as someone could accidentally remove the solar filter or otherwise cause damage. Make sure that any finder scopes are removed, have their caps kept on, or have filters installed themselves.
Never 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 and allow direct, unfiltered sunlight into the eyepiece. Additionally, you risk melting the innards of your telescope!
Do not use a solar filter on a truss or collapsible telescope with an open tube. Sunlight entering at an angle can still be focused enough to cause blindness, melt the telescope, or start a fire, even if the focused beam does not enter the eyepiece. Using a telescope with an aperture greater than 10” is pointless; your resolution is limited due to the turbulent daytime sky, and the larger light-collecting surface serves as little more than an unnecessary fire risk.
Failing to take these precautions can lead to eye injury and temporary or permanent blindness. Unfiltered sunlight coming through a telescope, finder scope, or binoculars can also set people or objects on fire.
What to Look For When Buying a Solar Filter
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.
Film solar filters have a crinkly appearance; pulling them taut would create a risk of tearing the film. However, this does not affect sharpness when looking through one.
Film filters are much cheaper than glass solar filters but are less durable; the film will eventually develop pinholes or tears over time and need to be replaced, which warrants constant vigilance whenever you use it. A glass filter is expensive and can of course be broken, but generally it will last far longer, and the weight helps with securing one to your telescope, though they usually provide a slightly less sharp image than a good safety film solar filter.
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. 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. Cheaper glass filters generally have less sharp images, while cheaper film filters will still have a sharp view but often unpleasant colors. 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.
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. 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.
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 below).
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.
Recommendations – Glass Filters
Recommendations – Solar Film
You can buy solar film sheets and mount them in your own holder to secure to the front end of your telescope. This requires diligence in checking for pinholes as well as not straining or tearing the film, but is cost-effective and, in some cases, might be the only way to create a secure enough attachment mechanism for your telescope. You should always buy sheets at least 1” bigger than your telescope and wrap the excess film around the lip of its holder like you are 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 you cut/trim yourself, 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.