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3 - A Guide to White Light Solar Filters

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A Guide to White Light Solar Filters
By: Brian Ventrudo and Manish Panjwani
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In this third article in the series on solar observing, you get an in-depth look at white light solar filters that let you safely observe the brilliant broadband light coming from the Sun's photosphere, the thin outer layer of the Sun where light emerges from the denser, hotter layers below. You will also learn about an alternative to a front-mounted solar filter—the so-called Herschel wedge or solar wedge—that removes light after the objective lens but before the eyepiece of a small refractor telescope.
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Fig. 1: The Sun's photosphere imaged with a white-light solar filter. Photo Credit: Solar Dynamics Observatory/NASA.

3.1 Front-Mounted White-Light Solar Filters for Telescopes

As explained in the second article in this series, observing the Sun with just your eyes and safe eclipse glasses or #14 welder's glass is good enough for seeing large sunspot groups or solar eclipses. But if you want to see the fascinating features on in the Sun's photosphere like sunspots or granulation or solar flares, you will need the resolution and magnification of a telescope. The most economical way to view the Sun directly with a telescope involves mounting a white-light filter in front of the objective lens or mirror to reduce the intensity of the visible, infrared, and ultraviolet light entering your telescope to a level of intensity that's safe for your eyes and your telescope. White-light solar filters are usually mounted in mechanical cells or holders that fit over the objectives of most types of telescopes, including refractors, Newtonian reflectors, and Schmidt-Cassegrain and Maksutov-Cassegrain reflectors with a wide range of apertures, from 2" to 14" or more.

White-light solar filters come in three main varieties: glass filters, mylar filters, and the specialized AstroSolar film manufactured by Baader. These filters, once safely mounted, are all you need to safely observe the Sun with a telescope. You can use your regular eyepieces with your telescope to observe the Sun with these front-mounted solar filters. No other accessories are required.

3.1.1 Glass Solar Filters

Glass solar filters are a great choice for budget solar observing with a telescope. 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. Thousand Oaks and Orion are the main manufacturers of glass solar filters. The cost is roughly $65 for a mounted filter for a small refractor to $200 for a mounted filter for a 14" Schmidt-Cassegrain telescope. Glass solar filters give good white-light views of the Sun at low to moderate magnification.

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Fig. 2: An objective-mounted glass solar filter. Photo credit: Orion Telescopes.

3.1.2 Mylar Solar Filters

Mylar solar filters can be even more economical than glass filters. They give the Sun an odd blue tint, which some observers don't like, but this tint can be removed by using a #23A red color filter at the eyepiece. Mylar solar filters are specifically designed for solar observing. They are not the same as low-grade Mylar sheets used for thermal blankets and packaging, which are NOT recommended for solar observing or for do-it-yourself solar filters.

3.1.3 Baader AstroSolar Filters

The company Baader Planetarium has moved beyond Mylar and designed a specialty AstroSolar safety film. It’s a high-strength polymer that’s metalized on both sides. Solar filters made with this material are durable and they give you a natural white image of the Sun. They also result in very good white-light solar images at high magnification. The filters are mounted in mechanical cells without added stress to help maintain high-image quality over a wide range of operating temperature. Don’t be surprised to see wrinkles in these thin-film filters when they are mounted in a cell. The wrinkling does not affect image quality.

These front-mounted Baader filters can fit a wide range of telescope apertures, and they are also available for binoculars, spotting scopes, and camera lenses as well. Agena also sells a few ready to use solar filters made from Baader Astro Solar Film.

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Fig. 3: An objective-mounted solar filter made with Baader AstroSolar film. Photo credit: Baader Planetarium.
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Fig. 4: An image of the Sun captured using Baader AstroSolar film. Photo credit: Radoslaw Ziomber.

You can also buy sheets of AstroSolar film separately and make your own inexpensive mounting cell out of cardboard. You can learn more about making your own solar filter out of AstroSolar film at this link: http://astrosolar.com/en/information/how-to/how-to-make-an-inexpensive-filter-cell

3.2 Tips For Using White-Light Solar Filters

With an objective-mounted filter, whether glass, Mylar, or AstroSolar film, get a full-aperture filter if your telescope's objective is less than 5-8 inches or so. The larger aperture will, in most cases help improve the resolution of the Sun's image.

However, with telescopes of aperture of 8 to 10 inches or more, a full aperture solar filter is not as critical. While you can still get a full-aperture filter for larger scopes, the image you obtain is often limited by seeing conditions during the day, which are usually less than ideal, and by currents in the telescope tube. So a larger telescope is often outfitted with an "off-axis" filter that has an off-center filtered aperture that's smaller than the full size of your telescope's objective. The remainder of the full aperture blocks the bright light with thick plastic. Because less filter material is uses, this approach keeps down the cost of the device.

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Fig. 5: An off-axis solar filter. Photo credit: Agena AstroProducts.

A few more safety tips for observing with white-light solar filters…

  • Make sure you double check to make sure the filter and its mounting cell are securely fixed to the front of your telescope tube before you aim at the Sun. Make sure you get a filter that fits the tube of your scope or binoculars. One size does not fit all. Most vendors have the filters mounted in cells to match the most common apertures of telescopes available on the market.
  • Cover the objective of your finder scope… and NEVER use the finder to find the Sun. Instead, aim your telescope in the general direction of the Sun and adjust its position to minimize the size of the shadow its tube casts on the ground. When the telescope is aimed at the Sun, its tube will cast a small circular shadow on the ground. Then—with the solar filter firmly in place over the telescope tube-- use your lowest power eyepiece to get the Sun into the field of view. Once you are lined up, you can move to higher power when you're ready.

3.3 Solar Wedges

Solar filters made of mounted glass, Mylar, and AstroSolar film are all examples of front-mounted solar filters than block 99.999% of the Sun's light from entering a telescope tube. They have the advantage of keeping the optics of the telescope cool, which can help with obtaining steady images, and they can be used with reflectors, compound telescopes, and refractors. However, front-mounted filters, especially glass and Mylar filters, can slightly reduce the contrast and sharpness of the solar image.

Solar wedges offer another solution for safely reducing the amount of sunlight reaching an observer's eye. These optical elements are "rear mounted" on the telescope after the objective but before the eyepiece, and they are usually mounted in a diagonal housing placed in the telescope's focuser. Solar wedges were first used in the mid-19th century, most notably by Sir John Herschel, so they are sometimes called a Herschel Wedge. If designed with high-quality optical elements, solar wedges can give excellent views and photographic images of the Sun in white light, even at high magnification.

When using a solar wedge, the full intensity of the Sun's light enters the telescope. This can be a dangerous proposition for telescopes larger than 3" or 4" aperture because the optics, especially the secondary mirrors of reflectors, can heat up and possibly suffer damage. For this reason, solar wedges are generally recommended for use only with refracting telescopes of aperture 6" or less. No additional optical elements, such as a color filter, should be inserted into the optical path before the solar wedge because of the risk of heat damage from direct sunlight captured by the telescope objective. The image below shows the optical layout of a solar wedge.

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Fig. 6: Three designs of a solar wedge for safe visual observing and imaging of the Sun in white light. Photo credit: Wikipedia.

A solar wedge consists of optically-flat but uncoated glass that reflects only a small amount, about 4.5%, of the Sun's light towards the eyepiece. The rest of the light is "dumped" into a darkened absorbent material, cavity, or screen within the housing of the wedge. The light that travels onwards towards the eyepiece is still far too bright to view safely. So an additional glass filter—a neutral density filter—is used to further attenuate the Sun's white light by a factor of 1,000 or more. Further attenuation by a variable polarizing filter is often recommended to reduce the Sun's intensity to a comfortable level.

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Fig. 7: Image of the Sun in white light using a solar wedge. Photo credit: Sergio Castillo.
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Fig. 8: A solar wedge for a small refractor telescope. Photo credit: Lunt.

Lunt Solar Engineering offers a 1.25" solar wedge for refractor telescopes up to 4" aperture and a 2" solar wedge for apertures up to 6" aperture, both for visual observing or imaging. Baader also offers two premium devices for photographic and visual use. The Baader devices include a small screen on the housing that can be used to safely find the solar image.

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Brian Ventrudo
About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites OneMinuteAstronomer.com and CosmicPursuits.com.

Manish Panjwani
About the Author

Manish Panjwani has been an active amateur astronomer since before Halley's Comet last flew by our neighborhood. A former wireless communications consulting engineer and management consultant to various Fortune 500 companies, Manish started Agena AstroProducts in 2003. Since then, Agena has become one of the leading online retailers of telescopes and astronomical accessories worldwide. Besides observing from his heavily light polluted backyard in Los Angeles, Manish enjoys conducting astronomy outreach programs in local schools. Manish also holds a Master's degree in Electrical Engineering from Virginia Tech and an MBA from the Kellogg School of Management at Northwestern University.

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