What is a Solar Filter, and How Do You Use It?

What is a solar filter and how do you use it?

 

A solar filter is an astronomical filter designed to protect both your eyes and your equipment from the intense brightness of the Sun. These filters block or attenuate more than 99.999% of sunlight, allowing you to safely observe solar phenomena such as sunspots, solar eclipses, and planetary transits. They are made from specialized materials that effectively eliminate harmful ultraviolet (UV) and infrared (IR) radiation while permitting a very small amount of visible light to pass through.

Although this warning is repeated often, it cannot be overstated: never look at the Sun without proper filtration and eye protection. A telescope concentrates solar heat and radiation just like a powerful magnifying glass. Even a brief, unfiltered view can cause permanent retinal damage before you have time to react. A solar filter is mandatory every single time you point a telescope at the Sun.

 

White-light Solar Filter

The most common and affordable type of solar filter is the full-aperture white-light filter. This filter attaches securely over the front of the telescope and blocks more than 99% of incoming solar radiation. The result is a comfortably dim, safe view of the Sun.

White-light filters reveal the full solar disk, sunspots and active regions, and granulation on the photosphere. They are called “white-light” filters because they pass the full visible spectrum, though the Sun often appears yellowish or orange depending on the filter’s material. White-light filters do not reveal prominences or features above the solar limb.

Celestron EclipSmart products use filters constructed of either aluminized polyester film (Mylar) or glass. Our solar glasses, eclipse kits, and telescope solar filters use Mylar, while our binoculars and solar telescope use glass filters built into the optical path. All Celestron EclipSmart solar filter products feature Solar Safe filter technology and conform to the ISO safety standard, ISO 12312-2:2015(E), Filters for Direct Observation of the Sun.

Full aperture filters are popular for viewing the partial phases of a solar eclipse and the totality of an annular solar eclipse. No filter is needed to view the solar corona during totality, but care must be taken to reinstall the full aperture filter seconds before totality ends. The filter must be in place once the sun emerges from behind the Moon to both protect the visual observer and prevent damage to photographic equipment.

 

Specialty Solar Filters

Specialty solar telescopes that pass the very narrow Hydrogen-Alpha wavelength and reveal solar prominences are available from aftermarket sources. Also available are solar filters that allow you to safely observe the sun at wavelengths emitted by Calcium, Magnesium, Sodium, and other elements in the solar atmosphere, each displaying a different part of the sun’s structure.

 

Eyepiece Solar Filters

In the middle of the 20th century, many entry-level telescopes featured a solar filter that fits over the eyepiece. This style of solar filter has been discontinued because of danger that the telescope’s concentrated solar heat may crack the filter, leading to permanent eye damage. Under NO circumstances use one of these obsolete over-the-eyepiece solar filters. For safety’s sake, destroy any examples found.

 

Determining the Correct Celestron Solar Filter for your Telescope

Celestron offers 13 telescope solar filters that fit a wide variety of telescopes from 60mm to 8 inches in aperture. Each filter’s name lists the telescopes it works with for easy reference. Celestron’s solar filters are custom-fit to attach securely to the telescope’s front end. If you do not see your telescope model listed, you can determine which solar filter you need by measuring the interior dimension of your telescope’s front end. Compare your results to the solar filters’ spec charts. If you need assistance, you can contact our technical support team.

 

Solar filters for Refractors

Solar filters for Newtonian Reflectors

Solar filters for Schmidt-Cassegrain and EdgeHD                             

 

EclipSmart Solar Filters

View all

 


Installing a solar filter on to a Celestron telescope

Celestron carries two styles of solar filters, those that attach with thumbscrews and those that use hook-and-loop safety straps. The thumbscrew filters work with refractors, and the hook-and-loop filters work with Newtonian Reflectors, Schmidt-Cassegrains, and EdgeHD telescopes.

 

Thumbscrew solar filter

For filters with thumbscrews – Each filter has three thumbscrews evenly spaced around the filter. Place the filter over your telescope’s front cell and tighten the screws to ensure the filter does not move. Do not force the filter onto your telescope. If your filter does not fit, back out the thumbscrews to ensure they are not getting in the way.
hook and loop solar filter

For filters with hook-and-loop straps – Each filter comes with two hook-and-loop straps and four hook-and-loop squares. To set up the filter straps, first place the filter on the telescope. Affix two hook and loop squares to the filter’s outer hard plastic—one on each side. Next, affix two more hook-and-loop squares next to your first two squares on the telescope tube. Lastly, lay the hook-and-loop strap across the squares you laid down (see image for detail). Do not force the filter on your telescope. If your filter does not fit, ensure it is the right one for your telescope and nothing is impeding it on the telescope’s front cell.

 

 


Important Solar Safety Note

Inspect your filter before every use. Do not use and discard if damaged, torn, punctured, or separated from the frame in any way. It is not advised to try to adapt the filter for one model telescope to another. Your vision is worth getting the proper filter for your telescope model. If there is a solar eclipse coming to your area in the future, order your solar filter now. Waiting until just before the eclipse may find solar filter inventories depleted.

Back to blog