Before using a solar filter
Inspect the solar filter thoroughly to ensure it is not damaged. Look for tears, creases, scratches and perforations, no matter how small. In case of doubt, play it safe and don’t use the filter.
Instruments with a solar filter: binoculars, telephoto lens or telescope
The most interesting and dramatic views of the Sun are through an optical instrument equipped with a certified solar filter. This combination provides the sharpest and most detailed image, however, only one person at a time can see it.
Certified solar filters are made of metallized polymer film; or optical glass covered with a thin metallic layer deposited by vaporization in a vacuum. Glass filters are usually mounted in aluminum rings that can be slid over the instrument’s opening. They come in different sizes to suit a range of telescopes and binoculars.
Polymer filters are available in similar mounts, and also come in large sheets that can be cut to fit any instrument. Both types are sold at stores that specialize in astronomical equipment.
How to install the solar filter
Solar filters should always be placed over the opening of the instrument to reduce the amount of light entering. In the case of binoculars, both lenses should obviously be covered. Make sure the filters are well secured with tape to avoid accidental removal.
Some older telescopes were sold with small solar filters that thread into the eyepiece. These filters are extremely dangerous and should never be used: They can crack without warning due to intense solar heat. These filters should be discarded!
Don’t forget to cover your finder scope, or better yet, remove it completely, or put a filter on it too!