Solar filter

A solar filter is a certified optical filter, typically made from coated glass or a metal-coated polymer film, that fits securely over the entire front aperture of a telescope or binoculars to reduce incoming sunlight to a safe viewing level before it enters the optical path. Full-aperture front filters are the standard, safe approach, since they block dangerous energy before it reaches any internal optics or the observer's eye.

Solar filters let observers safely see sunspots, and during specific events, a partial or total solar eclipse, features that are otherwise completely invisible because looking at the Sun directly, even briefly, through any unfiltered optical instrument causes permanent eye damage. A dedicated solar filter should always be checked for a snug, secure fit with no gaps, tears, or pinholes before every single use, since even a small flaw can let through a damaging amount of concentrated sunlight.

Never use a filter that threads onto the eyepiece end, since that placement lets full unfiltered sunlight travel through the entire telescope first, building up heat that can crack the filter or damage internal parts and expose the eye to a sudden burst of unfiltered light. When buying, choose a filter labeled and certified for direct solar viewing and sized to fit snugly over the specific telescope's aperture.