Filters for binoculars and telescopes should be placed in front of the main aperture of the instrument to reduce the intensity of solar radiation before it is concentrated by the optical system.
The Sun's intense radiation (visible light, ultraviolet and infrared) always presents a serious danger to the eye, especially to the central part of the retina known as the macula. The highly sensitive cells in this area can easily be burned since they absorb a maximum quantity of solar radiation which is then converted to heat. Retinal burns do not produce pain so there is no warning, and once damaged, these cells never regenerate.
Adequate solar filters must reduce the Sun’s infrared and ultraviolet radiation to within accepted safety thresholds, and they should allow no more than 0.001% of the Sun’s visible light to pass (equivalent to an optical density of 5). While some filters or “systems” appear to cut visible light to tolerable levels, they allow nearly all the invisible but dangerous infrared and ultraviolet radiation to pass through!
Do not use the following:
- sunglasses (even expensive ones);
- color negative film exposed to light and developed;
- slide film unexposed to light and developed;
- compact discs (CDs);
- glass plates covered with soot;
- crossed polarizing filters;
- color acetates;
- all other filters not specifically designed for solar observing.