Solar eyepieces
 
Although they are no longer in general use, special eyepieces have been designed for viewing the Sun. It cannot be emphasized too strongly that the Sun should NEVER be viewed by an ordinary telescope system, no matter how small the collector is. Without special precautions, PERMANENT DAMAGE can be inflicted on the eye, even with only the briefest glimpses of the solar image. It is also insufficient and dangerous to reduce the intensity of the image by placing a filter over the eyepiece; the heat that is generated in the filter can cause it to crack.
One of the special arrangements relies on partial reflection to reduce the brightness of the solar image. In such a system, only about 5% of the light is allowed to enter the eyepiece; the remaining 95% is usually reflected out of the system. In some designs, the solar image is viewed by the eyepiece in a direction at 90^◦ to the telescope’s optic axis and this attachment is called a sun diagonal.
For small telescopes, coated mylar disks may be fitted over the telescope aperture and these are commercially available. An alternative and convenient way of viewing the solar image produced by a small telescope, using a conventional eyepiece, is that of projection.