Solar disc phenomena: Visual observations
 

Danger to Eyesight
It cannot be emphasized too strongly that the Sun should
NEVER
be viewed using instruments without special precautions.
 

Ordinary telescope eyepieces sometimes carry a screw thread to allow absorbing filters to be attached to them. This system is not suitable for use with the Sun as either the filter may not be sufficiently absorbing or the solar heat may cause it to splinter suddenly.
Direct telescopic views of the Sun should only be attempted with special equipment designed for the purpose. An old-fashioned device is the solar diagonal whereby the converging beam from the telescope is split into two very unequal components. Only about 10% of the collected light is used for viewing through an eyepiece, which also requires the addition of an absorbing filter; and the remaining 90%, which is in the second beam, is passed out of the system. An alternative is to fix an absorbing filter securely over the telescope collection aperture. These are commercially available in the form of coated mylar sheets stretched on a ring which fits over the end of the telescope tube.