Many books tell you to project the Sun’s image using either binoculars or a telescope. However, while this is a standard procedure that has been recommended for many years, there are still dangers associated with it. If you have ever set fire to something using a magnifying glass you will know how hot the Sun’s focused image can be.
Many telescopes have a main lens cover that has a smaller removable cover, which will reduce the lens aperture down to about 40 mm or so. This is all that’s needed for a good solar projected image, so cut the aperture down in this way if the cover is of this type.
Even with such a limited diameter, the amount of light passing through the instrument is considerable, and anyone looking through it for only an instant risks severe eye damage. The image could be dangerously bright even if the Sun is partially cloud-covered at the time. Be especially careful if there are children around – they often do unpredictable things, even (or particularly!) if told not to do so. Make sure there is no chance of someone taking a quick peek even if you leave the instrument unattended for only a few seconds.
Don’t forget the finder telescope, if your instrument has an optical finder (rather than the red-dot variety, which will not be damaged). Keep a cap on it at all times, and line the telescope up using the shadows rather than the finder.
At one time, all eyepiece internal parts were made of metal, and the eyepieces could withstand the heat of projection. But today, many eyepieces have plastic parts, even if the external barrel is metal, and these can melt if subjected to heat for any time. Not just the field stop, which defines the field of view, but also the lens spacers, may be plastic. In addition, the components of the eyepiece may be cemented together using a material such as gum arabic, and this can break down in the heat. Any of these problems can make an eyepiece unusable.
So keep the Sun’s image within the field of view while projecting, rather than allowing it to drift off so that its heat is concentrated on the field stop. Only view for short periods, so that the lens has a chance to cool down between views. It’s hard to give a rule on this – often, people project the image for minutes on end without any problems. But some instruments will be more vulnerable, so if you are in any doubt cap the front lens during observations.