Some of the pioneers of telescopic observation made systematic observations of the Sun directly through the telescope without any protection, thus damaging their eyesight, often irreversibly.
Galileo (1564-1642) adopted instead a method devised by his pupil Benedetto Castelli (1577/8-1643), which consisted of projecting the image of the Sun through the telescope onto a sheet of paper placed about one meter from the eyepiece. To augment the contrast of the image, it is advisable to darken the room or at least to apply to the telescope tube a large cardboard shield that attenuates the light coming directly from the Sun. With this method, effective and perfectly safe, it was possible to draw the sunspots with great precision directly on the sheet of paper. For this purpose Galileo first traced a circle on the sheet, which he placed at a distance from the telescope's eyepiece where the dimensions of the image of the solar border exactly matched those of the circle previously drawn. The image thus obtained is however a mirror image; to obtain a correct representation of the solar disc, the sheet had to be placed frontally, turned over vertically and then rubbed or traced against the light onto another sheet.
The projection method, which was to be widely used later, was adopted also by Christoph Scheiner (1573-1650), who substantially improved the apparatus by providing it with a plane of rest for the sheet of paper integral with the telescope and, above all, by introducing a new type of mount, known today as the equatorial mounting, which allowed the Sun, once it had been centred in the instrument's field, to be followed in its daily motion by moving a single axis.