In his work on sunspots dated 1613, Galileo refers to the method invented by Benedetto Castelli for observing through the telescope and drawing—without damage to his eyes—the spots that he had discovered on the Sun's surface.

Galileo darkens the room and aims the telescope at the Sun. At about one meter from the eyepiece, he places a sheet of white paper. The solar disk is thus projected on the sheet. To heighten the contrast of the image and thus make it easier to observe, Galileo places a darkening screen around the sheet. He now draws a circle on the sheet with the compasses and, moving the sheet closer or farther away, he finds the position where the Sun's image exactly overlaps the circle. He then marks the sunspots on the sheet. Since the Sun moves, the helioscope itself must be moved continuously in order to keep the solar image in the circle.

The spots projected by the telescope on the sheet are reversed and upside-down with respect to the direct view. To obtain the real image, the sheet must therefore be turned around back to front and top to bottom.