The images of a celestial body recorded by various observers in the past can be compared with images that have been electronically processed to have a certain resolution. In astronomy, resolution consists of the minimum angle - measured in seconds of an arc - below which a detail in a celestial body observed through the telescope is not perceptible. Note that a second of an arc (1") corresponds to the angle subtended by the diameter of a one-euro coin placed at a distance of about 4.5 kilometres from the observer.

For evaluating the quality of the first telescopes, the case of Saturn is exemplary. This planet was perceived in very different ways depending on the resolving power of the instrument used. Based on comparison with the historic drawings, it can be estimated that Galileo, Pierre Gassendi (1592-1655), Francesco Fontana (c. 1580-1656), Johann Hoewel (1611-1687) and other observers had at their disposal telescopes with a resolving power on the order of 10". To distinguish significant details on the planet, telescopes with a resolving power of at least 5" are instead required. These instruments began to appear only around the middle of the 17 th century; with one of them, Christiaan Huygens (1629-1695) was able to recognise Saturn’s ring.