4 Refracting telescopes
Refracting telescopes can have different optical systems. The first telescope that was developed had an optical system that was called the Dutch system (because of its origin) or the Galilean system (after the man who made the instrument famous). In 1611 the German astronomer Johannes Kepler developed another system, the Keplerian system, described in his book Dioptrice. This system did not come into use until several decades later (Allen 1943: 303, Van Helden 1976, Degenaar 1988: 60) and was preferred from that time onward by astronomers over the Dutch system. This instrument was therefore also called the astronomical telescope. Looking at both systems we can understand why they preferred the Keplerian system.
In order to understand the two different optical systems, we must keep in mind the difference between convex and concave lenses. Convex lenses are thicker in the middle. The image seen through a convex lens is magnified but indistinct. These lenses were used to aid far sighted people and were available from the 13th century onwards (Van Helden 1977: 10, 17). Concave lenses are thinner in the middle. The image seen through a concave lens is reduced but sharp. These lenses were used to aid near sighted people but were not common in northern Europe until the 16th century (Van Helden 1977: 10-11, 17).
A telescope with a Dutch or Galilean system has a convex objective and a concave ocular (eye-lens). The stronger ocular is placed within the focal distance of the objective. A telescope with a Keplerian or astronomical system has a convex objective and a stronger convex ocular. The focal distances cross each other. (fig. 2) The Keplerian system therefore requires a somewhat longer tube than the Dutch system. The tube of the Keplerian telescope is as long as the focal length of the objective plus the focal length of the ocular (fo + fe). The length of a Dutch telescope is the focal length of the objective plus the (negative) focal length of the ocular (fo + fe, where fe is negative) (Pedrotti & Pedrotti 1993: 139-142). To increase the magnification of the telescope, the objective should be made weaker and/or the ocular stronger. In practice this meant that telescopes became longer and longer in the 17th century.
The advantage of the Dutch system is that it gives an erect image, in contrast with the Keplerian telescope, which gives an inverted image. The advantages of the Keplerian telescope over the Dutch telescope are that the image is brighter and that the field of view is larger (Pedrotti & Pedrotti 1993: 141). Because of these advantages, astronomers preferred the Keplerian telescope to the Dutch telescope. For astronomers an inverted image is not a problem. The image of a Keplarian telescope could also be made erect by putting in an extra convex lens, a so-called erector lens (Willach 2001: 382). This, however, also affects the quality of the image by increasing the chromatic aberration (Allen 1943: 303; Willach 2001: 383). It also lengthens the telescope (Pedrotti & Pedrotti 1993: 141; Willach 2001: 382).