Galilean
Astronomical
Refractor
Newtonian
Galilean Telescope
Parallel light rays come in from an object that is very far away. (In the scale of the drawing, we can't show an object plus how far away it is.) The first optical device in our Galilean Telescope is a convex lens.
Parallel light rays are converged by the concave lens and would form a real image of the distant object at its focal point.
However, before the converging light is allowed to reach its point of reversal, and before it gets a chance to form a real image, a concave lens gets in the way. This lens stops the convergence and causes the light rays to actually diverge - diverge as if they had come from a point much closer to the observer than the object he/she is looking at.
So the telescope behaves the way it does because it makes the object appear to be closer than it really is. Instead of parallel light rays arriving at the eye, diverging ones do. With the image being much closer than the object, we are able to see details that would otherwise be missed and it appears to be enlarged.
In fact, the image that is ultimately seen may not be markedly larger. However, it is brought much closer so one can see more detail. And, as it is closer, it will appear to be larger. Thus we have a telescope that is non-inverting, and makes the image appear larger -- a Galilean Telescope.
ASTRONOMICAL REFRACTOR
In an astronomical refractor, the observer is not likely to be moving during a given observation. Therefore, he/she is more tolerant of having an inverted image to examine. There will be two convex lenses in this telescope. Many of the same principles will be at work here as in a microscope.
For a distant object (object distance is essentially infinite) the real image will be formed at the focal point of the first lens, f1. It is inverted, real and reduced. Note that f1 is inside the focal point of the second lens, f2.
This first image serves as the origin of the light rays that now travel through the second lens, called the objective. Note that these rays diverge from one another on the right-hand side of the lens.
With the diverging light rays on the right-hand side of lens 2, we ask ourselves the usual question - where do they appear to originate? This location would be the location of the final image, I2.
The rays which are diverging, can be traced back to their apparent origin. This is done below to find the location of the ultimate image, I2.
The two light rays appear to originate at the location designated by I2. This image is inverted relative to the object, but not relative to the first real image. The image appears closer to the observer and thus he/she can see more detail and it will appear much larger than the object would naturally appear.
NEWTONIAN REFLECTORS
Telescopes using concave mirrors were invented by Sir Isaac Newton to replace or supplement the lens-type or refractor telescope invented by Galileo. Parallel light rays from distant planets or stars are reflected off the concave mirror and converge towards the focal point. As they do so, the small plane mirror redirects them to the side so that they can be seen by the observer or caught on a photographic plate.
Thre are numerous versions of the Newtonian reflector, but some of the biggest examples include the Hubble Space Telescope, Kitt Peak Observatory, and all the modern telescopes being built.
More information and more details will be included in the next edition of the Optics Bookshelf©.