Telescope Basics

Introduction

There are two major types of optical telescopes: refracting telescopes, in which the image is formed by passing through a lens, and reflecting, where the image is formed bouncing off a curved mirror. More modern variants can employ both reflecting and refracting elements. Such telescopes are usually called catadioptric. We will examine many different types, explain their basic components and configuration, and to a certain extent, their advantages and disadvantages.

Optical Components: Lenses and Mirrors

A Simple Galilean Telescope

Galileo was not the inventor of the telescope, but he was the first to apply it to astronomical study. He made refracting telescopes which consisted of a positive or magnifying lens as the main lens or objective, and an eyepiece which was a single negative or minifying lens as an eyepiece. This eyepiece was located inside the focus. The resulting telescope produces an upright image, and this simple optical design is still used today inside opera glasses. Telescopes with this design are known as Galilean telescopes.

Galileo's telescopes were rather small, maybe a two inch objective and the overall scope magnifying 10 to 20 times. Still Galileo was able to sketch features on the moon and Jupiter, discover sunspots, and carry out a wide variety of optical experiments. This type of telescope has a very small field of view however, and this telescope design has mostly fallen from favor, although it is an interesting project to construct one and try to duplicate Galileo's observations.

A Simple Refracting, but Inverting Telescope

Galileo's telescope design has one major drawback: it has an incredibly small field of view which makes it difficult to find objects in the nighttime sky.

The Modern Achromatic Refracting Telescope

All the refracting telescopes presented so far suffer from chromatic aberration. Because glass refracts light differently based upon wavelength, each lens in a telescope works to break the incoming light into different colors just like a prism would. Ultimately this limits the usefulness of these simple telescope because all images are blurred by this prismatic effect.

Chromatic aberration gets worse when one attempts to make telescopes with large aperatures or low focal ratios. Early astronomers knew this, and constructed long focal ratio refractors of very small aperature. In 1656, Christiaan Huygens constructed a refractor with an aperature of two to four inches and a focal length of 23 feet. This telescope magnified about 100x, and had a field of view of about 17 arcminutes. In an effort to gain even higher magnifications, Johannes Hevelius constructed a 140 ft telescope in the 1670s.

Further development of the refracting telescope would have to wait until the development of the achromatic objective. No lesser person than Isaac Newton proclaimed that achromatic objectives could not be fabricated. By the middle of the 18th centure, it became apparent that Newton was incorrect. British optical designer John Dolland devised an achromatic lens which used two different types of glass to provide much better color correction than the simple lens objectives that were used to date. For this work, Dolland received the Copley Medal of the Royal society.

Modern refracting telescopes (and indeed, most optical instruments that need to operate over a fairly wide spectral range) use achromatic lens elements. Typically these use two or three different types of glasses which have different dispersions: in other words, which spread light out into a spectrum differently. By carefully balancing the power of the lenses, color effors can be reduced signficantly, and telescopes of large aperature and reasonable focal lengths become feasible.

The Newtonian Reflector

The Newtonian reflector gets its name from its inventor: Sir Isaac Newton. Newton detailed the idea for a reflecting telescope to the Royal Society in 1670, but the idea didn't really take off until his treatise Opticks was published thirty years later. What made Newton's telescope unique was that he didn't use a lens to gether light and form the initial image, he used a mirror.

Mirrors have a number of advantages over lenses which make them ideal for telescopes:

The angle of reflection does not depend on the wavelength of light like the angle of refraction does in a lens. This means that mirrors are perfectly achromatic, and free from the color errors that plague simple refracting instruments.
Mirrors need not be made from expensive, high quality optical glass. The glass in a refractor objective must be very homogeneous and pure, without any bubbles. In a reflector, only the surface of the mirror matters, so it is not uncommon to find bubbles within mirrors, as well as variations in homogeneity. This also means you can make mirrors larger, and larger is generally better in telescopes.
You only need to grind and polish a single surface. A simple lens has two surfaces, and to make an achromatic refractor requires figuring four surfaces.

Mirrors also suffer from a number of drawbacks:

Mirrors must be covered with a reflective metal film. In Newton's time, mirrors were ground from speculum metal so they would be naturally reflective. Unfortunately, each time the mirror became tarnished or dull, it would need to be refigured to remove the tarnish. When chemical methods for depositing thin silver films were developed in xxx, mirrors rapidly shifted to glass, and were coated with reflecting surfaces of silver. Now it is more common to use aluminum for coating mirrors. While aluminum is less reflective than silver, it oxidizes to aluminum oxide, which is clear. This means aluminum coated mirrors are more durable.
Mirrors must be made into a parabolic shape to perform well. This can be more difficult than making spherical optics.

Nevertheless, Newtonian reflectors can perform excellently, are reasonably simple to fabricate, and can be made relatively inexpensively in large sizes. This makes them by far the most popular type of telescope amongst amateur telescope builders.

Components of a Newtonian Telescope

The first major component is the primary mirror. Newton's first telescope mirror was made out metal and was roughly two inches in diameter. Virtually all modern telescopes use mirrors made out of glass, quartz, or glass/ceramic composites. Amateur astronomers frequently use Pyrex blanks especially manufactured for optical fabrication, although ordinary plate glass such as portholes have also been used for primary mirrors.

The primary mirror has a concave shape, which makes it magnify like a shaving mirror. If you have a point light source infinitely far away, the mirror will form an image of the point some distance in front of the mirror. The distance between this image and the main mirror is the focal length of the mirror. If you divide the focal length of the mirror by the diameter of the mirror, you get the focal ratio. Reflecting telescopes are often described by the diameter of their primary mirror followed by their focal ratio. For example, a common telescope might have a primary diameter of eight inches, and a focal length of 48 inches. One could describe such a telescope as an 8" f/6 reflector.

The second major component of the Newtonian telescope is a flat mirror called the diagonal. Normally, you would place an eyepiece in front of the image formed by the primary, and would use that eyepiece to magnify the image. But if you tried to do that in a Newtonian telescope, your head would get in the way, and the primary mirror would not get any light. The diagonal mirror intercepts the cone of light before the focus, and sends it out the side of the tube where the eyepiece can magnify the image. The diagonal mirror must be very flat. Typically amateurs buy diagonal mirrors, because of their small size they can be relatively inexpensive.

The third major component is an eyepiece.

The Cassegrain Reflector

Mixes of Reflector and Refractor: Catadioptric Telescopes

Mark T. VandeWettering <markv@telescopemaking.org>