Flamsteed Astronomy Society

Today’s big reflecting telescopes

— April 3, 2006

page 1 of 4

Newtonian Reflector

Gregorian Reflector by Hadley

c 1721

Cassegrain Reflector

This short survey of today’s big reflecting telescopes follows-on from last year’s review of the world’s great refracting telescopes. Telescopes come in two principal varieties : ’reflectors’ and ’refractors’ :

‘Refractors’  are so-called because they use glass lenses (which ‘refract’, i.e. bend, light) as their primary means of collecting and focusing light. The earliest telescopes, as used by Galileo, were refractors.   In the 19th century, refractors were preferred by professional astronomers for their precision in making quantifiable measurements.  There are limits however, to the maximum size of a practical glass lens because the lens must be supported all round its edge, and above a certain diameter the glass will begin to bend under its own weight making the lens useless.   The largest surviving refractor, at Yerkes (R), uses a main lens (or ‘object glass’ OG) of one meter diameter.  The diameter of a lens is the measure of its ability to collect light and produce a bright image.

By contrast, ‘reflectors’ are telescopes which use a mirror as their primary means of collecting and focusing light.  As long ago as the mid-17th century, very early in the development of the telescope, it had been suggested that the use of a mirror to collect and focus light would be superior to a lens.  Mirrors don’t produce the colour distortions (‘chromatic aberration’) which afflict simple lenses.  Mirrors can also be mounted with support across the entire back of the mirror potentially allowing much bigger diameters than possible with lenses.  On the other hand, until the mid-19th Century, the mirrors in reflecting telescopes were of polished metal called ‘speculum’ metal, which absorbed much of the collected light and didn’t produce such a bright image as a refractor.

 

 

The principle of the reflecting telescope was proposed independently by three people in the mid-17th century :  James Gregory, Laurent Cassegrain, and Isaac Newton.   Newton was the first to build a successful ‘demonstrator’ telescope which he presented to the Royal Society around 1670.

Newton’s reflector used a small flat secondary mirror to turn the light path 90 degrees out through the side of the telescope tube to an eye-piece at its top.  Newtonian reflectors are simpler to construct because they only require one accurately-shaped curved mirror, the primary.  A simple flat ‘plane’ mirror is used for the secondary.  Many of today’s amateur telescopes are Newtonians.  Some very large telescopes use the Newtonian configuration by actually placing an observer where the flat secondary mirror would be in a smaller instrument.  This position is called the ‘prime focus’

By contrast, the configurations proposed by both Gregory and Cassegrain used a small curved secondary mirror to return the light path back through a central hole in the primary mirror.  Gregory proposed a concave secondary, and Cassegrain a convex secondary.   Cassegrain’s design produces a shorter telescope tube and is now favoured over Gregory’s in modern telescopes.

 

Newton’s ‘demonstrator’ for the Royal Society

c. 1670

Gregorian Reflector

The Schmidt telescope and the ‘Mak’ (Maksutov telescope —L) are examples of ‘catadioptrics’ which use both mirrors and lenses (or corrector plates) in combination, to collect and focus the image.    These designs can image wide fields of view compared to ‘standard’ cassegrains or Newtonians, and are therefore used for large survey cameras like the 4-ft Schmidt at Mt Palomar.  The Mak design only needs a short tube and is much favoured for small amateur telescopes.  The Schmidt can be configured as a Cassegrain for visual use (SCT—Schmidt-cassegrain telescope).

 

‘Mak’ Maksutov configuration

Big refractor — Yerkes 40-inch (1 meter)

by Mike Dryland