Primer - Telescope Types

[Rob]

Telescope Types

Ok, there are three principal telescope types. These being

Refractors

These are based on a straight lens system. Refractors have long, thin tubes with an objective lens at the front that collects and focuses the light. The earliest of this type of telescope was used by Galileo to study planets.

They are now used mostly for lunar, planetary, globular cluster, and binary star observing, they can perform admirably on brighter* Messier & NGC objects. Many amateur astronomers prefer the crisp, high contrast, diffraction free images of a good refractor.

A useful rule of thumb in astronomy is that a good 3" to 4" refractor will usually perform as well as an average 6" to 8" reflector or catadioptric for seeing details on the Moon and planets.

Refractors do not have a secondary mirrors or multiple optical paths to introduce light scattering or diffraction. These brighten the sky background and reduce contrast.

Refractors also have the highest light transmission, ie the percentage of the light gathered by the scope that actually reaches your eye and can transmit 90% or more of the light they collect, compared with the 77% to 80% transmission of reflectors and 64% to 75% typical of catadioptrics.

The light transmission of a refractor rarely deteriorates with age, as do reflectors and catadioptrics, since their mirrors slowly oxidise and lose reflectivity over a period of time.

With good seeing conditions, refractors can reveal subtle lunar and planetary detail within a good contrast range.

Diffraction spikes on a reflector's star images, caused by its secondary mirror spider vanes, are absent in an unobstructed refractor. With no diffraction spikes to hide faint binary star components or smear globular clusters, refractors can resolve closely spaced binary stars more easily than a typical reflector.

Since the Moon and planets are all brightly lit by the Sun, high light gathering power is not as important as high magnification within the solar system. The relatively small aperture of a refractor is therefore often an advantage for this kind of observing.

For lunar, planetary, binary and star cluster observing, a refractor may be perfectly adequate.

Shortcomings.

Refractors suffer from chromatic aberration, or false colours. This is an optical defect that produces a faint coloured 'halo' around bright stars, the limb of the Moon, and the planets. Chromatic aberration becomes more visible as the aperture increases and the focal ratio decreases, although modern optical systems can minimize the problem in two element achromatic refractors and virtually eliminate it in three to four lens apochromatic systems. Of course a price to fit this quality of lens step also applies

While they are light weight in smaller sizes, refractors become bulkier than reflectors or catadioptrics for apertures approaching 100mm (4"). A 100mm (4") refractor typically costs and weighs 2 to 6 times as much as a 115mm (4.5") reflector or 90mm (3.5") Maksutov-Cassegrain.

If high light grasp is not essential, ie for observing faint galaxies, where a larger reflector would have the edge, the clarity, contrast, and good image quality of a refractor is worth considering.

Newtonian Reflector

These are based on mirror systems, the most popular of which is the Newtonian invented by Isaac Newton in 1668. Newtonian reflectors offer more performance for your money than any other telescope type. Reflectors have no lens at the front. Instead, the light is collected by a parabolic mirror at the bottom of the telescope tube, which reflects light onto a secondary mirror near the top of the tube. This secondary mirror (called the flat) is used to reflect the light from the main mirror into the eyepiece at the side of the tube.

The reflector's large light gathering area and relatively short focal length can provide bright images of deep space objects that are too faint for any small aperture refractor. And the reflector's large aperture can resolve details within those objects with a precision that not many small scopes can match.

The penalty you pay for this performance is typically one of large size and weight, although not one of high cost, as reflectors cost the least per millimetre (inch) of aperture of any telescope type.

The reason is because a Newtonian reflector has only one mirror to grind and polish to a precise curve (with an accuracy of around 100 nanometres [100 x 10-9m ]( 4 millionths of an inch) or better.). A refractor, on the other hand, has two to four lenses ( not including the eyepiece ), with four to eight precision surfaces to shape. And those lenses might have to be expensive and esoteric formulations in order to provide satisfactory images. Similarly, a catadioptric scope has three or four curved optical elements to shape to a high degree of accuracy.

All that extra mirror, lens grinding and costly optical glasses in refractors and catadioptrics isn't cheap. It makes the one parabolic mirror and one flat mirror optics of the Newtonian reflector the least expensive to make, hence its lowest cost.

For the same money you get more aperture with a reflector than with any other type of telescope. All other things being equal, the bigger the aperture, the better the performance. An 8" reflector typically costs 50% less than a 4" refractor, and little more than 4" catadioptric, but will have four times the light grasp of either.

For purely visual deep space observing, Dobsonian reflectors are very cost effective and a good choice. These are Newtonian types with a simplified mount. With huge mirrors (up to 16" in diameter) to gather light, and inexpensive wood mounts, these new Newtonians have brought about the age of the "light bucket" in amateur astronomy. The deep space observer on a budget has never had it so good.

If astrophotography is going to be your thing, a large aperture equatorial mounted reflector is excellent for recording deep space objects in detail, as well as visual observing.

Shortcomings.

There are five:- diffraction, coma, size, weight, and added maintenance.

Light diffracted, or scattered, by a reflector's diagonal mirror reduces image contrast in lunar and planetary observing, and can mask subtle surface details. In addition, diffraction spikes on star images, due to the spider vanes that hold the diagonal mirror, can hide faint binary star components and smear globular cluster detail.

Because of the parabolic shape of their primary mirrors, all reflectors have coma, an optical defect in which stars appear triangular or wedge shaped at the edge of the field. The faster the focal ratio, the smaller the coma free field. This can be annoying in photos, where the entire field is available for leisurely inspection. It is usually unobjectionable visually, however, since objects of interest are normally kept in the centre of the field, where eyes and eyepieces are sharpest and coma is not a factor.

Since an 8" reflector can weigh 50% more than an 8" catadioptric, and its 48" long optical tube is not the easiest thing in the world to manage in an apartment / flat elevator / lift. A large reflector usually requires the elbow room afforded by a suburban environment. Also, since city light pollution compromises deep space performance by washing out faint nebulae and galaxies, dark sky observing sites are always recommended with large reflectors.

In addition, unlike refractors or catadioptric telescopes, a reflector can require frequent re-collimation or alignment of its optics. However, this maintenance typically averages only a few additional minutes of work per observing session.

These drawbacks aside, for serious visual observing of faint galaxies and nebulae, as well as for lunar and planetary observing, you'd be hard-pressed to equal, much less surpass, the price / performance ratio of a Newtonian reflector.

Catadioptric Reflectors

These are modern compound, or lens/mirror combination telescopes and combine many of the best features of refractors and reflectors into one package, with few of their drawbacks. Catadioptrics include Schmidts, Maksutovs and Schmidt-Cassegrains (SCT).

They allow the performance of a large aperture, long focal length scope to be folded into a reasonably lightweight and transportable package which can be very useful if the telescope is transported to dark sky sites.

Because of their optical design, catadioptrics are almost completely free of the coma found in reflectors and the chromatic

aberration found in refractors. Stars are point-like and coma free across the visual field of a catadioptric telescope, and there is no trace of coloured fringes around bright stars and planets to mask faint detail and colours. Some curvature of field is often visible in catadioptrics, particularly in fast focal ratio models, but it usually shows more at the edges of wide field photo's than in visual observing.

A catadioptric's setup and takedown time is short, due to its lighter weight, and compact size.

Shortcomings.

An 8" Schmidt-Cassegrain costs 50% to 300% more than an 8" reflector (although about the same as many 4" refractors).

Schmidt-Cassegrain catadioptrics do not have as wide a contrast range on the moon and planets as a refractor or most f/6 to f/8 reflectors, because of the extra light scattered by its more complex optics and folded light path, nor will it be able to resolve close binary stars as easily. However, a Schmidt-Cassegrain will usually outperform a fast (f/4.5) focal ratio reflector of similar aperture on the planets and binary stars due to the absence of secondary mirror spider vanes, which usually cause diffraction effects in Newtonian types.

A Maksutov-Cassegrain has better contrast than a Schmidt-Cassegrain, often equalling that of a refractor of similar aperture, because it has a smaller secondary mirror obstruction.

Although their large apertures allow detailed deep space observing, catadioptrics generally do not have as bright an image as other telescope types of similar aperture and power. A catadioptric can have a light transmission of 64% before secondary obstruction light losses are allowed for ( 70% to 75% with multicoated optics ), compared to 90% of a similar aperture multi-coated refractor and the 77% to 80% of a reflector.

Despite these shortcomings, if it fits your budget and you need a portability, then a good catadioptric is a wise investment

Clear Skies

Rob

Source: R.M. Clarke. The salopain web - edited by R Hughes. 2005

Edited December 8, 2012 by ant
correcting typo