Ken Hough's Website
Astronomy

Telescopes

There are two main classes of telescope, known as 'refractors' and 'reflectors'. Some telescopes use both refracting and reflecting components, and are often refered to as 'compound' telescopes.

Refractor telescopes use lenses to focus incoming light to a point as is done in a typical camera. Refractors all suffer to some degree from chromatic aberation. ie light of different colours is not brought to exactly the same point of focus. ACHromatic lenses typically use two elements made of different kinds of glass (typically crown glass and flint glass) to reduce chromatic aberation to sensibly low levels. True APOchromatic lenses use three or more elements to reduce chromatic aberation to practically negligable levels. By using special kinds of 'low dispersion' glass, near apochromatic performance can be achieved while using only two elements to make up a lens. These specialialised two element lenses are often advertised as being apochromatic, but are sometimes better described as being 'semi-APO'.

Needless to say, apochromatic telescopes can be expensive. The majority of refractors in use are achromatic and can give remarkably good results.


Reflector telescopes use concave mirrors instead of lenses. A concave mirror reflects light back towards a point of focus in the direction from which it came, in the same way that an ordinary shaving mirror will do. In the simplest form, a small flat secondary mirror is fixed just inside the point of focus and at an angle of 45 degrees, so as to reflect the focused light out sideways and into an eyepiece. This is the basis of the Newtonian telescope. Mirrors are silvered (actually aluminised) on the front surfaces so that no refraction of light can occur. Reflector telescopes do not suffer from chromatic aberation.



Compound telescopes:

Conventional refractors and reflectors are relatively bulky. This can be overcome by 'folding' the light path back on itself as is done in Schmidt Cassigrain telescopes (SCTs) and Maksutov Cassigrain telescopes (MCTs). Both of these types use a main concave reflector mirror, but unlike Newtonian telescopes where focused light is diverted sideways into an eyepiece, SCTs and MCTs, use a secondary convex mirror to reflect focused light back through a central hole in the main mirror. Focusing is achieved by moving the main mirror along the main axis of the telescope.

The secondary mirror of an SCT is supported by a thin glass 'corrector' lens fixed at the top of the telescope tube. The secondary mirror of an MCT is formed by an aluminised area on the central area of a relatively thick corrector lens. The corrector lenses cause negligable chromatic aberation. Although SCTs and MCTs are quite compact, they can be surprisingly heavy, especially MCTs. SCTs and small MCTs are very popular, but can be fairly expensive. They also have long focal lengths (~ 2000mm) when compared with refractors and 'fast' Newtonian telescopes (~ 1000mm).



There are other kinds of telescope, but these are mostly derived from the basic types described above. I'll leave you to find out about them. Just remember that the perfect telescope has yet to be made. Each design has it's own merits, but all have limitations.

Lens and mirror coatings

When light passes between the atmosphere and the surface of a plain (ie uncoated) glass lens, approximately 5% of the incident light is scattered away from the surface. A simple two element ACHROMATIC lens presents four surfaces at which light will be scattered giving a total scattering of approximately 20% of the incident light. This has serious effects on contrast of focused images. In the case of a 4 element APOCHROMATIC lens this figure could be as high as 40%!

This scattering of light can be greatly reduced by applying very thin layers of materials such as magnesium fluoride to the glass surfaces of a lens. A single coating can reduce surface scatter from around 5% to under 1%. All modern good quality lenses now use more complicated 'multi-coating' systems which typically reduce scattering to under 0.1% per surface! The difference in contrast achieved by use of these systems has to be seen to be believed!

Mirrors used in telescopes present a different problem. The reflecting surfaces of a telescope mirror is laid onto the front side rather than on the rear side as is the case with most 'domestic' mirrors. This means that light cannot pass into the glass substrate so that mirror based telescopes cannot suffer from chromatic aberration. However, the very thin exposed reflecting surface, which is typically of aluminium, is exposed to potentially corrosive agents present in the atmosphere. Some protection is achieved by overlaying the reflecting layer with a thin transparent coating of silicon oxide. This will typically allow a mirror to remain substantially free from corrosion for 10 years or more. All good quality mirror based telescopes are now made with mirrors that are protected in this way.

How big should a telescope be?

The size of the primary lens or mirror of a telescope has a direct bearing on the amount of light that the telescope can capture, and hence on the ease with which feint objects can be seen. It's perhaps not so obvious that as the size of the primary lens or mirror is increased, so is the ability to resolve fine detail. Theoretical resolving power (angular resolution) is inversely proportional to the diameter of the primary mirror or lens and does not depend on the type of telescope used.

The smallest telescopes that should be considered for general astronomy are refractors of at least 3"/75mm or reflectors of at least 4"/100mm. Don't be tempted by smaller telescopes. They will disappoint! Larger telescopes can provide improved viewing, but do think about the size and weight involved. 8" is probably a sensible maximum for systems that must be portable. I have a 10" refector which I used to take along to public events. It now stands on a static pier in my garden and is protected by a run-off shed. Enough said!

What about 'fast' telescopes?

Telescopes are often referred to in terms of their 'aperture number'. This is calculated by dividing the focal length of the telescope by the diameter of the primary mirror or objective lens, just as is done for camera lenses. The result is expressed as an 'F' number (eg F6.3). Typical SCTs, MCTs and some refractors have 'F' numbers of 9 or more. Many Newtonian telescopes have 'F' numbers of less than 5 and some refractors have 'F' numbers of 6 or less (sometimes referred to as 'fast' telescopes). Smaller 'F' numbers produce brighter images. However, there can be downsides having a 'fast' telescope.

As for ordinary camera lenses, fast telescopes demand very careful focusing. Fast Newtonian telescopes suffer from 'coma' which can make stars in the outer field of view appear to be streaked outwards. Not so much of a problem for normal viewing, but quite serious for photography. 'Coma correctors' are available.

Fast ACHROMATIC (but not APOCHROMATIC) telescopes suffer from significant chromatic aberration. This is visible as 'purple fringing' around stars. Again, not too much of a problem for normal viewing, but photogrpahers will need to use 'fringe killing' filters.

What about focal length?

You will also see lower case 'f' used to refer to the focal length of a telescope (eg as f=1200mm). More confusion?

The focal length of a simple/non compound telescope is the distance between the objective lens or mirror and the focal point. In the case of a compound telescope, an effective focal length is specified which is typically much greater than the physical length of the telescope.

Focal length of a telescope defines the magnification of images that will be seen via any given eyepiece or camera. That is, a long focal length will result in high magnification. However, there are practical limitations on the degree of magnification that can be usefully applied.

Don't pay any attention to claims along the lines of '500x magnification'. On rare nights when the atmosphere is very stable, 500x magnification might just be useful on a telescope with a 250mm (10") diameter mirror (or lens?). Under the same conditions, a small 60mm diameter refractor would have a maximum useful magnification of only 120x, no matter what it says on the box! Atmospheric turbulence often limits useful magnification of even a large telescope to maybe 200x.

An optimum focal length depends on the kind of subject to be observed. For example, planetary observation is best done at the highest practical magnification which suggests a telescope of long focal length. Conversely, such a telescope would be of limited use for studying wider subjects such as nebule. SCTs and MCTs (focal length of around 2000mm) are well suited to the observation of say Mars and Jupiter. The spectacular Orion nebula would not fit within the field of view. But my 250mm/f=1200mm Newtonian telescope works well with my digital SLR camera (at prime focus) to fit this stunning nebula nicely within the field of view.

Magnification via an eyepiece depends on the focal length of that eyepiece. (overall magnification = focal length of telescope / focal length of eyepiece). That topic is covered under the section about eyepieces.

What kind of telescope should you buy?

Unless you can be very sure of what kind of telescope you want, then consider buying a 100mm to 150mm (mirror diameter) Newtonian telescope. A Newtonian will definitely give you the 'best bang for your buck', especially in this very popular size range. If you should later decide to invest in something different or perhaps more ambitious, then you will not have lost much on the Newt', and it could still be useful for 'wide field' viewing, and at star parties, etc.

Do keep in mind that big telescopes can be HEAVY! I enjoy my 250mm/f=1200mm Newtonian, but with it's big equatorial mount and a total weight in excess of 50kg, this is not really a portable system. It's now permanently set up on a steel pier in my back garden! For portable use, I have a 127mm/f=1500mm Maksutov Cassegrain telescope and a 72mm/f=430mm semi-apo refractor which I can use on a small equatorial mount.


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07/01/13