Telescope designs and their optical layout
is by no means intended as an exhaustive guide, there are numerous designs
around but most will be based on one of these basic designs; often with
quite subtle differences.
Not wishing to buck a trend, in common with most text books on the subject we start with the basic refracting telescope. The optical layout for which is shown below...
is what everyone recognises as a telescope;
it has a lens,
called the Objective lens at the front.
In some designs, particularly smaller scopes, the air spaced doublet will
be replaced by a compound lens with the two elements cemented together.
A basic achromatic refractor design (achromatic meaning colour-free) is capable of producing superb high quality images, even a 60mm ( 2.4") telescope can offer good introductory image quality for Lunar and Planetary observation. However at focal ratios of below f/10 chromatic aberration becomes increasingly obtrusive; that is their images can be affected by coloured fringes around objects. Achromatic refractors bring both ends of the spectrum to almost the same focus, but some colour contamination can still be evident, particularly at higher magnifications.
Various apochromatic designs, often employing three elements, offer a vast improvement in terms of chromatic aberration and allow refractors of lower focal ratio to be produced. These designs are however relatively expensive and, for larger apertures, prohibitive.
The main advantage of the refracting telescope is that, unlike the other designs that follow, there are no obstructions in the light path. Any obstruction in the light path will modify the diffraction pattern compared to that of a clear aperture; this tends to reduce contrast in the image.
However contrary to the hype you may read elsewhere - if the obstruction is relatively small compared to the aperture the difference can be very marginal indeed and may only become noticeable when pushing the scope towards it's maximum useful magnification.
The REFLECTOR TELESCOPE (also known as a Newtonian) collects light using a parabolic mirror referred to as a primary. This primary mirror captures the light and brings it to a focus. Because the light is reflected and does not pass through the glass, it is effectively free from false colour. To bring the image to a position for the observer to view, a flat mirror is placed in the light path to deflect it to the eyepiece which is mounted on the side of the tube. As mentioned earlier the central obstruction cased by the secondary mirror slightly degrades the image in comparison to a Refractor of the same aperture.
Primary mirrors are cheaper to produce than objective lenses. A 150mm (6") reflector will be about a tenth of the cost of a 150mm (6") apochromatic refractor - Newtonian Reflectors offer a better ratio of aperture-to-the-£ than any other telescope.
For deep sky observation where light gathering power is needed, large aperture reflectors are readily available at a reasonable cost. The basic Newtonian, particularly those with larger focal ratios, also make superb planetary telescopes. For all round performance and versatility the Newtonian telescope probably represents the best all round choice for most amateur astronomers.
The biggest disadvantages are that in larger apertures they become rather bulky, especially in the larger focal ratios. The Newtonian also requires more maintenance and regular collimation.
design uses a spherical primary mirror, the secondary convex mirror
reflects light from the primary directly back towards the primary and also
serves to increase
the effective focal length of the instrument. The light passes though a hole in the centre
of the primary mirror and comes to focus at a point just behind it.
Because the primary mirror is spherical the scope requires a corrector
(the Schmidt corrector) in order to produce usable images.
This design offers large apertures in a compact and portable package whilst remaining considerably cheaper than an equivalent aperture refractor. These scopes are extremely popular among amateur astronomers, largely because of the compact tube sizes.
For deep sky observers/imagers the relatively large focal ratio/long focal length can be a problem, therefore they are often used in combination with focal reducers for this type of observation/imaging.
The SCHMIDT NEWTONIAN
also employs a spherical mirror and a Schmidt corrector but has a flat
secondary mirror as in the Newtonian. The design optics yield sharp
stellar images over wide fields, while reducing coma (an optical
condition, particularly troublesome in low focal ratio instruments, that
can degrade an image) by one-half. Designed specifically to operate at extremely low
focal ratios, the Schmidt-Newtonian displays very wide, well corrected
fields of view with bright, rich-field imaging of nebulae, galaxies and
star clusters, and fast speeds for
astrophotography or CCD imaging.
These scopes are however not as good for planetary use as their Schmidt Cassegrain counterparts, to which they are complementary. Their relatively short focal lengths force the use of shorter focal length eyepieces and/or an additional Barlow lens. They are also far more prone to collimation errors and focusing is considerably more sensitive.
is also a hybrid mirror-lens system and was developed before the
Schmidt-Cassegrain. In the recent past it was available, but at a high
proportion of Maksutov Cassegrain telescopes on the market today are of
the Maksutov-Gregory design - where the secondary is silvered directly
onto the Meniscus Corrector negating the need for a separate secondary
With vastly improved production techniques now available, its cost has been significantly reduced. Although they remain more expensive than the Schmidt, particularly in larger apertures - they offer similar advantages to the Schmidt Cassegrain, but with improved correction for aberrations yielding higher image quality.
offers cost savings over a Refractor of similar aperture whilst
maintaining the advantages.
The optical configuration allows aberrations to be reduced to an
insignificant level and also allows for a relatively small central
obscuration, giving improved contrast over other reflector designs. The
Maksutov Cassegrain is commonly produced in focal ratios of around f/6
making it good for
rich-field imaging of nebulae, galaxies and star clusters.
In smaller apertures at around f/6 their focal length is less than ideal for planetary use but they still make excellent planetary telescopes, and at 200mm (8") aperture or above are they rivalled only by the best apochromatic refractors of similar aperture.
is not to be confused with the Maksutov-Gregory design in common use
today, usually referred to simply as the Mak. The Maksutov Gregorian as
shown above is a true Gregorian design in the traditional sense, employing
a concave secondary mirror to reflect light back towards the primary.
As in Cassegrain designs, light passes though a hole in the centre of the
primary mirror and comes to focus at a point just behind it.
Optically the Maksutov Gregorian will perform identically to a Maksutov-Cassegrain of equivalent aperture, but the Gregorian design requires a longer tube assembly - This is the main reason the original Gregorian reflectors fell out of favour.
There is an awful lot of hype around
these days which could lead the unwary to believe that the only telescopes
worth looking at are apochromatic refractors. Claims like an 80mm
refractor can out perform a 150mm reflector etc. abound - Yet all else
being equal such claims are complete nonsense!
Only reflective systems are totally free of chromatic aberration and refractors are just as likely to suffer from field distortion and other aberrations as reflectors.
The effect of the central obstruction in reflectors is not nearly as noticeable as you may be lead to believe elsewhere. In most designs it only becomes noticeable at magnifications approaching the maximum useful magnification for a given aperture. Many of the 'differences' in performance of various designs are very marginal indeed and overstated by those who simply want to sell a certain line in telescopes.
When buying a telescope, keeping an open mind could save you a fortune!!