TELESCOPE TYPES
There are three basic types of telescopes - Refractors, Newtonian reflectors and Catadioptrics but which is best? Unfortunately there is no easy answer to this as all types have advantages and disadvantages to their design. Cost, durability, portability and type of astronomical application will all have to be taken into account i.e. a small portable refractor may be preferable over a larger heavy reflector if you have to travel to a dark site and do not mind the lower light grasp. We could go on forever about this subject so please read on for a brief description of the types.
REFRACTOR
Identified by the lens at the front, which is known as the 'objective', which is usually coated to increase light throughput and contrast. The eyepiece is at the other end of the tube. It is also sealed which decreases dust and air currents within the tube. Due to their construction they rarely need culminating (adjusting) and are very robust.
ADVANTAGES
1) Robust which makes them easily portable and will not require culmination.
2) Very good for planetary, lunar and double stars due to the sharpness and high contrast of the image.
3) Can be used for terrestrial viewing.
4) Needs little cool-down time compared with reflector.
DISADVANTAGES
1) Not really suitable for deep sky objects when compared to equally priced reflectors which will have more light grasp.
2) Very expensive when compared to similar size reflectors.
3) Awkward when viewing near zenith.
4) Colour aberration with achromatic designs unless a very high focal ratio is used.
NEWTONIAN REFLECTOR
The Newtonian Reflector uses two mirrors in its design. One being a large mirror which is concave and a small secondary mirror at a 45-degree angle near the top of the tube which reflects the light towards the eyepiece. A high quality Newtonian with hand made optics are considered by some to be the ultimate scope but are extremely expensive costing anything up to �2000 just for an 8" main mirror.
ADVANTAGES
1) Larger than equivalently priced refractor due to reduced manufacturing costs.
2) Great for deep sky objects due to large objective size and low focal ratio.
3) Little or no false colour aberration.
4) Easy to use when looking at zenith.
DISADVANTAGES
1) Needs regular culminating to get best performance due to fragile design.
2) Air currents in tube affect image.
3) Must be allowed to cool down for optimum performance.
4) Slightly decreased contrast due to the secondary mirror obstruction.
5) Mirrors are harder to clean than lenses and need it more often due to the open tube design.
6) Not easy to use for terrestrial viewing.
OVERVIEW
The refractor may produce slightly better high power images than the reflector but it can often cost three times as much for the same size. Usually, a refractor is used for planetary observing, a reflector with a focal ratio of about f6 used for general observing (planetary and deep space) and a fast reflector with a focal ratio of about f4 for deep space observing only. We must stress that this is only a guide though and it is assumed that all the different types of scope are of equal optical quality!
OTHER TELESCOPE DESIGNS
There are newer designs of telescopes, which are a mixture of mirrors and lenses, which are called 'CATADIOPTICS'. A mixture of reflector and refractor if you like and the two main designs are the 'Maksutov' and the 'Schmidt-Cassegrain'. The main difference between them is their front correction lens. In both scopes the focal ratio will be about f10 or higher.
CATADIOPTRIC ADVANTAGES
1) More compact and lighter than equivalent size refractor/reflector.
2) Often come in GOTO form (computer controlled).
3) Schmidt-Cassegrain is good for general observing (planetary and deep space).
4) Maksutov is extremely good for planetary/double stars and often compare to a good quality Apo-refractor.
5) Can be used for deep-space due to their brightness.
6) Little or no false colour.
7) Sealed tube so does not need cleaning.
8) Less coma than classical Newtonian design.
CATADIOPTRIC DISADVANTAGES
1) Do not offer wide field views.
2) More expensive than equivalent size reflector.
OVERVIEW
The maksutov design produces higher quality images than a Schmidt-Cassegrain which is mainly due to its high quality corrector lens being spherical. The disadvantage is that it is heavier