Which Telescope Should I Buy?!
Before looking at the range of telescopes that is available, consideration should be given as to what the telescope is to be used for. This is the most important consideration of all. The worst possible choice is the one that never gets used.
The telescope that is too complicated or too cumbersome to set up will spend its time at the back of a shed or garage and never be used. The first telescope should be easy to set up, easy to use yet give impressive views of the sky.
First, we must consider some of the physical characteristics of a telescope and how they affect our requirements.
The main specifications for a really useful first telescope should be: a minimum aperture of at least 60mm for a refractor or 130mm for a reflector and 900mm focal length for each. This combination will provide enough light grasp and permit a high enough magnification to see detail on the brighter planets.
One of the most important attributes for an astronomical telescope is the ‘light grasp’. This is an expression used by astronomers to describe the process of the main optic directing light from a distant object into the eye. The pupil of a young human eye is about 7mm in diameter when fully adapted to the dark. This is equivalent to 38 square millimetres (38mm²). A 100mm diameter telescope has an aperture area of 7,854mm². It is therefore capable of directing up to 206 times as much light into the eye of the observer. Put another way it will enable the observer to see objects about 200 times fainter than could be seen with the unaided eye. So a larger aperture will allow even fainter objects to be seen.
The focal length is also important. The FOCAL LENGTH is effectively the length of the telescope. It is measured as the distance from the main optic to the point where the image is formed. A short focal length will give a wide field of view but the objects in the field of view will appear small. A long focal length will give a narrow field of view (small area of sky) but the objects in view will appear larger. Short focal lengths are best for looking at star fields and larger objects. Long focal lengths are most suitable for small objects and studying fine detail, for example on the Moon and the planets.
Magnification, strictly speaking, is not an attribute of the telescope it mainly depends on the eyepiece being used. A telescope of a specific focal length will produce an image of a specific size and this cannot be changed. For example a telescope of a certain focal length may produce an image of the full moon 10mm in diameter. A longer focal length will produce a larger image and a shorter focal length will produce a smaller image. The eyepiece is then used, much like a microscope, to magnify that image. A larger image to start with will allow the eyepiece to produce a higher magnification.