Types of Telescope...
f all the many and varied telescopes Oavailable for use by amateur astronomers and nature watchers, all can be categorised into three types: the
refractor, the reflector & the catadioptric. Each have their relative strengths and weak- nesses, but they all have a common function: to gather and focus light from distant objects to produce a bright image that may be magni- fied. In this respect it is the aperture (i.e. the diameter of the main mirror or lens) of the telescope that performs a critical function. With larger apertures, more light is gathered so fainter objects may be perceived and the resolving power (i.e. the ability to see fine detail) is increased.
When comparing telescopes of similar type but of differing apertures, an instrument that has a main lens or mirror twice the size of another gathers four times as much light, not twice. We are comparing the collecting areas of the larger and smaller telescopes, which is proportional to the squares of the apertures. So, for example, a 120mm telescope gathers
2.25times (225%) as much light as an instru- ment of 80mm aperture (120/80 = 1.5, 1.5 squared = 2.25).
When it comes to seeing fine detail in an image, we are simply comparing apertures: a 120mm telescope will enable you to perceive lunar craters, for example, half the size of those visible in a 60mm instrument. No amount of magnification applied to the smaller telescope will show you that which will be visible in the larger instrument, though both images may appear equally sharp. The larger telescope merely forms its images out of smaller 'dots'.
However, the resolving power (as it is cor- rectly termed) of even the smallest telescope is awesome: a 60mm telescope is capable of resolving detail as small as a £1 coin at a dis- tance of 2.35 km - nearly 1.5 miles!
< The refractor:
This is the type of instrument that the lay- man thinks of when conjuring up a mental picture of a telescope.
At the end of the tube furthest away from the observer there is an objective lens (or rather two glass elements sandwiched together for reasons to be ex- plained in a moment) that gathers light from the object under scrutiny, to form an image at the other end which is viewed by means of an eyepiece.
The objective lens can- not be made of a single piece of glass since such an element is incapable of bringing light of differing wave- lengths to a common focus, introducing a prismatic effect that causes bright objects to be surrounded by false rainbow colours.
This undesirable quality of refractors is virtually eliminated by making the objective out of two glass elements with optical characteristics that effectively 'cancel out' the false colour.
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