ABSOLUTE BEGINNERS

This article featured in the September 2000 Beginners Magazine

BUYING YOUR FIRST TELESCOPE

To most people, the first question to ask about a telescope is "What magnification will it give?" but the important properties of an astronomical telescope are ‘the light gathering potential’ and ‘the accuracy of the optics’.

This means the bigger and more accurately configured the optics are, the more powerful the telescope.  There are however penalties in bigger and more accurately polished optics, the main one being costs.  As a rule of thumb, doubling the diameter of the primary optics will increase the cost by a factor of ten.  Therefore a 150mm (6inch) telescope might cost £300 a 300mm diameter mirror may cost £3000 and a 600mm diameter instrument £30000.

There are two basic optical systems used and the telescopes using them are called a REFRACTOR or a REFLECTOR.  The refractor uses a lens to focus light into an image and a reflector uses a concaved mirror to focus the image.  In both types the image may then be magnified and studied using an eyepiece rather like using a microscope.

There are also two types of mounting used for holding the telescope tube these are known as Altazimuth and Equatorial.  The Altazimuth is the type of mounting used on seaside telescopes, camera mounts and other tripod type mountings.  It has a rotating base like a turntable and a fork mounting to allow the telescope to be pointed up and down.  The simple mounting used on most amateurs telescopes is an Altazimuth type designed by an American named John Dobson, hence the name Dobsonian.  This mounting has a turntable which rotates on bearings.  The base has feet for stability on rough surfaces and the rotating box on the turntable support has bearings to support the telescope in its two side panels.

A more sophisticated type of mounting is the equatorial type.  This mounting is necessary if the telescope is to be used for photography.  The rotational axis is tilted at the same angle as the axis of the Earth.  This allows the telescope to be easily driven to track stars as they move across the sky.  The advantage is that because the telescope and Earth are rotating about the same axis only one axis needs to be driven, at the same rotation speed as Earth rotates (once every 24 hours).

There are cheap 100mm (4 inch) telescopes sold by main street stores and catalogues for about £200.  These are quite good but if astronomy is to be pursued as a hobby it is better to spend a bit more to get a 150mm (6 inch) for about £370.  It is possible to buy a second hand telescope through the adverts in astronomy magazines.  A 150mm may cost less than £200 possibly even as low as £150.  It is worth getting a larger second hand telescope rather than a smaller new one which will not have enough light grasp to enable the new astronomer to see some of the more interesting objects.

The most important advice is to buy the biggest telescope you can afford, even if it is second hand.

 

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