How to Pick a Good Telescope

Aperture

• 1

  Check, using a tape measure if necessary, the size of the aperture of any telescope you are considering. This is the the diameter of its main lens or mirror. The larger the aperture, the brighter and sharper the image it can present. According to "Sky and Telescope Magazine", a telescope with a 3-inch aperture can spot craters on the moon that are around two miles across. A telescope with a 6-inch aperture can spot craters that are around one mile across.

• 2

  Check the "power" or "magnification" advertised for the eyepiece of any telescope you are considering. Ideally, this number should be around 50 times the diameter of the aperture. More than that, and the image you see may be too dim, or too blurred.

• 3

  Compare the prices of telescopes with different aperture sizes. Although buying the largest aperture size you can afford is tempting, consider too that telescopes with apertures above ten inches will be more affected by atmospheric conditions and may be best used for viewing dim-and-distant objects such as nebulae and star clusters, rather than moons and planets in our solar system.

Weight

• 4

  Check the advertised weight of any telescope you are considering. Add to this the advertised weight of any tripod you may need to use.

• 5

  Put a rucksack on the bathroom scales and add tins of food to it until you achieve a similar weight.

• 6

  Carry your rucksack to the places where you will want to assemble your telescope and to the place where you intend to store it. Larger telescopes can be very heavy and you may find they are not portable enough for you to make good use of them.

Telescope Type

• 7

  Choose a refractor-type telescope---a long tube with a large lens at the front and a small magnifying lens in the eyepiece at the rear---if you are mainly interested in looking at moons and planets. Refractors are robustly made, but they can be very long when the main tube is fully extended, which can make them awkward to handle.

• 8

  Choose a reflector-type telescope---which contains mirrors to collect and focus the light and which has its eyepiece near the top, at right angles to the main tube---if you want the biggest aperture for your dollar. The length of the main tube is shorter than its equivalent refractor and even the longest ones are easier to manage, as they have a lower center of gravity since the eye-piece is towards the top rather than at the bottom of the tube. Other types of telescope reverse left and right or up and down on the image they present; reflectors avoid this problem too. However, the mirrors of the reflector often need realigning and cleaning and this maintenance can be awkward and fiddly.

• 9

  Choose a catadioptric or compound telescope---which uses both lenses and mirrors---if you want a compact, versatile, short telescope that is easily portable. This a medium-priced telescope, costing more than a reflector, but less than a refractor. It is suitable for observing nearby planets and more dim-and-distant objects, although the quality of its images may not be as sharp or clear as those given by refractors or reflectors. It does not require the maintenance of the reflector though, as it has a sealed tube.