Your First Telescope…
A Telescope is really a time machine. It`s job is to gather light from distant objects and magnify the view.While different telescope designs concentrate that light in different ways,the goal remains consistent.Light is collected by a primary mirror or lens and is focused to an image, which you view through a magnifying eyepiece.
The primary function of a telescope is to gather light. Beginners mistakenly place too much emphasis on a telescope`s magnification abilities. Think of it this way : if you`re looking for something small in a dark room, what you need most is light, not a magnifying glass.All other characteristics of the telescope (resolving power, maximum useful magnification, and so on) are directly related to the scope`s aperture. So you should buy a telescope with the largest main mirror or lens that`s practical for you. A 6″ Reflector or 4″ Refractor is the smallest size to be of any serious use in astronomy. Nevertheless, beware of going too large. O.K. large scopes gather more light, but they are a real hassle to carry in and out, and you may lose your motivation to take it outside.
Magnification is important, and although many of the objects at the top of a beginners observing list are quite large ( the Andromeda Galaxy is roughly eight times the apparent diameter of the Moon ), sometimes you`ll want to zoom in close to see detail on planets and the Moon or to resolve a tightly paired double star. You ca figure out a telescope`s magnifying power easily. First, find the eyepiece`s focal length. It`s shown on the side of the barrel. Next, find the telescope`s focal length, usually the “F” or “FL” printed on the specification sticker near the front lens, or the user manual. Both these numbers should be given in millimetres, and magnification ( or power ) is just the telescopes focul length divided by the eyepiece`s. For example, a telescope with a focal length of 1000 millimetres, coupled with an eyepiece of 25mm focal length, gives a magnification of 40x (1000 / 25 ). To change the magnification, select an eyepiece marked with a different number : Switch to a 10mm eyepiece and you`ll crank up the power to 100x ( 1000 / 10 ).
Resist the temptation to push the power too high in the beginning. Raising the magnification makes objects look larger in the eyepiece, but it also magnifies undesirable things as well, such as shakeness in the telescope`s support and blurring caused by the atmosphere. The trick is to use the proper magnification for a particular object on a particular night. There`s a rule of thumb that says a telescope should be good for a maximum power of about 50x per inch of aperture. As such, a 3″ telescope shouldn`t be pushed beyond about 150x and an 8″ scope beyond about 400x. Keep in mind that the maximum useful magnification will vary, depending upon local observing conditions and the quality of the telescope`s optics. But it`s a rare telescope that can give satisfactory views at 675x, no matter what the ad. says.
Which Type of Telescope
The Newton Reflector
Named after it`s inventor, Sir Isaac Newton, the Newtonian reflector collects light with a concave mirror and focuses it to an image near the front of the tube, where it`s redirected out the side by a small second mirror.
Reflectors have the advantage of being the least expensive, aperture for aperture, of all telescope designs. If pure value is what you`re after, look no further. They are also colour free, since mirrors do not introduce chromatic aberrations the way lenses do.
On the downside, reflectors tend to get awkward quickly at apertures above 10″. Also, they tend to need periodic optical alignment, called collumation, especially if they are transported around a lot. But collumation is just a 10 minute procedure, depending on your skill level and how picky you are.
The Refractor
A refractor is what most people think of when they heer the word ” telescope.” Light is gathered by a lens at the front of the tube, and the image is viewed through an eyepiece at the back of the tube. Refractors have the advantage of an unobstructed light path, unlike most other designs, there are no other components in front of the objective to block incoming light and cause image artifacts. Refractors also require little maintenance, especially when it comes to keeping the optics aligned. They tend to be tolerant of temperature changes that occur when a telescope is brought outside, so they deliver optimum images without collumation times. On the downside, refractors are, inch for inch of aperture, the most expensive telescope design. They also tend to get large and heavy at apertures of more than 4″. Finally all but the most expensive models suffer from chromatic aberration, or false colour, in the form of a blue-purple halo around bright objects. Refractors cost as much as £500 to £1000 per inch aperture at the high end of the market while at the bottom, ( department store instruments that, frankly, aren`t very useful ). However, the recent wave of low cost imports from the far east has made quality-refractor ownership a reality for many amateur astronomers.
