NightSky » Know-How » A Beginners' Guide to Telescopes

 

 Last month we discussed telescopes. This month, we examine the top ten features that define the best aparatus on which to mount them. There are two basic types of telescope mount (and some variations therein), each with a shared set of additional features that can make your viewing experience a delight:

1. The alt-azimuth mount is probably the most familiar type of telescope mount as it's the type suitable for terrestrial observing. It rotates in the azimuth (left and right) and the altitude (up and down). It's very easy to use but, unless motorized and computer controlled, does not make tracking the stars easy — one has to manage the up/down and left/right tracking and at high magnifications or for photography this is impractical.
2. The equatorial mount is specificially designed for astronomy and rather than having terrestrial axes it has one for declination and one for right ascension — the equivalent of up/down and left/right in celestial terms. It has to be polar aligned (see tip 7 below) but once achieved only one axis (not two) needs to be managed. Furthermore, add on a simple motor drive (see tip 8 below) and this enables easy tracking of objects and some impressive starter photography.
3. The Dobsonian is a special type of alt-azimuth mount. It does away with a tripod in favour of a large rudimentary pitch and yaw system. This is great for low powered viewing and thus ideal for deep sky objects but it's not forgiving for any kind of high magnification observing or photography.
4. The fork-mount is another special type of alt-azimuth mount which is a deluxe system for very short length telescopes — typically schmidt cassegrains — which are a delight to use. Because they are usually computer controlled they have the same benefits of the tricky-to-use equatorial mount, especially when fitted with a wedge (see tip 9 below).
5. For equatorial mounts, setting-circles provide a low-tech solution to finding celestial objects of interest. Once set up (by finding a star of known coordinates), they can be set to any (visible) coordinates reducing the time it takes to find objects of interest. They are also useful for noting unknown objects' coordinates which can later be looked up.
6. Fine adjustment controls are a cheap way of improving a telescope's set of features. These are turned by the observer and induce minute rotations in the axes; they circument the tricky job of tapping the telescope to make tiny adjustments to the object in the field of view and they do so without introducing wobble.
7. An equatorial mount needs a pole-finder for it to be useful for tracking celestial objects over long exposures or at high magnifications. The pole-finder (the astronomers' equivalent of a plumb-level) quickly identifies the Pole Star which the telescope's R.A. axis should point to correctly track stars. Without it, one finds themselves waiting for stars to drift out of view to know in which direction to rotate the mount and it becomes a pain for telescopes which aren't permanently mounted.
8. Nowadays, many mounts whether they be equatorial or alt-azimuth come with computer-controlled motors. Some request just one star, some two and others three to be identified by the user before the computers do the rest and know from that point onwards where the scope is pointing and thus where it should be pointing a tenth of a second later. For astrophotography, this is priceless. For navigation, a joy; it knows exactly where every other object is at any given time!
9. For photography of five minutes and greater exposures, a computer driven alt-azimuth mount (such as the Schmidt-Cassegrain's fork mount) exhibits a flaw. Because the telescope does not rotate (twist) on a polar axis, the photograph induces radial star trails (or blurring of aparent larger objects). For fork-mounts, one can buy a wedge which totally combats this problem: the wedge points the top of the scope to the Pole Star making it act like an equatorially mounted one and supreme photography can be achieved. (At this stage of our top ten, prices are out of the reach of most amateurs.)
10. Finally, and speaking of price, you should consider spending about a third to a half of the price of the telescope's body on the mount. If your budget is much less than this, it might be compared to buying a formula one car and then putting budget tyres on it — you're just not going to benefit from the scope unless it's on a mount worthy of the quality! (And this is even more important for high magnifications!)

 Whether you have £300 or £3,000 to spend, the fundamentals of a telescope are the same. Of course, the more you spend the bigger and better the scope. In this month's astronomy feature, we look at some of the basics of the optical tube assembly (or just "tube") — that is to say the tube and mirror or lens part of a complete telescope solution — as well as a taste of telescope mounts, finderscopes and eye-pieces. In subsequent features, we'll delve into each component in more detail. Here are the top 10 things you need to know about telescopes:

1. There are three main kinds of telescope:
   • A refractor has a convex lens (or combination of lenses) at the front of the tube and it focuses an image via an eye-piece at the back of the tube.
   • A reflector has an opening at the front of the tube, a primary concave mirror at the back, a secondary small flat mirror near the front and it focuses an image via an eye-piece at the side of the tube.
   • A hybrid has a combination of lenses and mirrors. The most common hybrid is the Schmidt Cassegrain. This has a corrector plate (a kind of lens) at the front of the tube, a primary concave mirror at the back, a secondary small convex mirror attached to the inner face of the corrector plate and it focuses an image via an eye-piece at the back of the tube.
   
   So, in brief: refractor = lens; reflector = mirror; hybrid = mirror & lens.
2. The main consideration when buying a telescope is the size — the diameter — of the primary mirror or lens. The bigger this value, the more light it collects, the more magnification can be achieved, and the more resolving power (or clarity) it attains. For a beginner's scope, a refractor should have at least a 4 inch lens; and a reflector or hybrid a 6 inch mirror. (Mirror telescopes need a larger diameter compared to their refractor counterpart due to light absorption of a mirror and the fact the the secondary mirror blocks some of the light.)
3. You get what you pay for. A telescope that has a big mirror or lens but is cheap might seem too good to be true. It probably is. The mirrors in reflector telescopes have a number of possible coatings and a number of different types of glass. Poor quality mirrors exhibit ghosting and inferior images. The quality of lenses in refractors is even more wide ranging. A cheap lens might only have one element. This means that the stars appear to be multicoloured with a red to blue gradient, or worse, a rainbow-coloured halo. Achromatic lenses have two elements which bring the colours of the light to a sharp focus meaning a much better image. Apochromatic telescopes are better still, at a much steeper price.
4. Knowing what you want to use the scope for is an essential question and ultimately the focal length of a telescope is an important consideration. This is the distance between the primary mirror or lens and the focal position at the eye-piece. (With hybrids that have a convex secondary mirror, an extended length is induced.) The longer the focal length, the more magnification can be achieved without having to use higher magnification eye-pieces. When the focal length of a telescope is divided by the diameter of the mirror (both in the same units), the focal ratio is given. So a telescope with f-length of 1200mm and mirror of 150mm has an f-ratio of 8. As a rule of thumb, f-ratios of under 6 are great for deep-sky objects. F-ratios above 8 are great for planets. In-between these values provide good general viewing. If you are more interested in astrophotography, you might consider a hybrid.
5. As previously alluded to, eye-pieces form part of a telescope. The eye-piece focuses the light that the rest of the telescope has manipulated and forms the image. The focal length of an eye-piece determines the magnification of the final image. The focal length of the telescope divided by the focal length of the eye-piece is the magnification. Thus, a scope with an f-length of 1200mm with a 10mm eye-piece provides a magnification of 120x. In May's feature, we'll look at eye-pieces in more detail.
6. We've now discussed all the elements that make up the magnification. However, magnification isn't everything! With every doubling of magnification comes a quartering of light. Furthermore, all telescopes are limited by their resolution and thus the maximum magnification any telescope can happily achieve is determined by the size of the mirror or lens. As a rule of thumb, the maximum magnification is equal to the diameter of the primary mirror/lens in millimetres times two. So, for a mirror of diameter 6 inches (150mm), 300x is the most you can comfortably see. So, if it has an f-length of 1200mm, a 4mm eye-piece (1200/4=300) is the smallest useful eye-piece. Go smaller than this and you might as well rub sand in your eyes.
7. More important than magnification is light gathering power. This is determined by the size of the primary mirror or lens measured by area. To calculate the light gathering power, this value is compared to the area of the naked eye. For example, an eye with a 10mm aperture (fully dark adapted) and a telescope with a 150mm diameter mirror receives 225x more light. That is 150²/10². Thus if you're comparing a 200mm mirror to one of 150mm, consider that the light gathering power is 1.77x more.
8. The resolution of the telescope is determined by size of the primary mirror or lens measured by diameter. This is measured by the Dawes limit (or sometimes by the Rayleigh limit). In simple terms it defines the clarity or sharpness of the image. To calculate the resolution, divide the number 116 by the diameter of the mirror or lens in millimetres. Thus, a diameter of 150mm has a resolution of 0.77 arcseconds. That is to say, a double star (or pair of lunar craters, or rings of Saturn, etc.) that are 0.77 arcseconds apart can just be resolved by the scope.
9. The finder scope is a tiny telescope that is attached to the main tube. It has a much reduced magnification and allows us to align the scope to an area of the sky before narrowing in with the main one. Without clever computer tracking of stars, it's an important piece of kit. A 10x50 finderscope will be more useful than a 6x30 one.
10. Finally, there are two main types of mount as we'll be seeing next month in our feature. These are alt-azimuth mounts which are simple to use but do not follow the path of the stars. And equatorial mounts which do. The latter is greatly preferred, especially for high magnifications or astrophotography. Come back next month to see our Top-10 tips!