Choosing A Telescope - Part 2

By Adrian Ashford

It's All Done With Mirrors

The second type of telescope, the reflector, uses (as its name suggests) mirrors to gather and focus light from the object under scrutiny. In its most commonly encountered form the Newtonian — which has been around for more than three centuries — there's a specially curved concave (dish-shaped) primary mirror at the bottom end of the telescope. Near the top of the tube a small, angled secondary mirror directs the light reflected from the primary to the side of the tube, where it's met by a conveniently placed eyepiece. If you want the largest aperture for your money, then the reflector is unquestionably the scope for you. When well made and maintained, a reflector can provide sharp, contrasty images of all manner of celestial objects at a small fraction of the cost of an equal-aperture refractor.

The tube of a Newtonian is considerably more manageable, too. Its length is rarely more than eight times the diameter of the primary mirror, and frequently less. This means an 20-cm Newtonian can be housed in a tube only 1.6 metres long, easily fitting in the back seat of a small car for transportation to dark, rural skies. Combine this with the Newtonian's generally low centre of gravity, and you end up with an instrument on a compact, stable mounting that presents the eyepiece at a convenient height for just about any sky orientation.

For the best value of all, much consideration should be given to a particular type of reflector known as the Dobsonian. This is a Newtonian on a very simple, very rugged mount. These extremely popular instruments are available in apertures from 20cm to more than 76cm and represent the ultimate in observer convenience for casual viewing.

Like all reflectors (there are other types, but we'll not consider them here because they tend to be specialist instruments not commonly encountered in amateur hands), Newtonians will require occasional maintenance. Unlike a refractor's solidly mounted lens, a reflector's mirrors can go out of alignment and hence will need periodic collimation (adjustment) to ensure peak optical performance, particularly if the telescope is moved frequently. This should not be overstressed, since the mirrors of the average Newtonian will not require tweaking for many months at a time. But for those observers not mechanically inclined, having to collimate a reflector even occasionally may be a frustrating or undesirable proposition. The open nature of the reflector's tube means that dust and dirt are more likely to accumulate on the optical surfaces, which will need periodic cleaning. Also, the aluminised surfaces of a reflector's mirrors will need recoating every 10 years or so — more frequently if you live in air-polluted urban areas or by the sea.

The Best of Both Worlds

Then there's the third category of instruments, the catadioptric or compound telescope. These came about in the 1930s out of a desire to marry the best characteristics of refractors and reflectors: they employ both lenses and mirrors to form an image. The greatest appeal of these instruments is that in their commonly encountered forms (the Schmidt-Cassegrain and Maksutov-Cassegrain) they are very compact — their tube lengths are two to three times the aperture of the scope due to the "optical folding" of the light passing through them. The smaller tubes can use lighter (and consequently more manageable) mounts and tripods. The practical upshot is that you can obtain a large-aperture, long-focus telescope that's very transportable.

But here too there are caveats. Like the Newtonian, the Schmidt-Cassegrain needs occasional optical collimation that lessens its appeal to those disinclined to tinker. Their fields of view can be quite narrow, too. In terms of cost, aperture for aperture, the catadioptric lies midway between the reflector and the refractor. Like a Newtonian, the popular forms of this compound telescope have secondary mirrors that lie in the light path of the instrument and slightly degrade performance for critical lunar and planetary observations. Even so, when well made, a Schmidt or Maksutov will deliver very fine images of a wide variety of celestial objects.

In common with refractors, the tubes of catadioptrics are sealed so that dirt and dust are largely excluded — a big plus for an instrument that you're going to take out into the country. But if you live in an area where dew is prevalent, some sort of collar or extension to prevent misting of the exposed corrector plate at the front of the tube is a good idea. In practice, many people seeking a highly versatile, very portable (for the aperture) scope that can be used for all sky subjects and astrophotography will tend to opt for some form of compound instrument. In short, they're excellent general-purpose scopes that can use a wide variety of accessories.

Telescope Mounts

The best telescope in the world — be it refractor, reflector, or catadioptric — will be rendered useless unless it's attached to a stable mount that permits it to be directed to any desired part of the sky and to follow a celestial object smoothly and precisely. A stable mount is one that, when you're using a moderate to high power, will not vibrate for more than a second or so after applying a small rap to the tube. While there are variations on a theme, you'll encounter two types of mount: altitude-azimuth (or "alt-az") and equatorial. Alt-az mounts operate like tripod-style pan-and-tilt heads, moving the scope up and down (in altitude) and left to right (in azimuth). Equatorial mounts also possess two axes, but one is aligned with the rotational axis of the Earth.

Those intending to use small telescopes for casual sky viewing or daytime use (say, bird-watching) will find the alt-az mount preferable. Better engineered mounts of this type will be equipped with finely threaded slow-motion controls that enable the scope to be moved smoothly by tiny amounts, especially when you're using high powers. The value of such refinements will be all too apparent when you are tracking a star or planet at higher magnifications.

The aforementioned Dobsonian is a form of alt-az mount that's quite popular. Unlikely telescope-making materials such as particleboard and Teflon figure in its construction, resulting in a lightweight, low-center-of-gravity mount that glides smoothly about both axes with fingertip control. A Newtonian reflector mounted in this fashion is not only extremely easy to set-up and intuitive to use, but very good value, too.

For a telescope dedicated to astronomical use and for which astrophotography is a future prospect, prime consideration must be given to some form of equatorial mount in order to automatically counteract the effects of the Earth's rotation. It's far easier to track a celestial object with a scope mounted in this fashion since observers need only concern themselves with turning the scope about one axis, not two simultaneously as in the alt-az. When these scopes are properly set up, turning the slow-motion control of the equatorial's polar axis is all that's required to keep an object in view. More sophisticated mounts (including some alt-az mounts) will have built-in electric motor drives to do this, freeing you to concentrate on observing. Another big plus of the equatorial is that it's frequently equipped with graduated circles about both axes that permit you to "dial in" the coordinates of celestial objects found in Australian Sky & Telescope, books, or on the Web.

So is one type of mount better than the other? Not really, since each has its own strengths. For the casual observer who desires a highly portable scope mount that can be quickly set up in a variety of locations, then an alt-az is actually preferable — especially a Dobsonian. Equatorials, while virtually mandatory for most forms of astrophotography and critical observations of the Moon and planets at high power, need to have their polar axes precisely aligned with the rotational axis of the Earth. While polar alignment is not a particularly difficult procedure and becomes easier with practice, it can take a little time at the start of your observing session. This is why large instruments on equatorial mounts are usually permanently housed in observatories.

"Go To" Scopes

Currently very much in vogue are the computer-controlled robo-scopes appearing on the market in various guises from several manufacturers. These are refractors, reflectors, and catadioptric instruments on mounts that are controlled either by a plug-in hand-held computer or remotely by an external PC. This allows you to direct the scope to any object in the computer's database. At first glance these "Go To" units would appear to be the answer to a novice's dream, since they ostensibly take all the hard work out of finding elusive objects like faint galaxies, star clusters, and minor planets (asteroids).

There's no denying that when well engineered, these robotic scopes are great fun to use, as they almost magically slew across the sky in search of whatever you've keyed in, zeroing in on the target to be presented in the eyepiece. But this technology is only just beginning to mature to the point where these scopes will automatically orient themselves when you take them outside and switch them on. Almost all Go To systems will ask you to enter the precise geographical location of your viewing site (or the nearest city) and the date and time at the beginning of each observing session. This enables the instrument's onboard computer to accurately calculate the positions of any celestial objects you may care to look at. Usually you'll also have to level the telescope's tube, point it south, and then launch into an alignment procedure that uses two bright stars (which you must know by name) to synchronise the telescope's coordinate system with that of the sky.

It's true that these setup routines are easily mastered with practice. But for someone completely unfamiliar with the sky, the vast majority of the current batch of robotic scopes have the potential to be very frustrating at first. Still, help is on the way. The newest crop of Go To scopes include their own Global Positioning System devices to tell you (and your telescope) exactly where you are and what time it is, thereby making setup a little easier.

Here's one last thing to keep in mind: the money spent on a Go To scope's electronic mount could be invested in a traditionally mounted scope of larger aperture.

Click here to read Part 3.