After satisfied of Obtaining sampled what the sky has to offer to the unaided eye, or experimented with taking pictures of the Moon, planets or stars with a digital camera, the next step is to buy a telescope. This isn’t a choice to be taken lightly, and this article is created to provide the essential information that you will need to choose a suitable instrument. Astronomical telescopes are of two main kinds, refracting telescopes, or refractors, and reflecting telescopes, or reflectors.
A refractor collects the light from its target object and passes it through a glass lens (also called an object glass or objective) where the rays of light are collected and brought into focus. The image is enlarged by a second lens known as the eyepiece or ocular. It is obvious that the larger the main object glass, the greater the amount of light that can be collected.
This in turn means a higher maximum magnification can be achieved. Different eyepieces can give various magnifying powers. An astronomical refractor gives an upside-down image which, if required, can be turned the right way up with the addition of a correcting lens. Since this slightly reduces the amount of light reaching your eye, correcting lenses are rarely ever used for astronomical purposes. One negative aspect of a basic refractor is that the different parts of the spectrum of light are brought to focus in different planes, so a bright image tends to be surrounded by gaudy coloured rings which may look beautiful but are a nuisance to the astronomer.
This difficulty is avoided with a reflecting telescope, where the light collection is done by means of a mirror. A popular reflector is the Newtonian, first designed by the great scientist Isaac Newton way back in 1651. The light falls on a mirror which is specially shaped and reflects back on to a smaller flat mirror at an angle of 45 degrees. This mirror diverts the light to the side of the tube, where it is enlarged by an eyepiece as before. In a Newtonian reflector you look into the side of the tube instead of up it. From the amateur’s point of view, these two types have their own advantages and drawbacks. The refractor needs very little maintenance and will last a lifetime if reasonably well treated.
On the other hand, a reflector has to have its mirrors coated periodically with some reflecting substance, generally a thin layer of aluminium.This has to be done regularly. In addition, a reflector is prone to going out of adjustment. Against this, a reflector is cheaper than a refractor of the same power because a large mirror is simpler to make than a large lens; all the world’s largest telescope are reflectors. The biggest refractor, the Yerkes, is 40 inches across; the largest single mirror reflectors have mirrors more than 323 inches in diameter. Modern manufacturing techniques have opened the market for new and improved, cost-effective telescope designs. These include large-diameter reflectors, colour-corrected refractors and hybrid telescopes known as catadioptrics, which incorporate both lenses and mirrors.
Choosing a suitable scope out of what has become a rather bewildering array of instruments has actually become more complicated over time because of the extended choice. In reality there is probably no one simple answer to the question of which telescope is best for you, but a simple bit of self-questioning may help to narrow your options quite considerably. The first question is one of budget and how much are you prepared to spend? For a first scope purchase this is a pretty serious question. Do you spend lots of money on a top-of-the-range model which you’ll keep and use for many years, or do you spend a small amount on a telescope to let you dip your toe in the water? The only person that can answer this question honestly is you, but experience suggests that the second choice is probably the right one. Spending a lot of money on what is essentially a blind purchase may lead to disappointment, an unused scope and wasted funds.
If you are looking to upgrade from an existing scope to a more advanced one, the choice is easier because you will know about the foibles and limitations of your current instrument and have a greater sense of the direction you want to move in. Once your budget has been set, the next decision needs to be geared toward what you want to look at in the sky. The choice ranges from the Moon and brighter planets to the stars, galaxies, nebulae and clusters that pepper the night sky. Let’s deal with the simple situation first, when your interest is either Solar System or deep-sky objects.
For Solar System observing you will need a large scope with a long focal length. The larger the diameter of the telescope the more resolving power it will have, enabling you to see smaller and smaller detail. Unfortunately, with increased aperture size comes increased cost, so this is where your budget limit kicks in! The brighter planets and the Moon require a reasonably high magnification to see significant detail. As magnification is calculated by dividing the focal length of the telescope by the focal length of the current eyepiece, it is clear that a telescope with a naturally long focal length will be able to get up to high magnifications more easily than one with a relatively short focal length. Similarly, if your interests are in imaging these objects, a long-focal-length instrument will give you a much better opportunity to use higher magnifications. As an example, consider a telescope with a 400mm focal length using a 10mm eyepiece.
Here the magnification will be 400/10 = 40x. Compare this with a telescope with a 2000mm focal length using the same eyepiece; the equivalent magnification is 2000/10 = 200x, which is much better suited for planets. You cannot go on increasing the power indefinitely though, and the rule of thumb for the maximum useful magnification you can achieve is obtained by multiplying the aperture in inches by 50. So for an 8-inch scope, the maximum useful magnification would be 400x, but this would only be useful if the atmosphere you were looking through was very stable. The ideal telescope for looking at the Moon and planets would be a long-focal-length, large-diameter, colour-corrected refractor, but here the budget limit is easily exceeded with alarming speed. An alternative would be a large-diameter, long-focal-length reflector, but these can be quite bulky and difficult to mount and handle. Scopes that use folded optics, known as catadioptric telescopes, can offer a good compromise here, combining a relatively large aperture with a long focal length, all for more realistic costs.
At the other end of the range, a telescope designed to look at the stars and deep-sky objects needs be large and have a relatively short focal length. A large reflector fits this bill well, and there are plenty of examples of this “light-bucket” type of telescope available on the market. One very popular and relatively cheap version of a deep-sky light bucket is the Dobsonian reflector. This is typically a large-aperture telescope on a simple left-right/ up-down (alt-az) mounting platform. The ethos behind this telescope, originally designed by John Dobson, was that this should be a telescope that was simple to make from readily available components. If you are good with your hands, there are many Dobsonian plans available online which would allow you to create a large-diameter scope for very little money. The difference between a planetary scope and the deep-sky scope is often made by referring to the speed of the instrument. This is a measure of the instrument’s focal length divided by its diameter using the same units. So a 200mm (8-inch) reflector with a 2000mm focal length has what’s called a focal ratio of 2000/200 = 10.
This is generally written as f/10 and is an indication of the speed of the telescope, in this instance f/10 is being considered slow. A telescope with a similar aperture of 200mm but a shorter focal length of 600mm would have a focal ratio of 600/200 = f/3, which is considered fast. If you’re wondering where the terms “speed”, “slow” and “fast” come from, they echo the days of film photography. The term “speed” is a measure of how quickly a lens or telescope can deliver a set amount of light to photographic film. A “fast” lens would achieve delivery quicker than a “slow” lens. Basically, a slow lens requires a longer exposure to achieve the same depth of image that a fast lens can deliver in a relatively short exposure. The basic rule of thumb is that slow instruments are good for the Moon and planets while fast ones are better suited to deep-sky objects and the stars. What are the definitions of slow and fast? Well, this is open to interpretation but a reasonable assumption would be that any scope with a focal ratio of f/5 or lower is considered to be a fast instrument, while any scope with a focal ratio of f/9 or higher is considered to be slow. Of course this leaves those with focal ratios in the region of f/5–f/9 uncategorized but these are the instruments which are suited to both camps.
If you regard your interests to be equally divided between Solar System and deep-sky objects, then the f/5–f/9 instruments are probably the ones that will likely interest you most. One further factor when considering the speed and focal length of a potential purchase is the use of an optical amplifier to alter these figures. An amplifier, such as a Barlow lens, can be used between the telescope and eyepiece to effectively make it seem that the focal length of the telescope has been multiplied by the power of the Barlow. For example, consider a 200mm-aperture telescope with a 2000mm focal length (f/10). Using a 2x Barlow increases the effective focal length of the scope to 4000mm but reduces its speed to f/20.
The caveat here is that amplifiers with powers greater than 1 can make viewing harder, the high focal ratios produced giving rise to dimmer images. Optical amplifiers also come with powers less than unity, called focal reducers. These reduce the effective focal length and improve the speed of the instrument, giving a wider, brighter view. Careful and considered use of optical amplifiers will mean that you can partially adapt a long-focallength scope for use as a deep-sky instrument and vice versa. However, the results achieved will not be as good as a dedicated scope.
To assist you further, below is a general indication of the best type of scope to buy for each budget group. It’s by no means meant to be a definitive list but hopefully it will point you in the right direction. Classify your budget
- Up to £100
This is probably too low a budget for a new telescope so the recommendation here would be to purchase a good pair of, say, 10 x 50 binoculars.
There are numerous small refractors available in this price range with apertures 60–90mm. Avoid computerized control and concentrate on optical quality and a good mount.
A 10-inch Dobsonian or equatorially mounted 8-inch reflector make great deep-sky telescopes. A 4-inch refractor is another option for lunar and planetary observing.
At this level look for good-quality optics on a robust equatorially driven mounting.
Optics and sturdy mount again come first. Colour-corrected refractors or catadioptric instruments for high-magnification views of the Moon and planets are recommended.
Try it !!
If you want to try out a particular telescope before you buy it, have a word with your local astronomical society to see whether anyone in the group has the same or a similar model that they are willing to let you look through and examine. If you have no luck here, have a word with your telescope stockist to see whether they know of anyone locally who may be able to offer the same service.
An internet search on a particular type of telescope, stating the manufacturer, type and size, is a good way to see whether there has been any user feedback about the instrument online. Here you can often pick up existing problems or even learn that the model you have chosen is regarded as a superb bargain.
Another alternative, Second-hand Telescope
There are a number of options available if you are happy to purchase a second-hand telescope. There are numerous online second-hand sites where you may be able to pick up a bargain. eBay is another potential source. As usual when buying anything privately, be cautious. Ask whether they are willing for you to see the scope. Even if you don’t inspect beforehand, this will help to verify that there is actually a telescope at the other end!
So far we’ve made little mention of telescope mounts despite these being a vital component to ensure that you get the best out of your purchase. Telescope mounts are analogous to speakers in a hi-fi system. After spending your money on a new amplifier, there can be a reluctance to spend a fortune on high-quality speakers.
Buying cheap here may result in a sound quality which doesn’t reflect the quality of the amplifier at all well. Similarly, if you put a decent telescope on a wobbly, cheap mount, you’ll degrade the quality of the view you get and your experience may be disappointing and rather frustrating. Some telescopes are sold with mounts included while others require you to buy the mount separately. Some scopes are also sold integrally connected to the mount. There are many different makes of mount available but, fortunately, the number of different types is actually quite small.
The simplest mount is called an alt-az mount. The term “alt-az” is short for altitude-azimuth and refers to the fact that the mount moves from side to side (azimuth) and up and down in altitude. A standard photographic tripod is an example of an alt-az mount. The problem with a basic manual alt-az mount is that it does not naturally follow the stars across the heavens. If you think about it, placing a small telescope on a photographic tripod allows you to move the telescope horizontally and vertically. The stars do not move horizontally parallel to the horizon unless you happen to be observing from the North or South Pole.
If you watch the stars on a clear night, they move in circular arcs centred on a stationary point in the sky known as a celestial pole. In the northern hemisphere the stationary point is known as the North Celestial Pole (NCP) and is located in the sky very close to the star Polaris. In the course of a day, Polaris describes a very small circle around the NCP. If you take an alt-az mount and tilt it so that the azimuth axis points towards the NPC, a telescope installed to the mount will, moving in what is now tilted azimuth, follow the curved arcs the stars naturally follow across the sky. This is the principle of the equatorial mount.
If you add a motor drive on the “tilted azimuth” axis which turns the axis at the same rate as the Earth rotates but in the opposite direction, the mount is able to compensate for the rotation of the Earth and anything in the eyepiece of an attached scope will stay in view and not drift out of sight. So to recap, so far we’ve explained an undriven alt-az mount, a tilted version of the alt-az mount known as an equatorial mount, and a driven version of the equatorial mount known as a driven equatorial mount.
Putting a drive on the azimuth axist of an an alt-az mount isn’t that useful, as the mount does not emulate the natural motion of the stars unless, as stated, you happen to be observing from one of the Earth’s poles. However, putting a drive on both the azimuth and altitude axes allows an alt-az mounted scope to follow the stars if a computer is involved to emulate their actual motion. Modern computerized mounts can offer some impressive functionality but it’s wise to treat them with caution for lower-priced telescope packages.
If you’re paying a set amount for a computerized telescope package, it’s useful to keep in mind that the computerization comes at a cost and may detract from the size and possibly the quality of the telescope that is supplied with it. Go-To capability is offered on many computerized mounts. Here, as long as the mount knows where it is in the world, what time it is, and the direction in which it’s currently pointing, you can select the target you’re after from a huge database of objects. The computer then does the rest, slewing the telescope round so that it’s then pointing in the right direction. The term Go- To describes the action of selecting your target and pressing a button to “go to” it. Although this may sound the perfect way for a beginner to find objects in the night sky, there are issues.
For example, letting the computer do the work will not help you learn your way around the sky. Here, there’s no substitute for a bit of practical map-reading using charts and the naked eye. If your goal is to learn where things are in the night sky, the use of a Go-To mount is best avoided or at least limited. Then there’s the issue of the onboard picking database, which for a typical Go-To system is measured in the tens of thousands of objects. Many generic Go-To systems are designed for a large variety of telescopes so using the database with a small-aperture instrument will invariably result in having access to many objects which simply cannot be seen through the eyepiece.
This is not to say that Go-To is all bad. If you know you’re a casual observer who isn’t that interested in learning your way around the constellations but would rather just “see” the items of interest that are up on any particular night, then Go-To is a good choice. Similarly, if you have very light-polluted skies so that navigating around the stars with the naked eye is tricky at best, then Go-To may help you out here too. Go-To also works well for imagers where locating objects by eye can be wasteful of precious imaging time. And Go-To may be the only way to quickly locate objects which are below your scope’s visual threshold. Go-To systems can be used with alt-az and equatorially mounted telescopes. Computerized alt-az mounts are not really ideal for imaging as they introduce an unwanted effect known as field rotation during long exposures. This makes the imaging target rotate around the central axis of the image frame.
Generally, any one type of telescope can be used for viewing any type of object. Even if, as described above, the scope’s focal length isn’t ideal for the job, you can usually get some sort of view of the object you’re after. By fitting a white-light filter to the front of the telescope so that the entire aperture is protected, and capping or removing the finder, it’s possible to turn a night-time telescope into an instrument capable of viewing the Sun in white light. However, this won’t reveal much detail beyond what can be seen on the Sun’s visible surface, the “photosphere”. For a look at the exotic features which inhabit the hot magneticallyinfluenced environment just above the photosphere, you need to use a special filter known as a hydrogen-alpha (H-alpha) filter.
These are available as a filter set used to convert existing night-time telescopes for H-alpha viewing or as dedicated telescope packages. H-alpha filters do tend to be quite pricey, and a dedicated H-alpha telescope cannot be converted for night-time use; this can only be done if using a filter set. The cheapest dedicated H-alpha scopes can often retail for around £500. Unlike their night-time equivalents, dedicated H-alpha scope apertures tend to be more restrained, the normal amateur range extending from 35mm up to 100mm. Larger amateur H-alpha scopes are available to you but cost tends to keep their numbers rather short.