Derby and District Astronomical Society

Telescope Buying Advice

Article by Mike Lancaster

If you are only just getting into star gazing then I would strongly recommend that you do not take the plunge of buying a telescope at first. Get to know the stars, constellations and brighter planets with the naked eye initially. If you have not already done so purchase one of the monthly astronomy magazines such as Astronomy Now or The Sky At Night magazine. These will have a star chart for the month showing what constellations and planets are visible etc., as well as tips and advice for beginners. The Philip's Planisphere is a handy low tech device with which you can 'dial up' what the sky looks like for a given date and time. If you own a PC or Mac then the free planetarium software called Stellarium is a must. If you have a smart phone then Google Sky Map for Android phones or StarMap for the iPhone are very good. Much of astronomy is about the beauty and grandeur of the night sky overall and does not require you to squint through a telescope!

Once you know your way around the sky invest in a good pair of binoculars. 10 x 50 is a good size (10x magnification and 50mm diameter lenses). These can be mounted on a tall tripod for stability and preventing neck ache! These will show many more stars than the naked eye and bring out the colours of the stars. You will be able to discern some craters on the moon, pick out the moons of Jupiter, resolve the Milky Way into magnificent star fields and even see the Andromeda Galaxy and the Orion Nebula etc.

Once you do decide to take the plunge with a telescope (or feel you have to even if you are new to astronomy!) then there are several types to choose from. The two main types are the refractors which use a lens to collect the light, and the reflectors which use a mirror. In either case go for as larger size (aperture) as you can afford. The larger the size of lens (for refractors) or mirror (for reflectors), the more light the telescope gathers and the brighter and better quality the image will be. There are two main types of reflectors in use, the Newtonian and the Schmidt-Cassegrain. The Newtonian, so named because it was invented by Isaac Newton, employs an open ended tube. Light passes down the tube and hits a mirror at the bottom of the tube. This mirror is concave such that the light reflecting off it comes to a focus. Before the light comes to a focus it is intercepted by a flat mirror suspended near the top of the tube. This mirror sends the light out of the side of the tube where the image is magnified by an eyepiece. The other commonly used type of reflecting telescope is the Schmidt-Cassegrain. This also has a mirror at the bottom of the tube but rather than being open the top of the tube is sealed by a glass 'window' or 'corrector plate' at the front. Instead of being sent out of the side of the tube the light from the main mirror is bounced straight back through a hole in the main mirror and the eyepiece is placed in the same position as one would find with a refractor. You may also come across Maksutov-Cassegrains. These are essentially the same as Schmidt Cassegrains but have a 'meniscus lens' instead of a corrector plate and a 'coated spot' behind this lens instead of a secondary mirror. Collectively the two types of Cassegrain are known as catadioptric telescopes. Some of the smaller Cassegrains are usually Maksutovs while the larger ones tend to be of the Schmidt type. The minimum useful size of refractor is a 60mm (or 2 inch) lens, for a Newtonian reflector a 150 mm (or 6 inch) mirror and for a Cassegrain a 70mm lens is about the smallest you can go.

The refracting telescope.
The refracting telescope. Light is collected by the objective lens which casts an image at point f. This is then magnified by an eyepiece.
The refracting telescope.
A refracting telescope mounted in altazimuth fashion on a tripod. A triangular 'diagonal' turns the light path through a right angle allowing the eyepiece to be used from a more comfortable position. Also note the small 'finder scope' attached to the main telescope.

The Newtonian reflector telescope.
The Newtonian reflecting telescope. Light passes down the open tube and reflects off the main mirror. This mirror is concave such that the reflected light comes to a focus. Before the reflected light comes to a focus it is intercepted by a 'secondary' flat mirror suspended in the middle of the tube and sent at right angles out the side of the tube. The image is then magnified by an eyepiece.
The Newtonian reflector telescope.
A Newtonian reflector mounted on an equatorial mount and tripod. Counterweights are required to balance the set up. Note the eyepiece is at the side of the tube near the top. A finder scope is also attached. The structure supporting the secondary mirror is visible inside the open end of the tube.

The Schmidt Cassegrain telescope.
The Schmidt Cassegrain telescope. Light passes through a 'corrector plate' at the top of the tube and reflects off the main mirror. This mirror is concave such that the reflected light comes to a focus. Before the reflected light comes to a focus it is intercepted by a 'secondary' mirror suspended in the middle of the tube behind the corrector plate and sent back through a hole in the primary mirror. The image is then magnified by an eyepiece.
The Schmidt Cassegrain telescope.
A Schmidt Cassegrain telescope mounted on an altazimuth mount with a computer controlled 'GOTO' system. Note how compact the telescope is due to the 'folded' light path. Note also the 'corrector plate' at top of the tube and the position of the eyepiece at the rear of the telescope. A triangular 'diagonal' turns the light path through a right angle allowing the eyepiece to be used from a more comfortable position. The black disk in the middle of the corrector plate supports the secondary mirror. A finder scope is also visible on top of the main tube.

In terms of cost you will generally find that refractors are more expensive per inch of aperture than reflectors, and refractors become prohibitively expensive at larger apertures. For anything above about 80mm in aperture most amateurs would have a reflector. The most cost effective are the Newtonian reflectors which have the eyepiece at the side of the tube near the top. More expensive are the Schmidt-Cassegrain telescopes by virtue of the more complex optics they use. There are many computer controlled telescopes available now which allow you to locate and track astronomical objects at the touch of a button. These computer controlled ‘GOTO’ scopes do add significantly to the cost and despite what you might think cannot do everything for you. You still need to know your brighter stars to set them up each time you use them. The GOTO scopes do work amazingly well once you’ve mastered them though.

You also need to consider what type of objects you want to observe. If you are mainly interested in observing the moon and planets then a refractor may be best for you. If you want to look at faint nebulae and galaxies (so called ‘deep sky’ observing) then you will want to gather as much light as possible so a reflector with a larger aperture would be best. Nevertheless a reflecting telescope of say 8 inch aperture (200 mm) represents an excellent instrument for both lunar and planetary as well as deep sky observing. If your main interest would be imaging objects through the telescope then a large aperture is not required as the CCD chip in the camera (either a digital SLR or a dedicated CCD camera for astronomy) does the work of collecting the light over a time exposure. For example many of the images of deep sky objects in our gallery were taken using an 80mm refractor.

With some adverts, especially some of those in the high street or in non-astronomical mail order catalogues, don't be taken in by claims of many hundreds of times magnification. Typically on most nights with any scope you are likely to use from the back garden in the UK (even the more expensive ones) you will only be able to usefully use up to about 150x magnification which is plenty adequate to see enormous detail on the moon, the cloud belts on Jupiter, the rings of Saturn, the wispy nebulosity in Orion, the magnificence of star clusters etc. Try and use a more powerful magnification such as 200x and things can get a little blurry and difficult to focus, except on the steadiest nights. The problem here is the Earth’s atmosphere, which is turbulent, and shakes the light from astronomical objects around. That’s why the stars twinkle. Go into space and they would shine with a steady brightness. The telescope magnifies these atmospheric disturbances so one is always limited as to how far one can push the magnification, quite apart from the inherent limits of the optics anyway.

Don't expect to see with your own eye through a telescope, any telescope, the glorious colourful and detailed views you get of nebulae, galaxies and the planets you see in magazines or on the web. They are long exposure images taken with digital cameras and processed on computers. You will see detail with your own eye and a telescope in the Orion nebula for example but it will be subtle. Jupiter and Saturn at 150x will appear no larger than say your half the size of your little fingernail held at arms length but you will see the cloud belts and the rings respectively. The moon is one of the few objects that does look just as magnificent as in photographs and star clusters for example are often quite magnificent when seen with the eye through a telescope. There is also nothing like the thrill of observing a distant galaxy through a telescope, even if it is only seen as a faint smudge!

Look at any astronomical object through a telescope that is fixed in position and you will soon see it drift out of sight. This is because the Earth is constantly rotating. In order to follow the object the telescope has to be moved. This can either be done manually – literally by pushing and shoving the telescope around, or by means of knobs called slow motion controls. Or the telescope can be driven by an electric motor set to counteract the Earth’s rotation. Many telescopes now come with computerized controls to find and track objects in the night sky – but you still need to know your brighter stars to set them up and use them effectively!

The ‘mount’ that a telescope sits on is very important in astronomy. This is not simply (and not always) just a tripod. There are essentially two types of structures that a telescope can be mounted to in order to move it around to point to and track objects in the sky. The simplest is the ‘altazimuth’ mount – literally meaning altitude and azimuth. These mounts allow the telescope to be moved around horizontally to point in different compass directions (the azimuth motion) and to point higher up or lower down in the sky (the altitude or 'elevation' motion). The other type of mount is the equatorial where instead of an altitude and azimuth axis there are a ‘polar’ and ‘declination’ axis. The ‘polar’ axis points due north and is angled up from the ground such that it is in fact parallel to the Earth’s axis of rotation. This angle is equivalent to your local latitude so for example from the Derby area of the UK it is set to 53°. The telescope can be moved around the polar axis to reach objects further east or west from north. The declination axis is at right angles to the polar axis and allows the telescope to be moved in a north-south direction. The telescope can be moved or driven about the polar axis to counteract the Earth’s rotation and keep an object in the field of view. The polar axis should point to a point in the sky known as the North Celestial Pole. All the stars in the sky appear to slowly rotate around this point. Fortunately and quite by accident for those of us in the Northern Hemisphere there just happens to be a reasonably bright star near the North Celestial Pole – that star is called Polaris (contrary to popular belief it is not the brightest star in the sky by a long way). Having a properly aligned mount that can track the stars is a must if you want to do long exposure imaging or photography through the telescope. A variation on the altazimuth mount is the so called Dobsonian. These are typically employed by larger aperture Newtonian reflectors and as mounts go are relatively inexpensive and simple to use. If you are looking for a large aperture reflector for purely visual observing of deep sky objects and not doing any photography or imaging with it then a Dobsonian may be for you. Setting up and aligning an astronomical telescope can be a little tricky and you must know where north is (i.e. know how to recognise Polaris). Even the computer controlled telescopes require you to center at least two known stars in them every time you set them up. So knowing how to recognise the brighter stars is a must. Just remember that patience pays off here! Note that computer control also allows altazimuth mounts to automatically track the sky, without the need for a polar axis.

The altazimuth mount.
The altazimuth (or 'altaz') mount.
The equatorial mount.
The equatorial mount.

The Dobsonian mount.
The Dobsonian is a variation on the altazimuth mount. Simple and relatively inexpensive. Although not unique to the Dobsonian note in this example the open frame design of the telescope so it has no tube as such, allowing it to be easily dismantled for storage and transport.

A final word of warning. NEVER UNDER ANY CIRCUMSTANCES LOOK AT THE SUN THROUGH A TELESCOPE! Permanent eye damage or even blindness WILL result! The only safe way to use a telescope for solar observing is either by taking the eyepiece out and projecting the image of the sun onto a white card, or by using a specially designed filter which fits over the front end of the telescope. Such filters should only be obtained from reputable astronomical suppliers. Even when projecting you must ensure no-one inadvertently looks into the telescope.

If you do finally decide to purchase a telescope I would recommend that you use a dedicated astronomical supplier such as one of those listed here and not simply go to one of your high street stores or shopping centres. If possible go and visit one of these astronomical suppliers and take a look at the telescopes before you buy. Although you may not always get completely impartial advice from these suppliers you will get some advice and much of it good as many of these companies are run and staffed by astronomy enthusiasts anyway. Further advice on choosing and buying telescopes may be found here.

Good luck!

Mike Lancaster
Derby & District Astronomical Society