Choosing a telescope

This is a very basic introduction. For much more detail and specific telescope recommendations at different price points, see the 'Telescope Buying' Powerpoint file at the bottom of this page - excellent information by Dimitrios Kechagias.

Telescope basics

·       Telescopes comprise two main components - the optical tube assembly (OTA) and the mount.

·       There are three main optical designs - the refracting telescope, the reflecting telescope and compound telescopes.

·       Refractors are the familiar ‘telescope-shaped’ telescopes with a lens, or ‘objective’ at the front and the eyepiece at the back, so the light from a distant object travels straight through the telescope to the user’s eye.

·       Reflectors use a mirror to collect the light instead of a lens. The mirror is at the bottom of the tube. Light from a distant object hitting the mirror is reflected back up the tube and is turned through a right angle by a secondary mirror near the top of the tube, directing the light to the eyepiece mounted on the side of the tube.

·       Compound or catadioptric telescopes use a combination of mirrors and corrector plates (specially shaped lenses) to fold the light path. Common designs are the SCT (Schmidt-Cassegrain Telescope) and Mak (Maksutov-Cassegrain). The eyepiece is at the back of the telescope, like a refractor.

Aperture, focal length and focal ratio

·       Telescopes are defined by three characteristics – aperture, focal length and focal ratio (f number).

·       The aperture is the diameter of the lens in a refractor or the mirror in a reflecting or compound telescope. The larger the aperture, the greater the light gathering power. A 200mm aperture telescope has 4 times the light gathering power of a 100mm telescope, so you can see fainter objects. Larger apertures also enable finer detail to be made out (resolution).

·       The focal length of a telescope is the distance from the lens or mirror at which the light comes to focus.

·       The focal ratio of a telescope is found by dividing its focal length by its aperture, e.g. a 150mm aperture telescope with a focal length of 1200mm has a focal ratio of 1200 ÷150 = 8, so the focal ratio would be quoted as f8. Low focal ratios of say f5 or f6 are often described as ‘fast’ and are capable of bright wide field views with low magnification. ‘Slow’ telescopes of say f12 and above are capable of higher magnifications but dimmer views.

Eyepieces

·       Beginners when looking at a telescope often ask “What’s the magnification?” and the answer is “Whatever you want it to be!” Eyepieces, like telescopes, come in various focal lengths, and the eyepiece can be changed to suit whatever is being observed.

  Magnification is calculated by dividing the focal length of the telescope by the focal length of the particular eyepiece being used. If we use a 25mm eyepiece in our f8 1200mm telescope the magnification will be 48 (1200 ÷ 25), which would give a good fairly wide field view of the Moon, the Andromeda Galaxy or The Pleiades. If we swapped the 25mm eyepiece for an 8mm eyepiece, this would give a magnification of 150 (1200 ÷ 8) – good for looking at individual craters on the Moon, the cloud belts of Jupiter or splitting double stars, but not good for looking at large faint objects like Andromeda.

The higher the magnification the fainter the image. The maximum usable magnification is sometimes quoted as 2x the aperture in mm, so for our 150mm telescope this would be a magnification of 300. However, the maximum usable magnification is often limited by the stability of the atmosphere (what astronomers call the “seeing”), so for our 150mm telescope a maximum of 150 to 200 would be more realistic.

·       When choosing extra eyepieces for a telescope, it’s worth making use of a Barlow lens to give an extra choice of magnifications. The most common Barlow increases the magnification by 2 times, so with our 25mm eyepiece and 1200mm focal length telescope, we get a magnification of 48 without the Barlow and 96 with the Barlow, which is the same as using a 12.5mm eyepiece without a Barlow. If you’ve got a 25mm eyepiece and a Barlow and you are thinking about an extra eyepiece, it’s not worth bothering with a 12mm, which would be too close in magnification to the 25mm used with the Barlow. Instead, you could think about getting a 17mm eyepiece, which would give magnifications of about 70 without Barlow and 140 with.

Mounts

·       It’s important that telescopes are supported on a sturdy tripod and mount. Mounts come in two types – AltAz and Equatorial.

·       AltAz mounts are the simplest. They move up and down (altitude) and side to side (azimuth). They are quick and easy to set up but have limitations (see below).

·       Equatorial mounts can follow the path of the stars by having one axis (the polar axis) aligned with the celestial pole (close to the North Star,  Polaris), so that this axis is parallel to the Earth’s axis of rotation. The other axis, the declination axis, is at right angles to the polar axis. This type of mount is good at tracking objects but takes longer to set up than an AltAz mount.

·       Either type of mount can be driven by motors to follow the stars and can have computer control to find and track objects in a database. These are GoTo mounts. Although an AltAz GoTo telescope can keep an object centred in the field of view for possibly hours, during that time stars will appear to rotate around the centre of the field of view (called field rotation). This is what makes AltAz mounts unsuitable for astrophoto exposures longer than about 30 seconds and why equatorial mounts are used for long exposure astrophotography of faint objects.

Brands and prices

·       That’s the basics covered, so what do you choose out of the wide range of telescopes on the market today?

·       First, it’s worth looking at well respected brands, like Celestron, Meade and Skywatcher (other brands are available!). Second, it’s best to buy from an established telescope dealer who will be able to offer a wealth of advice to help you make your choice and will give you after-sales support if you have any problems or queries.

·       You can get a reasonable starter telescope for under £100 which will give you good views of brighter objects like the Moon, Jupiter and Saturn, but at this price the telescope will only have a small aperture and therefore not very good for seeing faint objects like galaxies.

·       If you have a budget in the range of £150 and upwards you will have a much wider choice of various designs of good quality telescopes.

Various things to think about

·       Which you choose depends on what’s important to you. If you want something that is very quick to set up, you might want to look at some type of AltAz. The Dobsonian type of AltAz is particularly quick and easy to set up. If you intend to get into long exposure astrophotography you will need an equatorial mount. If you want to be able to quickly locate various night sky objects you might want to look at a GoTo.

·       At any given price point you will get a larger reflector than refractor or SCT. If you are particularly interested in looking at faint deep sky objects you will get maximum “bang for your buck” with a Dobsonian-mounted reflector, because the Dobsonian mount is simple and cheap to manufacture, so more of your money goes into a larger mirror. If you go for a GoTo SCT or Mak, at the same price point you will get a much smaller aperture because more of the cost is in the relatively expensive optical tube design and computerised mount.

·       You should think about storage. A big Dobsonian might be great for looking at faint fuzzies, but will your family be happy for it to be stood in the corner of the lounge if there is nowhere else you can put it?

·       Another consideration is portability. If viewing conditions at home are not good, you might want to travel to dark sky sites, so make sure your scope and all your other equipment will fit in the car boot. Many people like to take their scopes on holiday with them to take advantage of dark skies in places like the Canaries. In this case a small short focus refractor is hard to beat because it can be taken on planes as carry-on baggage.

·       If you think you are going to get serious about deep sky astrophotography, the first thing you should be thinking about even before the optical tube design is a heavy duty equatorial mount. For deep sky work a fast focal ratio is more important than a large aperture, and many deep sky imagers use fairly small aperture, e.g. 80mm fast focal ratios, e.g. f6 or lower.

The best thing to do before making a choice is to talk to people with telescopes at HPAG or other astronomical societies about their experiences and get advice from a specialist dealer.

 

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Pete Collins,
23 Nov 2017, 15:37
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