Flamsteed Astronomy Society

Introduction

So you’re interested in doing some fun star-gazing and you’re thinking of buying your first telescope.   What are the choices and how much will you need to spend?  The main difficulty with deciding what to buy is that you’re spoiled for choice.  Ask 3 astronomers for suggestions and you’ll get 5 ideas!

This web feature is intended to help those interested in doing some fun star-gazing and thinking about their first investment in a telescope or binoculars.   It assumes little or no previous experience with small astronomical telescopes.  While it is possible to get started with basic equipment for under £100, we have targeted a budget of £250 so that some of the broader choices and considerations can be explored.

Begin by reading the overview which provides a background summary, then dip into the sections which recap  and add more detailed information.

Overview

What do you need to consider when choosing your first telescope, and how much will you need to spend?  It is possible to get started for under £100 but we’re going to assume you have a budget of about £250 which will give you quite a few alternatives to choose from.  Here’s a bit of background and some suggestions to point you in the right direction.

First, what choices do you have?  For fun star-gazing your choices are broadly these –

Reflector

Mak

Refractor

Bins

Dob

1.  Binoculars – ‘bins’ have lots of advantages for your first investment in star-gazing.  Binoculars show the image the right way up (an ‘erect’ image) so you can also use them for bird-watching etc as well as astronomy.  They’re very portable, extremely easy to use, reasonably robust, easily stored, and let you concentrate on the stars rather than the instruction manual and the nuts, bolts, and knobs.  Bins are highly recommended as a starting point.  They come in different sizes eg 10×50, where the first number (10x) tells you the magnification, and the second number (50 in this case) tells you the lens diameter in mm, generally speaking the bigger the brighter.  For astronomy, people normally recommend 10×50 or bigger (but see below about image-stabilising models which are quite effective and allow use of a smaller model).   Remember when holding bins in your hands you will shake and this causes the image of stars etc to jump about like sparklers, the more magnification, the more shakes. From 15x upwards you will probably need to mount bins on a tripod so losing the portability advantage. Bigger models get pretty heavy and need a tripod anyway. Image stabilisation makes a huge improvement.

2.  Telescopes – You point it at the target and look through the eye-piece.  Bear in mind that a serious astronomical telescope shows you an inverted (upside down) image.  They all do this.  It’s not a problem when you’re looking at stars and planets but you have to get used to it to move the telescope around the sky to find the target.  They’re not much use for looking at terrestrial targets (birds etc) unless you fit an ‘erecting lens’ (supplied with some starter scopes).   Telescopes come in different sizes, the important size quoted being the ‘aperture’ or diameter of the main lens or mirror eg 90mm.  This is a measure of the light-gathering power of the scope and, broadly speaking, the bigger the better (and dearer!).   You will also come across two other dimensions being quoted — (1) the focal length of the scope (eg 800mm), and (2) the focal ratio or f/number (eg f/10) which is broadly a measure of its ability to show wide bright fields of view (low f/numbers eg f/5), or support higher powers of magnification (high f/numbers eg f/12) — more later.

Telescopes come in three flavours —

‘Refractors’ – contain lenses like a kid’s telescope.  On a refractor you’ll find the eye-piece at the bottom (back) of the tube.  Refractors are fairly light and robust and good for beginners.  Go for an 80mm or 90mm refractor if you can.  For a refractor the actual length of the tube will be about  the same as the focal length.    When you touch any telescope to move it or re-focus, it will vibrate and cause the image to jump around annoyingly for a while.  Starter refractors tend to have longer tubes eg 700mm to 900mm, and so will suffer more from vibration (another reason to look for a firm mounting — see later).  Refractors also suffer to some degree from an effect called ‘chromatic aberration’ which means you will likely see a fringe of false colour around the image (except with better, more expensive, optics called ‘apochromatics’ or APOs).  Watch out for restricting ring baffles called ‘stops’ put deliberately inside some cheap refractors to disguise false colour fringes.  You can see a stop if you look down the tube from the objective end, opposite end to the eyepiece.   A stop reduces the effective aperture, making a 70mm into a 50mm for example.

‘Reflectors’ – contain mirrors.  Most common are ‘Newtonians’ and ‘Dobsonians’.  On these you’ll find the eye-piece at the top (front) of the tube which can be more convenient to use.  The Dobsonian or ‘Dob’ is actually a Newtonian on a kind of simple mounting (see below).  More experienced astronomers like reflectors because you can get more for your money (ie bigger apertures) and they don’t produce false colour fringes (chromatic aberration), but they are more fragile, and need re-alignment periodically (called collimation) — might be a bit tricky for a complete beginner.

‘Maksutovs’ (also called ‘Maksutov-Cassegrains’ or just ‘Maks’).  You’ll find the eye-piece at the bottom (back) of the tube.  Maks use a shorter sealed tube with both a lens and mirrors, but are more robust than reflectors, don’t produce false colour fringes, and don’t need re-alignment.  They have shorter tubes than most refractors and so suffer less from vibration and are more portable.  They are a good choice for a beginner but tend to be dearer.

Maks are in fact one type of a class of telescopes called ’catadioptrics’ meaning they use both lenses and mirrors.  The other common type of catadioptric is the Schmidt-Cassegrain or ‘SCT’.   You probably won’t be able to afford an SCT on a starter budget of £250.

Dobsonian mount

Altazimuth mount

Equatorial mount

GO-TO mount

3.  A telescope needs a ‘mount’ or mounting, to hold it steady and enable you to steer it around the sky.  Make sure any telescope you buy comes complete with the mount and a tripod or stand to which you bolt the mount.   A sturdy, firm tripod or stand is very important to minimise the vibration a telescope will suffer when you touch it to focus or move position.  Avoid flimsy tripods even though they may be lighter and more portable.

Starter scopes will come complete with a mount in one of three flavours:

Altazimuth or ‘alt-az’ mount – this is the simplest up-down, left-right mount, just the same as a pan and tilt photographic camera mount.  They are the easiest to use, don’t need any special set-up, and are probably the most portable, but the hardest to keep the target in view and can become annoying for this reason — the sky appears to be moving continuously and an object will quickly drift out of the small field of view of a telescope, so you need to move the mounting to follow it across the sky and keep it in view.  It’s not possible to fit motors to traditional alt-az mounts but there is now a type of motorised alt-az called an ‘autotrak’ or ‘supatrak’.   A ‘Dobsonian’ or Dob is actually a cheap but effective way of mounting a Newtonian reflector on an alt-az mount.    It can be tricky to follow an object across the sky with a non-motorised alt-az mount or Dob but easier with an ‘equatorial’

Equatorial or EQ mount.   These allow the telescope to be moved ‘left-right’ parallel to the celestial equator, thus following the apparent path of a star or planet more easily.  Control knobs called ‘slow motions’ are fitted which you turn to adjust the mount up-down and left-right.  Equatorials need to be set-up at the correct angle equal to the latitude of your observing location, and with the mount aligned pointing at the celestial pole (ie north in the northern hemisphere).  This isn’t all that difficult to do for casual observing and, once you’ve learned how to set it up, an equatorial mount can be less annoying to use than an alt-az. One ‘but’ — the eye-piece, controls and finder can end up in difficult positions to use as you track across the sky during a session with an EQ mount.  Equatorials can be fitted with small electric motors (integrated on some models) to drive them across the sky and help keep targets in view.  They are preferred if you plan ultimately to do any astro-photography.

A ‘GO-TO’ mount – the easiest of all to use once you’ve done the set up.  These can be types of alt-az or equatorial mounts driven by battery-powered motors controlled by a tiny on-board computer that has a database of star and planet positions.  You use a hand-held controller to select the target and the telescope moves itself to where it thinks the target is, and then keeps tracking automatically.  Set-up can be tricky but they usually have good instructions and once you’ve mastered it a go-to should be the easiest to use.  They’re also the most pricey (of course).

You can now also buy go-tos with built-in GPS satellite positioning technology (like the sat-nav found in some cars).  These can automatically align themselves north and make set-up a lot easier.  As you might guess, at present they cost more than the starter budget, but you may feel like pushing the boat out one day!

Eye piece

Star diagonal

4.  As well as the mount and tripod or stand, a starter scope should also include one or two eye-pieces (essential), and a small finder telescope (or red-dot finder) which attaches to the side of the main telescope and is important to help you get the scope pointed at the target.   Most starter scopes come complete with all this stuff but do make sure, otherwise you’ll need to buy the missing bits.

The eye-pieces are very important.  They control the magnification you will see and can make a big difference to the quality of the image.  Eye-pieces are labelled with a size in millimetres which is their focal length.  All you need to remember is that the smaller the eye-piece focal length then the higher the magnification eg a 4mm eye-piece will magnify 5 times more than a 20mm.  Also, the more magnification then the smaller the field of view (and the harder it is to find and keep a target in view) so you will often begin an observing session with a longer (lower power) eye-piece like a 20mm or 25mm then switch to a higher power (eg 10mm or 4mm) for more magnification.  One tends to switch eye-pieces a lot when moving from target to target during a session.  Eye-pieces slot in and out of a holder on the scope near the focussing knob.  A starter telescope will include one, two, or three eye-pieces, and you can buy additional eye-pieces in a wide variety of focal lengths, qualities, and prices.

On a refractor or Mak the eye-piece holder is usually part of a piece called a ‘diagonal’ or star-diagonal which contains a mirror or prism to turn the light path through a right-angle so the eye-piece is held at a comfortable viewing angle for you.   Starter refractors and Maks usually have a diagonal included.

The eye-piece in use determines the magnification you will see.  To calculate the actual magnification divide the focal length of the main telescope (eg 700mm) by the focal length of the eye-piece – so a 25mm eye-piece will magnify x28 in a 700mm scope, and a 10mm eye-piece will magnify x70.   Adverts for reputable scopes will seldom mention magnification but always tell you the focal lengths (or focal ratio) of the main scope and eye-pieces included.  You can safely ignore ads for cheap telescopes that talk for example about ‘675 times magnification’.  In fact there is a practical limit to the maximum magnification that a telescope can deliver which you can take as 2 times the aperture in millimetres.  So a 90mm aperture would have a practical limit of 180x magnification.

Finally, by way of background, to have fun remember some other essentials –

• Dress properly for the conditions.  It can be very chilly after dark and star-gazing is no fun if you’re freezing.  In cold weather a hat is essential.
• Never look directly at the Sun or via binoculars or a telescope without a proper solar filter!
• Have something to help find your way around the sky – a star chart from a book or magazine (eg Astronomy Now) or a planisphere.  You can download some excellent free sky chart software from www.stellarium.org.   If you don’t have some sort of sky chart you’ll soon run out of things to look at.
• People differ in their eye-sight.  Some have much better sight and will be able to see things that poor moles like me can only imagine!
• The condition of the atmosphere and local lighting makes a big difference to what can be seen through a scope.   In towns and cities or near golf ranges etc watch out for light pollution..  You’ll see a lot more from a dark site compared to London where you can’t even see the Milky Way.

What can you get for your money?

What can you get on a budget of £250?   Here’s some suggestions to get you started.  We suggest you think about 4 different ways to proceed –

(1)  Try someone else’s!   There’s no substitute for trying out different types of telescope and bins before you shell out your hard-earned cash.  The best place to do this is at the local astronomy society — there are dozens up and down Britain.  They will be delighted to let you peer through a selection of scopes and give you the benefit of their experience — check the listings at the Federation of Astronomical Societies website.  Also if you have the time and inclination to do more research, study some of the excellent books and websites available (see our ‘useful links’ page).

then / or —

(2)  Just buy a good pair of binoculars.   To repeat, binoculars are a great place to begin your star gazing and a good pair will give you excellent views of the Moon, star fields like the Pleiades, nebulae like the Orion nebula, globular clusters etc, and planets (but probably not detail like Saturn’s rings, or cloud bands on Jupiter).  You won’t lose any time or sleep trying to set-up a complicated mount and you can concentrate on learning your way about the sky and just looking.   They’re really easy to use and very portable.  Arrange a sun-lounger to lay on when you want to observe objects high up and over-head..  Also you can use them for other things like bird-watching and boats.   There is a huge choice available – look at the websites listed in useful links.  10×50 or bigger are normally recommended for astronomy, but heavier types will need a tripod to hold them – they’re too heavy to be hand held.  Take a serious look at image-stabilising binoculars.   The starter budget can’t quite run to Canon 10×30 image-stabilising bins but these Canons have brilliant optics and can be hand-held while the IS system removes most of the shake that will spoil viewing using normal bins.

or —

(3) spend the minimum on a reasonable starter telescope and a cheap pair of 10×50 binoculars as well.  Even a cheap starter refractor will give you a chance to learn how to use an equatorial mount and introduce you to the annoying aspects of astronomy as well as the fun.  This way you can discover if you enjoy it before you spend any more than necessary.  A starter refractor will show you some planetary detail like Saturn’s rings and the bins will give you a view (albeit shaky!) of nebulae and the Pleiades etc.

or —

(4)  if you’re determined to dive in at the deep-end, blow your budget on a GO-TO mount, or an equatorial, with the best Mak or refractor you can afford.

Go visit a reputable dealer to chat and see the range.  For a list of reputable dealers see the listings in magazines like ‘Astronomy Now’

You pays your money and takes your choice.  As we said earlier, although not included in the suggestions, you can get more for your money with a reflector so long as you feel up to tackling the maintenance.  A Dob offers a lot of aperture for the price — that’s why they’re sometimes called ‘light buckets’.

With option (4) you could be blowing all your budget before you find out if you like star-gazing or not.

Remember, the best telescope is the one you use the most.

Whatever you decide, have fun, and “Clear skies!”.

Strengths and weaknesses of different telescope types

Strengths and weaknesses of different mount types