So, you would like to buy a telescope...?

Choosing a telescope for the first time can be a daunting experience.  There is a mountain of information to digest then the choice may still not be clear.  So let us start with a few questions.  They may look obvious and straight forward but these questions and the following information will give you an idea of what to look for.

How well do you know the night sky?
How far will you have to carry it?
How long will it take to set up?
What do you want to do with it?

How well do you know the night sky?

Do you know where ot find the planets in the sky?  Can you point out the brighter constellations.  Do you know a few star names?  Understanding how the sky changes throughout the year and being able to recognise the major "signposts" in the sky will lead you to a more rewarding experience with your telescope.  There are many objects that can't be seen with the unaided eye but can be found with the aid of a star chart.  Unless you know where to start in the sky you will most likely have a frustrating and discouraging time trying to find anything.  If you know absolutely nothing about the sky but would like to get into astronomy some good advice which is often given by amateur astronomers is "get some binoculars and a good starchart." 

Why binoculars?  Firstly they are a lot cheaper than most telescopes and if you decide that astronomy is not for you you've still got something you can use during the day for other purposes.  Secondly, they will magnify the sky enough to see some faint objects while providing a wide field of view that makes them easy to find.  They will introduce you to using star charts and navigating your way around more easily than a large telescope will.

How far do you have to carry it?

Here you need to think about where you are going to use the telescope.  Are you going to leave it set up somewhere?  Will you only have to carry it outside to the lawn or driveway?  Or are you going to put it in your car and take it to a dark site for observing?  Can you easily physically lift it?  Does it take two or more people to set it up?  Do you have to twist yourself into an awkward position to use it when it is pointing at certain angles and will this affect your back or neck?  Do you have the space to store it when it is not in use?

How long will it take to set up?

A telescope that takes a long time to set up and put away or is too big for you handle without assistance is less likely to be used.  If the set-up time is less important than the results at the end then you may decide that this question will have little impact on your decision.  Favourite telescopes are often the ones that you can have outside and in use in a few minutes rather than the biggest or best made ones. 

What do you want to do with it?

Look at the stars!  But there is much more up there than just stars.  There are double stars, star clusters, nebulae, galaxies, the moon, planets and more.  Perhaps you are thinking of doing astrophotography?  Perhaps you find the moon and the planets more interesting than faint nebulae and galaxies?  Or perhaps the challenge of finding and observing a deep sky object is more exciting.  Different types of telescopes will perform better on certain objects than others. 

There is no perfect telescope that will do everything, there is always a trade-off in performance.  The "perfect" telescope for you will be the one you use the most. 

So many questions!  It is recommended that you look through a variety of other peoples telescopes, if you can, before buying your own.  If you don't belong to a club, watch out for when they hold public nights, or visit one of the observatories near Perth (or near where you are).  Go to our Events page for forthcoming public nights and the Links page to find the clubs and societies.   This way you will learn about the different kinds of telescopes on the market and how they perform compared to each other as well as their advantages and disadvantages.  Following is a description of  the three most common types of telescopes available on the market and some of their pros and cons.
 

How Important is the Focal Length and Aperture?

The focal length of a telescope will determine the amount of magnification a telescope will give.  The aperture (the width of the primary mirror of a reflecting telescope or the width of the front lens in a refractor) will determine how bright the object appears at that magnification. 

The simple formula to be applied to determine magnification is to divide the focal length of the telescope by the focal length of the eyepiece being used.  The focal length of the eyepiece is usually given in milimetres on the side of the eyepiece barrel (e.g. a 25 mm eyepiece).  The focal length will be given millimetres/centimetres/inches or as an f-ratio.  If only an f-ratio is given the focal length of the telescope will then be the diameter of the mirror times the f-ratio.  For a 200mm f/6 telescope the focal length will be 200 x 6 or 1200mm.

If you do a few sums you will quickly see that a telescope with a short focal length wont be able to magnify as much with a telescope with a longer focal length.  Short and long focal lengths both have their advantages but if you are after a telescope for general use an f/6 or f/8 is a good length.  If you are looking at a nice telescope but you are worried that it's focal length is too short, a good quality Barlow lens will help you achieve the magnification you are after. 
 

A note on Magnification and Power

Astronomical telescopes don't have power ratings.  This is because the power of magnification depends on the eyepiece you are using in the telescope and eyepieces are readily interchangable.  Astronomical telescopes are not fixed in their power-rating as would a rifle sight or a spotting scope for bird watching.

It should be clearly stated that magnification in a telescope isn't everything.  The size of the aperture, the quality of the optics and how you use it is just as important.  In fact, there are some objects that are so large that they are best seen at low magnifiaction, such as in binoculars.  Sometimes you will see a telescope advertised as being able to magnify 500 times.  In theory, yes, this telescope can achieve this but the image becomes so small and faint you wont be able to see anything!  In small telescopes the general rule is the smaller the aperture the less it will effectively magnify.  As you get progressively bigger the telescope can handle the higher magnifications more easily, but the effective limit in amateur telescopes under 40cm (16") in size is still 300x or less.  A magnification of around 50x to 100x is adquate for exploring the deep sky and to see some galaxies.  A magnification of 100x to 150x will let you observe detail on the planets (Jupiter, Saturn and Mars near opposition), 200x will let you see some finer detail.  There is no need to go overboard on magnifying power.
 

Some Other Misconceptions About Telescopes

How deep can it see?  Telescopes "see" brightness, not depth.  The closest star to earth, Proxima Centauri, is too faint to be seen with the unaided eye, yet the Andromeda galaxy, 500,000 times further away is easily seen if you know where to look.  If you would like to get an idea of how strong a telescope is you should ask "How faint can it see?"  As a general rule, the larger the aperture the fainter it will be able to see. 

There is a big comet coming - I must get a telescope to see it!  When a bright comet appears in the sky interest in astronomy increases rapidly and some people start to think "I'm going to need a telescope to see this comet!"  Well, you don't. I'm not trying to spoil your fun but if the comet is bright enough to be seen with the unaided eye it is not going to fit in the field of view of a telescope.  Binoculars are a far better choice for enjoying the full majesty of these brief visitors.  It may be better to invest in a pair of good binoculars and then, if you find you wish to see more than binoculars can reveal, look into buying a telescope.  Don't be tempted into buying something cheap that you may only use the once; you'll only hate it for taking up space in a cupboard  afterwards and you will be unlikely to able to resell it for a good price.
 

Refractors

Refractors are the classical telescope.  They are made by having a pair of glass lenses at the front of the telescope which focuses light at the far end of the tube.  You look at the focused image at this end through an eyepiece.  These telescopes are commonly what you see in camera and department stores but they are often poorly made items that can be frustrating to use. 

When looking at bright objects , such as the moon, you will often see a bright line of colour around the rim of the object.  This can be corrected for but these telescopes will cost a lot more.  An acromatic telescope has partial colour correction, an apochromatic is more fully colour-corrected.  You may decide that a bit of false colour is not so imprtant and that resolution of images in the eyepiece is, as this is what refractors are good at.  Some light is lost as it passes through the lenses at the front, which provides good contrast in the image and enhances fine detail on bright objects such as the planets but means that fainter objects such a galaxies and nebulae may be harder to see, so if you like the deep sky you will want to look at the other types of telescopes.  At the top end of the market the best apochromatic refractors cost thousands of dollars.  Refractors are also excellent for astrophotography. 

You can now buy short focal length refractors and long focal length refractors (see the section on focal length below), there is quite a selection available.  The shorter focal lengths wont magnify greatly but they will give you excellent wide-field views and can be used during the day with the addition of an erecting prism.  They are also very portable.  The longer focal length refractors will give you greater magnification but a narrower field of view.
 

Reflectors  (Newtonians)

Reflectors can be made in many different ways but the most common type around is the Newtonian, named after Sir Isaac Newton, who first applied the design for use.  Light enters at the top of the tube and reflects off a parabolically shaped primary mirror.  After hitting a smaller, oval shaped secondary mirror the light is reflected at 90^o out of the telescope through an eyepiece to your eye.  They are popular for their value for money versatility and they are easy enough to make yourself if you are dedicated.  They are the choice of deep sky observers due to their light-gathering capabilities and with modern materials and ingenuity size is no longer a limiting factor.  100cm (40") portable telescopes can now be made.  A 20cm (8") Newtonian is quite  common but you wont see many 8" refractors around!  Newtonians can be mounted on an equatorial mount, which can have a motor attached to it for tracking the sky, or a Dobsonian mount, a cheap but very effective alternative. (There are many types of mounts that can be used but the two most commonly seen for Newtonians is the german equatorial and the Dobsonian mount.) 

The Dobsonian mount is a kind of altitude-azimuth mount, meaning it moves in two directions, up and down (altitude) and around left to right to all points of the compass (azimuth).  These mounts can be rigged up to track the stars but it becomes a little more complicated than an equatorial mount but certainly possible if you decide to pursue the matter.  Dobsonians are very easy to set up and use as you don't have to align them with the celestial poles and there are no knobs or latches to turn or tighten.  If you don't mind having to give them a little adjusting push every so often, which doesn't take long to get used to, you may want to consider buying one of these.

You can view the moon and planets easily with a Newtonian, although they won't give as good a contrast of image as a refractor.  This is caused by the need to have a central obstruction, the secondary mirror, and something to hold the secondary mirror in the middle of the tube, commonly called the spider.  Light will be diffracted or bent around the spider vanes and this effect is seen in the eyepiece (and sometimes around bright stars in photographs) as long spikes flaring off the stars.  Newtonians are best for deep sky observing where their large size and small light loss will capture many faint details that the other kinds of telescopes wont.

Another disadvantage is that the optics, set in an open tube, will need relatively frequent cleaning and collimation (realigning of the mirrors) but those who take on a Newtonian will accept this as a necessary task.  With a bit of advice and practise this is not difficult and the rewards are worth it.
 

Schmidt-Cassegrains& Maksutov-Cassegrains

A Schmidt-Cassegrain telescope is also known under the title of a catadioptric telescope.  this means it is a combination of a reflecting and a refracting telescope.  It has a spherical primary reflecting mirror at the back (as opposed to a parabolic mirror in a Newtonian) that reflects light to a curved seconday mirror that then reflects the light out through a central hole in the primary mirror (the Cassegrain design) and out to your eye.  The difference between the Schmidt and the Maksutov is the shape of the corrector plate at the front of the telescope.  The Maksutov has a deeply curved plate and the Schmidt has a seemingly flat plate at the front.  This plate is actually very precisely shaped but cannot be seen by a casual glance.  These plates "correct" the path of light entering the telescope to provide optimum performance for the design.

Schmidt-Cassegrains have only been on the commercial market for several decades but have been very popular.  They can offer an aperture as a large Newtonian while being more compact and portable.  The smaller sizes may also be lighter in weight but if you are looking at one of the larger ones, say 25cm (10") or more, get an idea of how much they weigh first.  When collimated correctly they give good images and can be used for astrophotography.  They hold their collimation very well and rarely need recollimating.

Before you declare "money is no object!" (they can be expensive) and run off to buy one,  there are a few disadvantages.  The corrector plate at the front of the telescope is a great dew collector, so you will have to deal with this problem sooner or later.  The secondary mirror presents a large obstruction to the primary mirror, which means it wont collect as much light as it could for that aperture.  As a result there is some loss of contrast with fine detail on planets and deep sky objects wont appear as bright as they would in a Newtonian.  The long focal length also means a smaller field of view but greater magnification. 
 

Mounts

A good mount is half of a good telescope. You can have a really fantastic telescope but put it on an inadequate mount and it is likely that you will not get the results you hoped for and may become discouraged from using it.  Things to look for in a good mount are:
 

• The telescope doesn't wobble about on the mount when you shake it.

• It can be balanced correctly so that it holds the telescope pointing to where you want to point it when you let go of it.

• Smooth motion as you turn the telescope about - it doesn't "stick."

• Ability to support the weight of the telescope correctly.  This can be seen as the amount of time it takes a telescope to stop shaking fter you let go of it.  A good mount will dampen vibrations quickly.

There are a few extra items you should check on before you buy your telescope.
 

• Does it have eyepieces?  You may want to buy more eyepieces or better ones.  a good eyepiece will make a lot of difference to the quality of the image in the telescope.

• Does it have a finderscope of some kind?  Is it the one you would like?  Sometimes these are an optional extra.

• If it is on an equatorial mount, is a motor drive an extra?  If you want a telescope that is going to automatically track the stars across the sky a motor drive is essential.

Making a Choice

Having read all this information you may not be any wiser about which telescope is right for you but you should have a better idea about what to look for in a good telescope and what you can expect to do with it.   Go back to the questions posed at the beginning and run through them again once you have had a look around and seen a few telescopes first hand.

Remember, a good telescope is one you are going to use the most.