PLEASE NOTE THAT THE FOLLOWING ARTICLE WAS PRODUCED BY THE CASTLE POINT ASTRONOMICAL SOCIETY [http://www.cpac.org.uk/] AND HAS BEEN REPRODUCED HERE WITH THEIR PERMISSION
Guide to Buying a Telescope
There are lots of telescopes on the market, but how to choose which one? What should you look for in your first telescope? Read on and hopefully your mind will be enlightened before your wallet!
Why do I want a telescope and how will I use it?
This may sound like a silly question, but it is probably the most important that the prospective purchaser must ask him or herself before going to the store. Your (honest) answers to this and other questions below will be used to guide you in choosing the right type of equipment (if any).
Firstly, do you have realistic expectations of what you will see in an amateur telescope? Many of us are first inclined to buy a telescope because we see wonderful colour photos in magazines and on the internet and television. Let´s be clear about this. You will not see anything like this through a realistic amateur telescope. These pictures are taken using specialist equipment which is able to produce long exposure photographs which enhance the amount of detail present (including the wonderful colours). With few exceptions, the view through a telescope cannot rival the impression gained from these pictures.
Do not however, be disheartened as the view is still quite stunning (albeit, not what you may have expected). You have also just learnt your first valuable lesson. "Try before you buy". The best way of doing this is to come along to the various events held by the Castle Point Astronomy Club, including our open nights at the Hadleigh Country Park, or our observing sessions at the Club (see our programme for details). Here you will get the opportunity to look through various types of instruments and talk to experienced owners and observers. Best of all, it´s free and can in the long term save a very expensive mistake. If having done this, you are (like us) completely overwhelmed by the stunning beauty of the heavens and still believe a telescope is for you, read on.
The next question you must ask yourself is about your anticipated observing habits. This is a lifestyle kind of thing. Do you have much spare time in the evenings or will commitments limit the amount of time you are able to spend observing? Are you a night bird prepared to stay up all night until dawn to see all those goodies available (some of us frequently do), or do you just want to grab an hour or so before retiring to bed in the evening? If the latter, there is nothing wrong with this, but it will influence the best choice of equipment.
Astronomical equipment is often very heavy and bulky and if you only have an hour to spend, you don´t want to waste most of it setting up the equipment. There is a well known adage which runs that "The best telescope is the one you use most". Consider this before putting down your money on a large, equatorially mounted reflector which weighs more than you do.
Another important question is in what type of astronomy are you interested? Will you just observe visually, or do you hope to take long exposure photographs. For wide-field images, very simple equipment may be used to allow your camera with its basic 50mm lens to track the apparent motion of the stars across the sky. However, taking pictures through the telescope (called prime focus) requires sophisticated equipment which is generally expensive to buy. Prime focus astrophotography is not for the fainthearted (or impatient). Beware that most beginners and entry level scopes are not well suited to the task.
Finally, the all important question of budget. One glance at one of the several astronomy magazines on the newsagent shelves will tell you that astronomy equipment is not cheap to buy. If you are on a tight budget, your options may be limited and in fact it may be better not to buy a telescope at all. For the cost of a cheap telescope which as we will demonstrate, is usually not worth the money paid, a good pair of binoculars may be purchased which will open up a whole world of observing opportunities and also provide for easy daytime terrestrial use. In fact, most astronomers consider a pair of binoculars an essential viewing tool, especially for those wide sweeping vistas of the Milky Way and larger star clusters and galaxies. The wide field of view of binoculars (~5° to 7°) is all but impossible to achieve in most telescopes.
What should I look for in a pair of binoculars?
The most important thing is light gathering. Most binoculars are labelled something like 7x30, or 10x50. What does this mean? Well, the first number is the magnification. This is how much larger objects will appear in your field of view compared to just looking with your eyes. The second number is the diameter in mm of the front lens element (the objective). This is important because the bigger this is, the more light gathering they will provide and the fainter stars you will be able to see.
However, be careful. Remember that most things are a compromise. If you buy something with a large objective, such as 80mm, you will not be able to hold them comfortably for more than a few seconds and a tripod mount will be essential. This tripod fixing point is a useful feature anyway and should be looked for even in smaller binoculars as the more stable view will allow you to see more and for longer than if you try hand holding. One more thing to look at is the exit pupil (again in mm), given by the ratio of the numbers, i.e. 50/7 = 7.14. This should be between 4 and 7 to provide a useful match to the size of your eye´s pupil at night. Lower than 4 and the image will be somewhat dim. Higher than 7 and the light will not all enter your pupil and some will be wasted. We would suggest that 7x50 or 10x50 are good all round instruments for astronomy and also for daytime use.
Finally, remember that for astronomical use, some optical errors (called aberrations) which can be tolerated in terrestrial instruments look particularly bad on the night sky. Buy your binoculars from a good telescope dealer as they will have a better idea of which instruments will perform best for astronomy.
But I hear you say "I don´t want binoculars, I want a telescope". Do not despair, help is at hand. Even on a modest budget it is still possible to purchase a telescope which will provide all the required functionality without breaking the bank. It is just that selections must be made more carefully to avoid the usually very poor "department store junk scopes".
What should I look for in a telescope?
As in all things, quality counts (and as usual, costs). In order to get something which is going to be worthwhile optically, it is best to make sure the money is spent where it counts. Watch out for exaggerated claims for maximum magnification. The very best scopes will be limited to a magnification equal to approximately 50x per inch of aperture and in most practical observing conditions to more like 30x per inch. The 2.5" department store refractor is not therefore going to be useful at much above 100x magnification, let alone the 675x or more often claimed.
For most uses a much lower magnification will be best anyway. Astronomical objects are not necessarily small; they are just far away and faint. For this reason, light collection is more important than magnification. This leads to another old adage, "Aperture wins". The largest practical sized telescope (remembering the rules about lifestyle) will almost always show you more than a smaller one. Unfortunately, as collection area grows, so does the weight of the scope and the mount required to hold it steady. The need for a sturdy mount cannot be overstated. It is what keeps the optics pointed where it should be and if it is too shaky the whole experience can become very unpleasant.
So, we have looked at the basics, and we can now begin to consider the specifics of different types of telescope. There are a number of suppliers in the UK, mostly offering US and Japanese or Chinese imports, although a little home grown product is available (and is good too). Out of fairness, we will not (generally) recommend specific brands over others, but stick to discussions of general types of instruments and provide notable examples of what is available. For a less impartial view, come along to the club and talk to the owners who will tell you their views on specific brands and models!
This is often the choice of entry level scope and is usually a mistake! That is not to say that they are all poor, just that making a good refractor costs more per inch of aperture than any other type of telescope. So when on a budget, they are not the best choice of instrument. There are exceptions to the rule. A good 4" refractor such as offered by Celestron or Vixen, will be an excellent choice for lunar and planetary observing, particularly where quick set up time is essential.
There are also some premium brand 3" class refractors (such as the Televue range) which give excellent results with wide field images. The Celestron short tube 80mm is a good example of a modest priced refractor which will be easy to travel with and quick to set up. One thing to note about refractors is that all but the very best will show some (or lots) of colour fringing around bright stars and the moon and planets. If this is annoying to you, avoid refractors unless you have lots of money to spend. The main down side to refractors is that due to their expense, they are limited in aperture and will not show as bright an image of deep sky objects (galaxies, nebulae and star clusters etc) as a larger scope.
This is a very important class of instrument and is often split into two groups, depending on how they are mounted. The simplest use a type of mount called a Dobsonian (after its founder John Dobson). This has no electric drive and must be pushed by hand to follow the stars. Fear not, this is not as difficult as it sounds. The good thing is that the mount is cheap to make and more of your hard earned cash is going to go on the things that count (such as the quality of the mirror and accessories). The other type are the equatorially mounted versions which include motors for tracking the stars. Beware, good equatorial mounts for heavy reflectors are expensive and also very heavy in themselves. Don´t buy before you have seen one of these beasts. Whatever type of mount, the scope will almost certainly be of a type known as a Newtonian (after its inventor Sir Isaac Newton).
The best thing about reflectors is that they give by far the best "bang for the buck", particularly when on a Dobsonian mount. For a give aperture, they are the cheapest type of telescope and when well made, give terrific views. They are not in general however particularly compact, but a 6" or 8" aperture Dobsonian mount telescope is a very good investment which will take a long time to grow out of. American imports are available from Meade and Celestron, and home grown models from Orion Optics, Darkstar are also very good. The Russian imported models are also extremely good value such as the 4.5" TAL-1 and larger 6" Sovietski models. For the beginner on a tight budget, a 6" Dobsonian mounted Newtonian or the 4.5" equatorial mounted TAL-1 would be our strongest recommendations.
This is another important class of instrument as it provides a lot of aperture in a compact package which is portable and may be purchased at a reasonable cost. These are mostly of the Schmidt Cassegrain (SCT) or Maksutov Cassegrain (MCT) type (such as the popular Meade ETX). Notable suppliers are again the big US brands, but there is a growing market of imported Russian manufactured equipment (which offers excellent value and very high quality, but can be less easy to find).
The 8" SCT is a good intermediate to advanced level scope which is very suitable for visual observing and is well equipped for prime focus astrophotography (at a price). They can give excellent performance on lunar and planetary observing and have sufficient aperture for good deep sky performance. They can be slightly bettered in all individual aspects of their performance by other types of instrument, but none can give such a good all round performance for all types of observing. This, along with their small storage space requirements explains their rapid growth in popularity in the UK where huge gardens with purpose built observatories are not the norm. However, there are a number of aspects which make these scopes more suited to the dedicated observer and you should talk to existing owners before purchasing one.
Computer Controlled Telescopes
Many telescopes now come with computers which can control their motorised drive. They have a database of objects in the sky and as long as they know the current date, time and their location, they can automatically point the telescope at the correct part of the sky to see the object. Some even have built in GPS and self levelling devices so they can be completely self setting!
The newer ones can be updated from the Internet with new objects objects such as comets. Objects can also be entered manually. Some can be remote controlled from a computer and controlled from the various planetaria software that now exist.
Opinions on these devices are divided. Some say they are very good for beginners as they can easily find objects for you. They are also handy if your observing time is limited and you do not have time to find the objects by eye using star charts and setting circles.
Others say that they make observers lazy in that they do not have to learn the night sky, constellations and star positions. This means you may never get a feel of where things are in relation to each other.
In practice, it is best to have some knowledge of the constellation layouts and the location of the brighter stars in order to at least make sure your computer is working properly and pointing at the right object" Also many of them require you to know bright star locations in order to align them.