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Traps for Young Players: Buying the Right Telescope

Astronomy can be a very expensive pursuit. Name a price and someone will build you a telescope for that amount of money. An (upper) estimate of six billion dollars U.S. could have been spent on the Hubble Space Telescope but it captures amazing images.


(Photo from the Huble Space Telescope)

When buying a telescope it is a good idea to work out how much you can afford, get good information and then buy the best telescope that you can for the price that you can afford.

Here are few important terms that you will need to learn if you are new to astronomy.

Aperture: The aperture of a telescope is determined by the size of the diameter of the front of the telescope. That is either the size of the main lens (for a refractor) or the hole at the front (for other types of telescopes), the larger the aperture the better the image.

Focal length: This is the total distance that the light travels from the front of the telescope to the eyepiece. If the telescope has mirrors then this is the total distance that the light travels (which includes the distance bouncing between the various mirrors to the eyepiece).

Tripod / Mount: A telescope is mounted on a tripod. There are three main types of tripods, Equatorial, Dobsonian and Altazimuth, (more about tripods later).

There are plenty of cheap telescopes out there that are advertised as having powerful magnification yet are disappointing when it comes to using them. That’s because when it comes to seeing clear and bright images, it is not the magnification that counts but the size of the aperture.
Large apertures let in more light than small apertures and more light means brighter images. High magnification just makes dim blurry images, dimmer and blurrier and the cheap tripod that usually comes with a cheap telescopes makes the dim, blurry images very shaky as well (especially when there is a breeze).

Binoculars

I really don’t like using binoculars for astronomy. Not everyone shares my dislike but there are a number of downsides to binoculars. Firstly you are paying for twice the number of lenses that you pay for when you buy a telescope. Large binoculars (at least ones that are of a good quality) are expensive and they are heavy and unless you can get a mount for them (or at least something to lean on) then it is very hard to hold them steady enough to stop the image from shaking. Personally I’d rather spend the money on a cheep telescope than expensive binoculars.

Telescopes

There are three main types of telescopes, Refractors, Reflectors (or Newtonian) and Schmidt-Cassegrain.

Refractor

Let’s take a look at a few types of telescope starting with a refractor. A refractor telescope is a telescope that has a glass lens at the front. The light travels (basically) in a straight line from main lens to the eyepiece at the other end.


Refractor with an equatorial mount.

The advantage of refractors is that there is no chromatic distortion (a rainbow like blurring of the image that is noticeable around its edges), but the disadvantages are that they are more expensive to make and heavy to carry. That is why you will not often find a refractor telescope with a very large aperture. With a good refractor telescope you should be able to see some of the larger craters on the moon, the rings of Saturn and the four main moons of Jupiter. (Saturn is side on to the Earth in 2008 and it will not be a good time to see Saturn’s rings with any amateur telescope for a few years.) If you are not planning to become serious about astronomy then a small refractor (3 to 4 inch or 75mm to 100mm) might be suitable for you.

Possible disappointments of a refractor telescope

You will not likely be able to see faint objects, such as planetary nebulae or distant galaxies through a small refractor. Binary star systems, i.e. stars that appear to be one star but are actually two (or more) stars, will probably still appear as a single star.

Newtonian

A Newtonian telescope uses mirrors. Instead of a main glass lens at the front of the telescope there is a hole and at the back of it there is a main mirror that focuses the light on to a second smaller mirror that in turn directs the light to the eyepiece that is on the side of the telescope.


Newtonian with a Dobsonian mount

Newtonian telescopes are cheaper than an equivalent refractor, so they tend to have larger apertures and so are better for seeing faint objects.

Possible Disappointments of a Newtonian Telescope

It is not always easy to look through the eyepiece at the side of the telescope; sometimes you may need a stool. A Newtonian Telescope uses mirrors which can lead to chromatic distortion. A Newtonian Telescope with a large focal length (i.e. a long tube) may be heavy and hard to move around (or fit in a car). Images may be dim and blurry with smaller apertures. I would not recommend a Newtonian with an aperture smaller than 200mm or 8 inches, though casual astronomers might be happy with a 150mm/6 inch telescope.

Here are some examples of what some objects look like through a six inch Newtonian.

 Binary star system - Alpha Centari
 The planet Jupiter
 Staturn (when the rings are visible)
 Craters on the Moon
The Moon appears much brighter than this photo shows

Schmidt-Cassegrain

A Schmidt-Cassegrain telescope is like a Newtonian except the eyepiece is at the back of the telescope. The light travels further to eyepiece at the back giving this design a larger focal length for a relatively shorter tube.

Possible Disappointments of a Schmidt-Cassegrain Telescope

This is the kind of telescope that I would recommend. The only down side would be the price but if you purchase a recognized brand then you should not be too disappointed. But in the end no matter how much you spend (assuming you find astronomy interesting) you are always going to wish you could see further. If you are buying a second hand telescope that has a motor and Go To (see below), make sure the gears are not worn because if they are then the telescope will not track properly (if at all).


Schmidt-Cassegrain Telescope

Telescope Mounts

Equatorial: An equatorial mount is designed to make it easy to manually track objects as they move through the sky. Once the mount is correctly aligned, all you need to do is twist a single control to follow an object through the sky. However, the setup process is complicated, especially for people living in the Southern Hemisphere and with some equatorials; it is difficult to point the telescope at objects that are close to a celestial pole (a point that would be approximately directly overhead if you were near the south or north poles).

Dobsonian: Dobsonian mounts are cheap and they are easy to use manually. There are two directions of rotation. You just spin the base until the telescope is facing in the right direction and then swivel the tube up or down. No setting up is required.

Altazimuth: An Altazimuth mount is similar to a Dobsonian except that it is mounted on a tripod.

Telescope Accessories

Tracking Motor: A telescope with a tracking motor will automatically follow objects as they move through the sky. That makes it easy when showing object to friends and family. Without a tacking motor, especially at high magnification, the object that you are trying to show someone could have quickly moved out of the telescopes field of view and inexperienced people will have a hard time trying to keep the telescope pointed in the right direction. Tracking motors also make it easy to take time exposures with a camera or a CCD. (A tracking motor stops the stars from appearing as streaks of white lines, smeared across the photograph).

Go To - Computerized Object Finding:

Telescopes with computerized object location make it easy to find objects, especially faint objects. It requires a quick set up, by pointing your telescope at three objects to align the system but after that it will find objects for you (even faint ones) and all you have to do is type in the code for the object. The code is normally the Messier or NGC (New General Catalogue) number of an object.

Star Charts and Astronomy books

Its not much fun just pointing you telescope at the sky and wondering what it is that you are actually seeing. By using good star charts and books and learning to identify the major constellations and various objects within them, you can not only enjoy your viewing more but you can also enjoy sharing your knowledge with friends and family. Some of the more amazing things to view are the moon, the planets, galaxies, globular clusters, nebulae and planetary nebulae, a good book will help you find them. In fact you don’t actually need a telescope to do astronomy. There are lots of things you can see without a telescope and a good book will help you identify them.


The right telescope for you

So in summary if you are a very casual astronomer and don’t want to spend a lot (and don’t care that you will not be able to see faint objects such as nebulae) then a refractor with an aperture of 75mm to 100mm (3" to 4") might be good enough for you.

If you are more serious but don’t have a lot to spend and don’t need to track objects through the sky (and you don’t mind learning how to find these objects yourself) then a Newtonian telescope with an aperture of 200mm/8" with a Dobsonain mount will be easy to use (especially for someone who can’t be bothered setting up an equatorial tripod). Note: that if you live in an urban area and you would like to see faint objects such as galaxies or nebulae, then you will need to be able to fit your telescope in a car. Any telescope with a tube longer than 1m (a little less than an adult’s arm span) will be very hard to fit in a small car.

The more serious astronomer with the cash to spare will enjoy using a Schmidt-Cassegrain telescope with a tracking motor and a computerized object finder (Go To). I recommend an aperture of 200mm/8" or larger. If you want to find objects easily and be able to take long exposure photographs then this is the kind of setup you will need. To take photographs, you will need either a CCD to use with a laptop, or a special T adaptor for an SLR style camera.

Conclusion

Try to think long term and get the telescope that will suit your long term needs. If you think it might be a passing phase then think about how much you can afford to spend on something that will just sit there gathering dust in the basement. If you are keen and want to pursue this interest then buy a telescope that either has all the features you need or one that can be upgraded.

Clear Skies.

This note is directed at Christians (or anyone who might be interested), I am a Creationist and I have posted a page with links to resources related to Creation.

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