Purchasing Your First Telescope

Purchasing Your First Telescope
By Michael Nelson and George Greenfield

Introduction

We are writing this article to give a little guidance in buying your first telescope. We are assuming you have little or no knowledge of telescopes and need a place to start. We are also assuming you are interested in astronomy or you wouldn't be reading this. A couple of dos and don'ts Having said this, there are few initial questions you need to ask yourself. For example, be sure you know what you want to do with a telescope.

What do you expect to see?
How much do you know about the night sky and how much are you willing to learn?
How much do you want to spend?
Where will you be using your scope?

After you think you have answered these questions and your expectations are set, talk to an expert. Go to a few star parties and ask questions and look through the different types of scopes. These guys love to talk about their stuff. Keep in mind that their bias will be towards the type they have. Also, buy a copy of "Sky and Telescope" and "Astronomy" magazines. Check out the Internet. However, the best source of information will be with the experts at your local astronomy club.

After you have decided what you want to buy, don't expect to go to a department store or camera shop find what you want. You won't. Most department stores and camera shops sell only low-end telescopes. The capabilities of these scopes are generally overstated and they are generally of poor quality. The majority of good telescopes are sold either by mail order or over the Internet by people who specialize in telescope and are passionate about them. But, have a good idea of what you want to buy before you talk to them.

Mounts

So, what do you want to buy? Well, let's first talk about the mount. The mount is just as important as the actual telescope. The bigger and heavier the mount is the more stable it is. It's as simple as that. Assuming you're going to buy the mount and telescope as a package from one vendor, most vendors match the scope to the mount and they will perform pretty well.

The next consideration has to do with your knowledge of astronomy. If your knowledge is limited and you don't particularly want to spend a lot of time learning the night sky, we would suggest you buy a telescope with a fully computerized "Go To" mount. Meade and Celestron both make low to medium end "Go To" scopes priced between $250 to over $5000 depending on type and aperture. If you know the night sky fairly well, and you don't mind spending the time with star hopping to find objects, you don't need a "Go To" mounted telescope.

If you should ever want to do astrophotography, you will also need an equatorial mount or an alt-azimuth mount that can be adapted to polar alignment. Since objects seem to arc across the night sky due to earth's rotation, the telescope needs to arc or track with them in order to take a picture with exposures longer than a few minutes. An equatorial mount will be at an angle with respect to earth. The angle is equal to the latitude of where you are on earth. The most basic of mounts is an alt-azimuth mount. It moves in two directions (side ways and up and down) but does not "arc" with out an adapter of some sort. It is also recommended that you have at least a motorized clock drive for astrophotography. A motorized mount is also handy at star parties so you don't have to continually reposition your mount when a long line of people want to look at what's in your eyepiece.

Aperture

The next thing to look at is aperture. No doubt about it, the bigger the aperture the more light gathering capability of the scope. The more light gathering capability, the more you will see and the better the star separation. It's as simple as that. However, the bigger the aperture, the heavier and bulkier the scope will be. So there are tradeoffs with size, price, and ease of transport.

Types of scopes for the Amateur

There are many different kinds of telescopes. The following three types represent the most common in use by amateurs today.

Refractors:

$Refractor block diagram$

Refractor on an equatorial mount:

$Refractor on a german equatorial mount$

First is the refractor. This is the type of scope Galileo used. This is the most recognizable scope and the simplest design. It is a tube with an objective lens on one end and a focuser and eyepiece on the other. Light comes in through the objective and is passed directly to the eyepiece. The distance between the objective lens and the eyepiece is the focal length and is determined by the shape of the objective lens. Typically, the longer the focal length the higher the magnification the scope can attain and also the narrower the field of view. The opposite also applies; shorter focal length scopes have nice wide fields of view but can not be pushed to higher magnifications., Without getting too technical, let me give an example. If the focal length is 1200mm and the eyepiece is 20mm the magnification is 60 times. (1200 divided by 20). If the focal length is 600mm and the eyepiece is 20mm, the magnification is 30 times. We say "theoretical magnification" because all telescope have a practical limitation on magnification that greatly depends on the quality of the optics, the seeing conditions, and the mount. We mention this because it is important and because the cheapest department store scopes are refractors. More often than not, the magnification is well overstated. Additionally, Refractors are also among the most expensive telescopes. It is very hard to make large lenses and make them well.

Depending on the quality of the optics, a large refractor can cost as much as $10,000 and you would have to wait a couple of years for it. However, there are a lot of mid to low end refractors available from $250 - $1000.00 that perform extremely well. Many include a reasonably good equatorial mount. Some mounts are motorized and some are computerized "Go To" mounts. And, of course there is everything in between. A good low to medium end refractor gives very pleasing high contrast pinpoint images, is easy to transport and set up and requires very little maintenance.

Newtonians

Newtonian Telescope

Next is the Newtonian. This is a reflector telescope with a mirror instead of an objective lens. There is a primary mirror at the back end of the tube, a secondary mirror suspended in the middle of the tube at the other end. The secondary mirror is at a 45-degree angle that directs light captured by the primary mirror to an eyepiece mounted on the side of the tube.

Equatorial mounted Newtonian:
Newtonian with a Dob mount:

Newtonian on a dobsonian mount

Newtonian telescopes can come with equatorial mounting, but by far, the most popular mount is an alt-azimuth mount called the dobsonian. Invented by John Dobson, the dob mount also makes Newtonian telescope package very inexpensive - in fact, this setup gives you the biggest aperture for the price. You can get a mammoth 10 inch dob for less than $600. A 6 inch for less than $250. You can also spend up to several thousand if you want to. However, while there are motorized equatorial platforms that dobs can sit on, they do not lend themselves well to astrophotography. Further, the larger you get in aperture, the larger and heavier the scope. They can get large and heavy quickly and, therefore less transportable. The more expensive dobs have trusses rather than a tube. The scope can be broken down for transport. The down side is that it can take a while to setup and it isn't something you want to do in the dark. Some of these are so large you need a stepladder to look into the eyepiece. 30inch mirrors are not uncommon, but they are spectacular.

Schmidt-Cassegrain

SCT Block Diagram

And last is the Schmidt-Cassegrain or SCT. The SCT is a reflector telescope of the catadioptric design, which uses a lens and a mirror. They are very compact in length because the eyepiece is mounted at the back of the scope and the light path is folded twice. Light comes in through the front "corrector lens" and is reflected off the primary mirror in the back of the scope to the secondary mirror mounted on the corrector lens in the front. It is then reflected back through a hole in the primary mirror to the eyepiece. SCT's are fairly sophisticated instruments and can be rather expensive. They start at about $1000 and go up from there. They are available on equatorial mounts and altazimuth fork mounts that can be polar aligned with the addition of a wedge. Most are computerized "Go To" mounts. Because of their compact length, they can be easily balanced and are the best amateur telescope for astrophotography. Apertures up to 14 inches are commonly available for the amateur. But, they can be quite heavy. A 12" SCT with the fork mount weighs over 70 lbs. You have to be able to lift this up onto a tripod that weighs about 70lbs. This doesn't sound like much but it can be a hand full!

Fork mounted SCT with an equatorial wedge:
Fork mounted SCT without the wedge:

SCT on an altitude azimuth mount

Focal Length

Now let see if we can adequately explain focal length with out getting too technical. Actually there are three terms that work

together. They are focal length, focal plane and focal ratio. Focal length is the length of the light path from the objective lens or primary mirror to the focal plane. The focal plane is where the light comes into focus. When you turn the focus knob on a telescope, you are moving the focal plane of the eyepiece to coincide with the focal plane of the scope. Focal ratio is the ratio of the focal length to the aperture and is expressed as an f/#. So a 12inch scope, like I have, has a focal length of about 3000mm and an aperture of about 300mm. 3000 divide by 300 equals 10 so the focal ratio is f/10. Focal ratio is also described as speed. The lower the f/#, the faster the scope. Fast telescopes, say f/5 or 6 give a wider field of view, but the optics have to be very good or there will be a lot of false color and distortion. Slower scopes, f/10 or 11 are much more forgiving. Additionally, a slower scope will have higher magnification than a fast scope using the same size eyepiece. But, and this is a big but! Most observing is done at relatively low magnification - 40x to 120x. And most scopes will handle this range just fine.

So which the best scope for you?

The best scope is the one you will use. That means that you need to choose a scope that you can move from where you will store it to where you will use it to observe and feel comfortable with. Choose a scope that you can carry outside to your favorite viewing area and that doesn't need so much set up that you may shy away from setting it up.

Think of how you will use your scope. Do you have a nice observing spot in your backyard or driveway that doesn't require much transport? Or will you need to take you scope to a good observing area and it will need to fit in your car. Or maybe you will want to take your scope with you on vacation, or out to the coast whale watching, or bird watching? Will you take it on an airplane? Pick a type and size of scope that will fit your needs.

Also, choose a scope that you can afford. The cost of the scope and mount is just the start. You'll want to purchase a few eyepieces, a flashlight, a book or two, etc…and believe me, there are many ways to spend money in this hobby!

If you are very much a beginner you may want to start with a pair of binoculars. These are wonderful tools, easy to use, and quite inexpensive. Use your binoculars for a couple of months observing the night sky, visit a few star parties with your local astronomy club, and try out several types of scopes to see what really interests you. And ask lots of questions. Then you will be ready to choose a scope wisely that will fit your needs and that will provide your with a wonderful viewing experience for a long time.

Whether it will be a refractor, newtonian or schmidt-cassegrain; there's a scope out there that's just right for you!