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Buying a Telescope for Astronomy

by William T. Peters
Former Director of the TELUS World of Science

It isn't easy to select a telescope which is both affordable and will do a good job of showing sky objects. Here are some ideas and things to look for which will make the selection easier.

What makes a telescope good?

The telescope has two jobs, to magnify distant objects, revealing more detail in them and to make faint objects appear brighter. The diameter or aperture of the main lens or mirror of the telescope determines the ability of the telescope to perform both these jobs.

Think of the telescope as a kind of funnel with the large end pointed toward the sky and the small end at the eye. The larger the big end of the funnel the more light will be collected and "funneled" into the eye, As the aperture - the big end of the funnel - increases in size, fainter and fainter star clusters and galaxies will become visible. As the aperture increases the ability of the telescope to show fine detail also increases.

So let's just buy the biggest diameter telescope available. Right?

Almost, but aperture is only half the story of what makes a telescope good. The other half is quality. If the telescope is to show things with satisfying crispness its mirrors and lenses must be crafted so that the light rays passing through deviate no more than 1/4 of a wavelength of light from their ideal path. The best available optical fabrication techniques can just barely achieve this level of quality in mass produced instruments. The manufacturers specifications may give a wavelength value or simply say “resolves to the theoretical limit" which indicates that the 1/4 wave criterion is met or exceeded.

What if the quality is a little lower?

Then the images will be fuzzy and unsatisfying, no matter what the size or the power.

It is easy to measure aperture, but how do I find good quality?

The same way as in cameras or other technical goods; buy from a maker with a reputation for quality, one who will stand behind the product and make it right if the product fails or doesn't live up to specification. Celestron and Meade are two of a number of makers with good reputations. If in doubt talk to an amateur astronomer who has a passion for telescopes. Avoid the small oriental made telescopes unless you understand how to test and evaluate telescopes. Their quality is highly variable.

In addition the user friendliness and the mechanical. stability of the telescope should be considered. A telescope magnifies every vibration in addition to magnifying the stars. One mounted on an even slightly shaky stand is very frustrating to use.

How about magnification? Can I get a 1000 power telescope?

Yes. But it will have to be at least half a meter in aperture. That's a size starting at about $5000! Even on a half meter telescope 1000 power won't be very useful. The maximum power is limited by the steadiness of the earth's atmosphere. Few nights are perfect enough to permit more than 400 power, even on the world's largest telescopes, before the images become uselessly blurry. Regardless of size of telescope the most useful powers fall in the range of 30x to200x.

On good telescopes the power can be changed by changing the eyepiece. Avoid telescopes that zoom or change power by pushing the eyepiece in or out. The extra optics to permit this convenience seriously degrade optical quality. Also select a telescope that takes North American standard 1 1/4 inch diameter eyepieces. The smaller 0.965 inch diameter eyepieces supplied with most oriental telescopes are generally poor quality.

To calculate the power that any given eyepiece yields, divide the focal length of the eyepiece into the focal length of the telescope. For example an 18 mm eyepiece on a 900 mm. telescope gives a power of 900/18 = 50x. The focal length is always engraved on the eyepiece.

A useful rule of thumb for determining the maximum power of a telescope is to multiply the aperture, expressed in centimeters, by 20. Thus a 7.5 cm telescope would magnify 20x7.5 = 150 times before the images get too fuzzy. Incidentally, a 7.5-cm telescope is the smallest aperture that can be recommended for general purpose sky watching. Smaller telescopes do a good job only on a rather limited selection of big, bright sky objects.

By now you've noticed that l classify telescopes by their aperture size, rather than their power. That is
the standard way of doing it since aperture is what counts.

So what is a good, large diameter, telescope going to cost?

That depends on the type of telescope you choose. Here are the three main types in order of increasing

Reflecting: The most common variety is the Newtonian reflecting telescope which has its mirror
at the bottom of the tube and an eyepiece on the side near the top end. This is a classic design that
can perform very well. It is inexpensive because only two precision optical surfaces, the main mirror
and the little secondary mirror that reflects light into the eyepiece have to be fabricated. In sizes up to
15 cm reflectors are quite portable and easy to use.

Disadvantages are that the optics are hard to clean and most makes have mirrors that get out of alignment easily, necessitating some fussy adjustments to keep the telescope performing well.

Catadioptric: The most common ones go by the name Schmidt-Cassegrain rather than catadioptric.
Like a Newtonian reflecting telescope the light first falls on a mirror at the bottom of the tube. A little secondary mirror then reflects the light to an eyepiece through a small hole in the middle of the main mirror. A glass plate covers the front end of the tube, protecting the optics and improving optical correction.

Advantages include compactness and easy portability in sizes up to 25 cm. and good adaptability for sky photography. Celestron's 20 cm telescope, the C8, is the most popular telescope for amateur astronomers and schools. C8s start at about $1500 CDN.

Disadvantages: Optics can get out of line, though this isn't as much of a problem as in reflecting telescopes.

Refracting: This is the “classic telescope" with a lens at the front and an eyepiece at the bottom of
the tube. Affordable only in small sizes. A 10 cm refractor typically costs as much as a C8. One the size of a C8 can go for as much as $15,000. Prized, even in relatively small sizes for very crisp, sharp images.

Advantages: The best design for planet observing. Optics never get out of alignment.

Disadvantages: Long, awkward tube.

Are there any good telescopes under $1000?

l like Celestron's "First Scope 80", an 8cm refractor, or similar.

The 9 and 10 cm catadioptrics from Meade and Celestron for about $ 800 to 1200, depending on features, are also very nice, though not quite as crisp on the planets as the refractor.

How about way under $1000?

Now we are down into the range of the small oriental telescopes. Usually theses are either too small or too low in quality to recommend.

Now-a-days, there's good quality Newtonian reflecting telescopes for around $300. They is partly made in Japan or China, but to quality designs and specifications and sit on solid, but low-tech mounts.

Is there anything under $200?

Yes. Binoculars! Moderate to good quality binoculars in the 6 to 10 power range will show an amazing amount in the sky. Try a pair on the Moon and you will be amazed at the craters that show up. Best when mounted on a tripod -some camera stores have a gizmo that permits this. Better than most telescopes in the same price range. l like 10 power x50 mm. aperture ones best but some of my friends prefer 7x50s for sky watching. Avoid zoom models, but start with what you have.

How do I put a camera on the telescope?

~That's a whole other story! For more on every aspect of telescopes and sky watching, read Terence Dickinson's marvelous book "NightWatch," its a Canadian best seller - really! Try also the fllow-up book he wrote with Alan Dyer, "The Backyard Astronomer's Guide." Scan the articles and ads in "SkyNews", "Astronomy" and "Sky and Telescope" magazines.

After you've done some reading and thinking, members of the Royal Astronomical Society of Canada will be
happy to help answer the deeper questions.

Back to the question about cameras. Most telescopes can be adapted to an SLR camera that has interchangeable lenses if the telescope is rigid enough to carry the extra weight. The telescope
effectively becomes a very long telephoto lens for the camera. Amateur sky photos and information on
shooting them appear in every issue of the magazines mentioned.

Final thought:

Don't dither too long. Get a telescope. There is a whole universe to explore out there. So many stars - So little time!

Here are two brochures you might want to look at:
RASC Choosing a Telescope.pdf
RASC Choosing Binoculars.pdf

Table of Contents

Page title Most recent update Last edited by
Orion: the Hunter in the Winter Night Sky December 30, 2011 2:56 PM Roland D.
Trial by Fire: the amazing story of Comet Lovejoy December 18, 2011 11:53 PM Roland D.
Buying a Telescope for Astronomy October 16, 2012 9:29 AM Roland D.
Autumn and Early Winter Constellations December 1, 2011 7:05 PM Roland D.
Light Pollution Abatement in Calgary November 24, 2011 6:00 PM Roland D.
Astronomy Social Media in Calgary August 1, 2013 5:00 PM Roland D.
About The Calgary Astronomy Meetup Group April 7, 2014 12:02 PM Roland D.

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