Beginner's Guide to Building a Telescope
Section I - Choosing the telescope Design
1. Reflector vs. Refractor
1. Reflector vs. Refractor
The first step in building a telescope is determining what type of telescope to build. This will vary depending on many factors including what type of observing is to be done. There is no type of telescope superior to any other type in all respects. The two major types of telescopes are reflectors and refractors.
Refractors use a lens as the objective. This is the typical type of department store telescope. It's usually limited to about 8 inches for the amateur, since refractors have a long focal length they require a tube length roughly 12 to 15 times the aperture. For a 8 inch telescope, the tube is approximately 10 feet long... a rather cumbersome package to set up and use.
Refractors are typically used for lunar and planetary work where the long focal length is an advantage. Additionally, the tube of the refractor is sealed at both ends so convection currents do not form in the tube, so the observed image is steadier than would be the case for a comparable reflector.
Another disadvantage of the refractor is the fact that light at
different wavelengths will come to focus at slightly differing points, thus the color of the image is affected. Even though the surface precision of the objective is only 1/4 that required for a reflector, the glass must be of very high quality and is very expensive. This limits most amateurs to small refractors, generally 8 inches or less and typically under 5 inches.
Reflectors use a mirror as the objective. Because the quality of glass isn't as important, the cost of materials for a reflecting telescope are much lower. The glass is only used as the substrate for the actual reflecting surface which is typically a very thin coating of Aluminum.
Another advantage is the fact that all wavelengths of light will come to focus at the same spot, so reflectors do not suffer from the problems of color we discussed in the previous section. Reflectors are easier to figure and do not require high precision optical reference elements.
The lower cost and easier construction of the reflector means that the amateur can afford a much larger instrument. This allows a much larger objective and the typical beginning reflector has an objective of 6 or 8 inches, about the upper limit for an amateur's refractor and some amateur reflectors are over 36 inches in diameter!
The disadvantages of reflectors include convection currents in the tube, since (usually) one or both ends of the tube are open. Reflectors also typically require that a secondary mirror be placed in the optical path. The spider which holds the secondary mirror introduces diffraction spikes in the image, usually these are not too objectionable however.
Larger reflectors are usually built to a smaller focal length, so that a typical 8 inch reflector will have a tube length of around 50 inches or so. A much easier instrument to setup than the 8 inch refractor mentioned above with it's 10 foot tube!
Whether a reflector or refractor is chosen, one must decide just how much of the telescope will be constructed from scratch. Some ATMs simply assemble their telescope from parts purchased from suppliers and some go as far as to construct each part. If you're mechanically inclined and you're willing to spend some time scrounging for materials, you can construct even a large
telescope on a fairly tight budget.
In the case of an amateur building a reflector, the average amateur will grind his mirror and then purchase a tube, a mirror cell, secondary mirror and spider, a focuser and assemble the telescope. If a Dobsonian mount is chosen, the amateur with even novice woodworking skills can easily construct the mounting. If an equatorial mount is desired, it helps to have access to a
machine shop and metal working equipment, so many amateurs will purchase this type of mounting. Be forewarned however, that a quality equatorial mount can be quite expensive to purchase and difficult to construct. Fortunately advances in personal computers and software have made it much easier to use an alt-azimuth mounting and guide it under computer control making it
nearly as capable as the equatorial mount for many applications.
The objective is the heart of the telescope. A poorly made mirror or lens means a poor telescope! Unfortunately many commercially built telescopes have mirrors or lenses that have been made hurriedly and are of low quality. A novice mirror grinder, with a little help, can produce a telescope of
high quality that will quite often outperform a commercially made telescope of similar aperture.
There are many good books written for the amateur that will enable even the beginning ATM to produce a well-built telescope.
Some of these include:
How to Make a Telescope by Jean Texereau
Build Your Own Telescope
Making & Enjoying Telescopes:
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You are visitor Last Updated NOvember 7, 1999 by Star Wanderer
1.1 Refractors
1.2 Reflectors
2.0 Assemble or Grind
2.1 Grinding the Objective
ISBN 0-943396-04-2
Published by Willmann-Bell
http://www.willbell.com/
ISBN 0-943396-42-5
Published by Willmann-Bell
6 Complete Projects & A Stargazer's Guide
ISBN: 0-806912-77-4
Published by Lark Books
http://www.larkbooks.com/
More sections and ATM Suppliers Lists for North America, Europe, and Australia will be added to this guide in the near future... so check back often.
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