© Norman Sperling, November 25, 2011
Part of a series on Educational Star Parties:
Star Parties Designed for Students (July 7, 2012)
7 Spectral Types in 1 Big Loop (April 15, 2012)
Trading Cards for Telescopes and Celestial Objects (September 20, 2012)
For decades, I have been proclaiming that focal ratio is one of the most important characteristics in choosing a telescope. Most authorities tout aperture instead. But none of us has ever conducted a true visual test, isolating the variables of focal ratio, aperture, and eyepieces.
I propose that 3 triplets of Newtonian telescopes be made to demonstrate the effects of focal ratio, aperture, and eyepiece. They can be used for classes and at star parties to teach about the properties of the telescopes themselves. Mount each triplet so that viewers can easily shift among all 3 eyepieces to instantly compare views.
The "focal ratio" triplet should consist of 3 telescopes, all with the same aperture and eyepiece. Make one f/5, another f/10, and another f/20. For this triplet, I think 3-inch (76 mm) apertures are best: even the f/20 would be a manageable 5 feet (1.52 m) long. Users will see that Jupiter looks best at f/20, and the Great Andromeda Galaxy best at f/5. Trying this battery of telescopes on the sky's enormous variety of targets will probably reveal very few objects that look best at f/10.
A second application of this same telescope set will use different eyepieces that all result in the same magnification: a long eyepiece on the long scope, a short eyepieces on the short scope, and a middling eyepiece on the middling scope. How different are the views of different targets?
The "aperture" triplet should consist of 3 telescopes, all with the same focal length (perhaps 4 feet = 1.22 m) and eyepiece. Make one 3 inches (76 mm) aperture, the second 6 inches (152 mm), and the third 12 inches (304 mm). Users may be surprised how much even the 3-inch shows.
The "eyepiece" triplet should consist of 3 identical middling telescopes, perhaps 4-inch (102 mm) f/8. Insert eyepieces of equal focal length but different optical designs (such as Huygens versus orthoscopic versus Nagler). A second application of this same telescope array will use eyepieces of equal design but different focal lengths (perhaps Plossls of 6 mm, 12 mm, and 25 mm ...).
Make each triplet so the scopes, and their eyepieces, can also swivel to allow 2, or even 3, different people to watch through one of the scopes at a time. This is because, perhaps once a decade, some sky event brings out throngs, and the host needs to move a whole lot of eyeballs through the scopes in minimal time.
These triplets could be built by amateur-telescope-making workshops, such as several clubs run, or perhaps by a veteran scope-maker. Most are quite small, only one is large. Try hard to hold all but one factor constant so they really test that single variable.
A whole metropolitan area probably needs only one set. Telescope triplets can be passed around among nearby colleges, astronomy clubs, planetaria, etc., to use at their classes, star parties, and member-events.