|How large a telescope
should I build?
If you are a first-time scope builder, this might be the easiest decision you’ll ever make: you should make an 8 inch f/6
dobsonian. I will the discuss the tradeoffs and you can decide for yourself, but I think the choice is simple and clear.
Here’s why. There are three considerations: cost, ease of use, and useful lifetime. First cost—8” f/6 mirrors are
widely available and quite inexpensive for the aperture. They are one of the best bargains around. 6 inch mirrors
don’t cost much less, and there is a big bump in price when you go up to mirrors 10” and larger. The rest of the
materials cost is mostly in tubing and mount material—cardboard form (or mailing) tube and plywood. Cardboard
tube is cheap, so the diameter doesn’t matter much. You will need to buy a full 4X8’ sheet of plywood whether you
intend to build a 4-1/4”, 6”, or 8” telescope (although you will have different amounts of material left over, of course).
For a 10” telescope you will have to buy two sheets of plywood.
The real cost of the telescope, however, will be the hours of loving labor you invest in building it. And here is a little
secret: it takes just about the same amount of time to build 4-1/4”, 6”, 8”, and 10” instruments of the same overall
design. The classic dobsonian design for telescopes in this range uses the same parts whatever size you build; only
the size of the parts differ. The time cost of a part is related to the number of cuts you need to make, not to its size—
making a groundboard for a 10” telescope takes the same amount of time as making the groundboard for a 4-1/4”
The ease of use is related to two factors—how much effort it takes to bring the scope out under the stars, and how
comfortable it is to look through. Setup effort increases directly with aperture: larger telescopes tend to be both
heavier and more unwieldly. An 8” telescope is about as heavy an instrument that most people can haul out and
carry comfortably. My 70 lb. 10 incher was a real strain, and consequently didn’t get much sky time—and time under
the stars is what it is all about.
Viewing comfort is most closely related to eyepiece height, which is a function of the focal length of the primary
mirror. For the most commonly available focal lengths of 4-1/4” mirrors the telescope comes out a bit too short to be
used comfortably by most adults—too much stooping and craning of the neck to put in long hours at the eyepiece.
The most common sizes of both 6” and 8” mirrors have focal lengths of 48” (with focal ratios of f/8 and f/6 for 6” and
8” apertures, respectively). This is the perfect height for a child to use standing, or an adult sitting. (By the way,
being able to sit increases greatly what you are able to see with a telescope, whatever the aperture, because it
steadies the head and lets you look for much longer periods without fatigue.) Since 8” instruments gather more light
and resolve finer details, of the two it is clearly the preferred choice.
10 inch instruments are another matter. If the focal length is short enough to be used sitting it will be finicky to
collimate accurately enough to take advantage of it’s greater theoretical resolving power. On the other hand, if the f/
ratio is longer it will be too tall to use sitting, but still probably a little short to be used by adults standing comfortably.
Once again, for comfort the 8” f/6 wins.
The useful life of a scope will depend on how much you can see with it. If the scope is too small you will quickly
exhaust the number of interesting objects you can observe with it. Remember, the number of things to see and the
quality of the view increases directly with aperture. If the instrument is too small you will either (1) quit the hobby out
of boredom, or (2) soon replace it with a larger instrument. And if you really get hooked, will you continue to use your
old instrument once you graduate to a really big telescope, like that 18” Obsession you are lusting after? Here is
where 8” f/6 instruments really shine, folks, and perhaps this is all you really need to know to make your decision. 8
inch scopes can see a lot. Back in the golden age of plumbing-fitting mounts and home-ground mirrors 8” was
considered an awful lot of aperture. That’s enough to resolve many globular clusters into balls of stars rather than
just faint round smudges, and show the rings of Saturn, the belts and red spot of Jupiter, and the changing seasons
on Mars. And the views of the moon are simply spectacular (and often on a night with poor seeing more satisfying
than the shimmering and boiling craterscapes you see in larger telescopes).
And if you do buy that really big scope, the 8” f/6 will still have grab-and-go appeal for those That’s-a-pretty-moon-I-
think-I’ll-have-a-look nights when you just don’t have the energy to wrestle with 120 lbs of glass and plywood and
eight long aluminum tubes. In fact, your old trusty 8” dob is likely to continue to get more sky time than your new high-
maintenance expensive giant. I’ll bet you will spend more time in its company than with any other telescope you will
ever own. That’s why you need to start with an 8” f/6 telescope.
Copyright Ross Sackett 2009
Ross Sackett's amateur telescope making
Ross Sackett's amateur telescope making