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 The basic parts of a typical Dobsonian telescope: The business end of our Optical tube is aimed to the upper left in the picture. Light from the image in the sky goes down the tube around the Secondary mirror, (held in position by the 4 vanes) and hits the Primary mirror inside the bottom of the tube. The image bounces back up the tube to the Secondary mirror, which angles it to the Eyepiece. The dial on the Focuser is turned until the image is in clear focus. With this size telescope, (10-inch diameter) a 6-foot man would stoop slightly to look into the eyepiece. Some Dobs are so large (24 inches or more in diameter) that an orchard ladder is required to get high enough to look in the eyepiece.
Photo labeled by The Belmont Society
The Finderscope is a very low-power "spotting scope", which the observer can use to accurately align the main Optical tube at the target. A Dobsonian's Base is an Alt-az design which swivels in two axes (Altitude and Azimuth). It is common practice to drive both axes with a computerized "Go-to" system for smooth automated operation. With such a system, observing and astrophotography can be accomplished with ease.
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A telescope's "f/number" is its "focal ratio". A scope with a focal LENGTH of 1000mm and an aperture (diameter) of 100mm has a focal ratio of 10, and is designated "f/10" (divide aperture into focal length). An aperture of 125mm and a focal length of 1000mm would yield f/8.
You will see f/numbers mentioned in telescope advertisements, and you'll read or hear that one telescope is "faster" or "slower" than another. This term and its specification are NOT IMPORTANT VISUALLY. The ancient myth that longer telescopes "see" farther and better just isn't true. All this f/number-stuff means is, the configuration of some scopes will allow astroPHOTOGRAPHY at differing exposure times — it has nothing to do with how bright or how dim images will appear in the eyepiece. For instance, an f/5 telescope is considered "faster", than an f/10 of equal quality, and will produce a photograph of the same object four times faster. If you don't plan on taking astrophotos, then you don't need to be concerned with this term or its specification. The main advantages of a faster telescope are storage and portability, and that's usually why people buy small (short) refractors.
Pictured at left: An example of similar scopes with different f/numbers is Orion's AstroView 120 ST EQ refractor. It's available in both a long and a short version. The long tube is an f/8.3, and the shorty is f/5. They both yield the same quality image in the eyepiece. However, slow (longer focal length) achromatic refractors will exhibit less inherent chromatic aberration than fast ones.
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What you can see... and what you WON'T see
The Cost of Amateur Astronomy
Finderscopes, Telrads, etc.
What is "GO-TO"?
Recommendations - GO-TO Systems
Misleading Astronomy
How things REALLY look in the eyepiece
Light Pollution