Telescopes come in many different shapes and sizes. In recent years, "short tube," or "rich-field," reflectors and refractors have become popular. Their short-focal-length optics fit in stubbier tubes, making them lighter and more portable than "long-tube" scopes. (Rich-field telescopes have low f-ratios ? focal length divided by aperture ? typically f/5 or lower.) The physical differences are obvious, but how do short-and long-focal-length scopes differ in performance?
Field of View
The main distinguishing characteristic is the field of view. The shorter a scope?s focal length, the wider its field of view (for a given eyepiece). For many of the deep-sky treasures we so enjoy looking at ? diffuse nebulas, open clusters, and big galaxies ? a wide field is desirable to frame the whole object in surrounding starfield. Long focal lengths show a narrower patch of sky, which is ideal for viewing small objects like planets and lunar surface details.
Example: the Orion AstroView 120 (f.l. 1000mm, f/8.3) used with a 25mm eyepiece shows a 1.25° field of view (2.5 full Moon widths), while the AstroView 120ST (f.l. 600mm, f/5) boasts a 2.1° field (more than 4 Moon widths) with the same eyepiece. A wide viewing area also makes it easier to find objects, since you?re seeing more of the sky at once.
Image Quality Short-focal-length optics introduce certain optical aberrations. Short refractors have more visible chromatic aberration than long refractors. You?re likely to see purple or orange halos around bright planets and the lunar limb. With short reflectors, "coma" comes into play: stars appear like "commas" or "seagulls" near the edge of the eyepiece field. For most people, a little color fringing or coma is no big deal; for purists, they?re undesirable. These aberrations are less noticeable at low magnifications, which is why you shouldn?t push the power higher than about 100x in typical short-tube telescopes. Of course, high-end versions made with exotic glasses or with corrector lenses can handle higher power.
While a short-tube scope is likely to be more easily portable, choosing between a short-and long-tube telescope really boils down to this: If your main interest is in viewing or photographing the planets and Moon, go with long focal length. If deep-sky objects are what you?re after, the wide field of a short, rich-field telescope will do the job wonderfully ? and more portably, to boot.