Our Rating System

In the May 2004 issue of Sky & Telescope we introduced a five-star rating system that tells at a glance our opinion of a telescope's optical, mechanical, and overall performance. When you use this rating system to compare different telescopes, keep in mind that such comparisons are valid only for telescopes of similar design and aperture. The ratings alone will not tell you how much you can see using very different telescopes. But the combination of the ratings and an accompanying performance diagram will.

The diagram graphs the theoretical performance of a telescope as a function of aperture. We plot the test telescope's position on two curves: one giving the visual magnitude limit, and the other the visual resolution. The former indicates the faintest star visible in the telescope at a magnification of 150× under good sky conditions, and the latter gives the minimum resolvable separation for a pair of stars of equal brightness (the so-called Dawes limit). Each is dependent on aperture — bigger scopes generally let you see fainter stars and finer details. But quality matters too, and that's where the ratings come in.

So, for example, a mediocre 8-inch reflector with a two-star rating will run circles around a jewel-like 4-inch refractor with a five-star rating when it comes to viewing faint deep-sky objects, because the 8-inch gathers four times more light. But for high-resolution observations of bright targets like the Moon and planets, this refractor may beat the reflector, because exceptional optics can be used effectively at higher powers than poor ones.

The rating system is not an assessment of the merits of one optical design versus another. Those subtleties, which usually involve some consideration of the types of observations you might wish to make with a given telescope, will be dealt with in the text of the review. But for observations of the same targets with, say, two 6-inch reflectors, a four-star instrument will outperform a scope with a three-star rating.

Our rating system is a bit different than those often found with reviews of other products. Most significantly, our rating scale is nonlinear. A typical five-step scale might mean poor, fair, average, good, and excellent. But we generally review equipment intended for serious amateur astronomers, so our selection process tends to filter out instruments that might warrant a poor rating if we were reviewing the full spectrum of telescopes sold today. Nevertheless, we did reserve a one-star rating (defects so severe that the equipment is virtually unusable) just in case.

The remaining four steps in our rating system offer a precise assessment of the equipment we review. At the top is five stars for a telescope so perfect that we can see no room for meaningful improvement. This would be a rare instrument indeed, but since we are judging each telescope by its own set of standards (that is, by standards appropriate to its particular optical design and aperture), every scope we look at has the potential of achieving a five-star rating — reflector, refractor, or catadioptric; large or small aperture; mass produced or custom made.

A four-star telescope, while not perfect, is still a superb instrument. This rating means we can detect imperfections with our testing procedures (and we'll mention these shortcomings in the review), but they will go unnoticed in normal use. A good example is minor spherical aberration of the optics. Even tiny amounts of this common optical defect — the one made famous by the Hubble Space Telescope in 1990 — are revealed with a simple star test. But it takes more than the minimum detectable amount of spherical aberration to render fuzzy star images that don't "snap" into focus and to produce planetary views that lack contrast.

A three-star rating signifies a scope with defects that are visible to a trained eye during normal use but do not significantly degrade performance. An example would be a scope with enough spherical aberration to degrade low-contrast planetary features and show slightly "soft" in-focus stars.

Two stars signify defects that compromise performance. In this case we'd be talking about enough spherical aberration to fuzz close double stars to the point where they are difficult to resolve in an instrument that would easily split them if the optics were better. Such degradation would also significantly impact views of the Moon and planets.

When it comes to telescopes, optics aren't the whole story. Even the best optics are ineffective if they're not supported by a steady mount. So we rate optics and mechanics — which include "fit and finish," the smoothness of motions, tracking accuracy (if applicable), and related features — separately. And because a telescope is more than the sum of its parts, we include a third, overall rating that incorporates such things as price, versatility, and other factors that we'll point out in the accompanying "bottom-line summary." This overall rating is not the average of the optical and mechanical ratings. Indeed, a telescope that gets three stars for optics and mechanics may nevertheless end up with a four-star overall rating — if, for example, it is priced extremely attractively and thus represents a terrific value for consumers.