Rather than provide only an overall five-star rating, it is more helpful to rate different aspects of a telescope separately. Also, because telescopes can be more than the sum of their parts, the overall five star rating is not necessarily the average of the individual performance rating schemes. Here is how we rate individual aspects of telescopes in our reviews.
- Optics: The quality of the main optics of the telescope.
- Focuser: How good is the focuser mechanism?
- Mount: Sturdiness and accuracy of the mount.
- Moon & Planets: Is the telescope intended for moon and planet viewing at high powers?
- Rich Field: How good is the telescope for wide-field viewing of deep-sky objects?
- Accessories: The quality and number of accessories, such as eyepieces and finders.
- Ease of Use: How easy is it to use this telescope?
- Portability: How easy is it to travel with the telescope?
- Value: How affordable is the telescope, and is it worth any of its other flaws?
Telescopes without mounts will not receive a score of 1 for the mount; the mount score will simply not be counted. Telescope optical tubes sold with no accessories will be scored the same way.
A more detailed rubric follows:
Optical quality should be determined by a star test but need not be exact.
- 1 star: Terrible at any magnification, worse than the unaided eye.
- 2 star: Substandard, fuzzy optics, not suitable for high power observing.
- 3 star: Not diffraction limited, but not unusable either, good to about 1x-1.5x per mm of aperture.
- 4 star: 1/4th wave of spherical or other aberrations, or better.
- 5 star: Absolute optical perfection; indistinguishable from perfect optics.
- 1 star: Loose, wobbly, impossible to maintain collimation or focus position, or so rough/imprecise that exact focus position can not be ascertained.
- 2 star: Wobbly enough that collimation may not be maintained, or rough enough to make it difficult to find focus.
- 3 star: Acceptable focuser, but occasionally difficult to use.
- 4 star: Focuser is stable enough that it never tilts and messes with collimation.
- 5 star: Smooth and rock-steady motion, plenty of precision; easy to find focus.
- 1 star: Unusable mount, can not even keep the Moon in the field of view properly at low power, or is too wobbly and shaky, or too unbalanced, to reliably aim at deep-sky targets.
- 2 star: Mount is either shaky and unbalanced and difficult to reliably point at targets, but is not totally impossible to use. It may be a decent but undersized mount, for example.
- 3 star: Mount is usable, but shaky at high power or tough to get the balance right, or difficult to use.
- 4 star: Mount is stable, sturdy, balanced, and easy enough to use, but has some design flaw which occasionally gets in the way.
- 5 star: Perfectly balanced, stable, sturdy mount. If it’s a Go-to computerized mount, it is very accurate.
Moon & Planets:
High-power observing of the Moon, double stars, and planets, requires a lot of things to go right. The telescope must have a sturdy mount which does not wobble too much, it must have a good focuser, and the optics must be very good. Having a long focal length helps as well, because it means it’s easier to find affordable eyepieces which deliver high magnifications. Any telescope with good optics may be capable of showing great views of the Moon and Planets, but telescopes with longer focal lengths may be given a higher Moon & Planets rating, to signify that the telescope was probably intended for this purpose.
- 1 star: Optics rating of 1 or 2 stars guarantees a 1 star rating for Moon & Planets. Alternatively, telescope might be too wobbly, collimation may be impossible to maintain, or focuser may be too rough.
- 2 star: Telescope has mediocre optics or mounting which make it unsuitable for high power viewing; attempting high power viewing may be possible but frustrating.
- 3 star: Telescope has some issues (including but not limited to wobbliness, shakiness, short focal length) which make it possible to use for planets, but not ideal.
- 4 star: Telescope shows really nice planetary views but may still deal with some flaw, optical or mechanical, which can be worked around.
- 5 star: Telescope has a long focal length which makes it easy to reach high powers, and optics rating of 4 or 5. Mount is stable and does not vibrate too much. Focuser is usable.
Field of view relative to aperture. Rich field telescopes provide brighter, wider-field images, depending upon the eyepiece provided, which will usually have to be an aftermarket eyepiece. Because Rich Field and Moon & Planets ratings are both related to focal ratio, a good Rich Field scope might not be a great planetary scope, and vice versa.
- 1 star: Focal ratio greater than f/10 with a 1.25” focuser or f/15 with a 2” focuser.
- 2 star: Focal ratio of f/6.1-f/9.9 with a 1.25” focuser or f/9.6-f/14.9 with a 2” focuser.
- 3 star: Focal ratio of f/5-f/6 with a 1.25” focuser or f/8-f/9.5 with a 2” focuser.
- 4 star: Focal ratio of f/4 or less with a 1.25” focuser or f/6.4 or less with a 2” focuser.
- 5 star: Focal ratio of f/4 or less with a 2” focuser.
- 1 star: Low quality, garbage accessories, including eyepieces which encourage extremely high magnifications in a telescope which can not deliver.
- 2 star: Bare-bones accessories which are usable but still poor quality.
- 3 star: Useful but basic accessories, for example a red dot finder in a scope that needs a finderscope, or cheap Kellners.
- 4 star: Useful, quality accessories, but one or two may need to be upgraded eventually, or is lacking in something needed.
- 5 star: Everything you need to get started; accessories which won’t need replacing or upgrading soon.
Ease of Use:
- 1 star: The reviewer can’t even figure out how to use the telescope effectively.
- 2 star: Extremely difficult to figure out, constantly tough to use even once it’s learned.
- 3 star: Steep learning curve. Difficult to learn, easy to use after learning how.
- 4 star: Gentle learning curve, but not entirely trivial.
- 5 star: Good for young children and technophobes. Telescope is simple in operation and easy to understand.
- 1 star: Really needs to be a permanently mounted observatory setup, or needs a dedicated trailer to move.
- 2 star: Fits in an SUV or other large car/trunk.
- 3 star: Can fit in a car, but takes up a lot of space in it.
- 4 star: Can fit in the trunk of a small-ish car or in a single seat in the car, or in a suitcase or backpack.
- 5 star: Would fit in carry-on luggage in an airplane.
- 1 star: There are other options which do everything this does at a much better price.
- 2 star: There are other options which do most of what this does at a better price.
- 3 star: Average value. Other options might be better, but it’s not a bad value.
- 4 star: Good value, does more than other similarly priced items or is cheaper than comparable items.
- 5 star: Absolute best bang for your buck. It does better than anything else in its price range.