How scores work

Every score on Backyard Scopes is worked out from the specifications the maker publishes, compared with every other model listed here. No score comes from a night under the stars, and a missing figure leaves its score empty rather than guessed.

The five scores

Light grasp

Aperture alone, on a logarithmic scale: a 20 mm lens scores 0 and a 160 mm one scores 10. The wider the lens or mirror, the fainter the galaxy it can gather.

Detail

The coarser of two limits: what the aperture can resolve (116 divided by the aperture in millimetres, in arcseconds) and what the sensor can record (two pixels' worth of sky, from the pixel size and the true focal length). A limit of 1.2 arcseconds scores 10 and one of 12 arcseconds scores 0. When the maker leaves out the pixel size or the true focal length, the score stays empty.

Field of view

The longest side of the main camera's frame, from the maker's own figure: half a degree scores 0 and five degrees scores 10. A wide field suits large nebulae and the Milky Way.

Portability

The weight you carry, without the tripod: 0.8 kg scores 10 and 25 kg scores 0, on a logarithmic scale.

Ease

The mean of four parts, each out of 10: battery life (one point per hour, up to 10), autofocus, a dew heater, and light pollution handling (10 for a built-in filter, 7 for processing in software). A part the maker does not state is left out; without a battery figure, or with fewer than two parts known, the score stays empty.

From scores to a verdict

The mean of the scores that have data gives the overall figure out of 10, once at least four of the five can be worked out. The verdict then reads like the seeing on an observing log: Clear night from 6.5, Good seeing from 5.5, Thin cloud from 4.5, and Wait for a clearer night below that. When fewer than four scores can be worked out, there is no verdict at all: a gap in the maker's figures is shown as "not stated by the maker", never filled with a guess.

Binoculars apart

Smart binoculars do a different job from a telescope, so they carry no scores and no verdict, and they are never ranked against the telescopes, on any list. Their pages give the maker's figures and say what the binoculars do.