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How to work out your telescope's specs

Two numbers describe the optics of any telescope, its aperture and its focal length, and from those you can derive everything else you need to choose eyepieces and accessories.

Aperture

Aperture is the diameter of the main lens or mirror, given in millimetres or inches. It sets how much light the telescope gathers and its maximum useful magnification. A telescope named 200P or C8 is telling you its aperture: 200 millimetres and 8 inches, which is about 203 millimetres, respectively.

Aperture is often the first number in a model name, so reading the model is usually enough to recover it. A larger aperture gathers more light and resolves finer detail.

Focal length and focal ratio

Focal length is the distance over which the optics bring light to focus, in millimetres, and it sets magnification with any eyepiece. Focal ratio is focal length divided by aperture, written as an f-number. A 200 millimetre telescope with a 1000 millimetre focal length is f/5, because 1000 divided by 200 is 5.

If the model gives aperture and focal ratio, multiply them to get focal length. If it gives aperture and focal length, divide to get focal ratio. Either pair recovers the third number.

Where the numbers are written

The specs are usually printed near the front of the tube, on the focuser, or in the model designation itself. Celestron NexStar SE numbers, Sky-Watcher and Orion model names all encode the aperture, and the focal length or focal ratio is on the tube or in the manual.

Once you have aperture, focal length and focal ratio, you can compute magnification for any eyepiece, the exit pupil at each power, and the maximum useful magnification for the scope.

Turning specs into choices

With the three numbers in hand, eyepiece selection follows directly. Divide the focal length by an eyepiece focal length for magnification, and divide aperture by that magnification for exit pupil. Aim for eyepieces that keep the exit pupil in a comfortable range and stay under the aperture-set magnification ceiling, and match eyepiece quality to the focal ratio.

Enter your telescope and see what fits

Open the Fit My Telescope gear finderPick your telescope to see its specs and matching eyepieces.Find telescope eyepiece and filter kit on eBayaffiliate

Focal length and magnificationUse your specs Exit pupil and focal ratioMatch to your f-number Eyepiece barrel sizesWhat your focuser takes Celestron AstroMaster 130EQSpec-labelled example

Sources: Celestron telescope specification documentation · Sky-Watcher telescope model reference