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Refractor telescopes: how they work and what they suit
A refractor gathers light with an objective lens at the front of the tube and brings it to a focus at the back. It is the design most people picture when they think of a telescope.
How a refractor forms an image
A refractor uses a lens, called the objective, at the sky end of the tube. Light passes straight through the glass and is bent to a focus near the eyepiece end. Because the light path is a sealed straight line with no central obstruction, a refractor delivers high contrast and sharp, steady images, which is why the design is prized for the Moon, planets and double stars.
The trade-off is cost per millimetre of aperture. A lens must be supported only at its edge and made from two or more precisely figured glasses, so refractors become expensive and heavy quickly as aperture grows. Most amateur refractors sit between 60 mm and 120 mm of aperture.
Achromat versus apochromat
A simple lens brings different colours of light to slightly different focus points, leaving a faint colour fringe around bright objects. An achromatic refractor uses two elements to bring two colours together and largely control this. An apochromatic refractor uses three elements or special low-dispersion glass to bring three colours together, cutting the residual fringe to a level most observers never notice.
Achromats give the most aperture for the money and are excellent all-round instruments. Apochromats cost far more but are the choice for demanding lunar and planetary work and for wide-field astrophotography, where clean colour matters most.
What a refractor is best on
A good refractor excels on the Moon, the planets, double stars and bright star clusters, where its high contrast per unit of aperture shows fine detail. A short focal-ratio apochromat also makes an outstanding wide-field imaging scope.
Its weakness is faint deep-sky objects. Distant galaxies and nebulae reward raw aperture, and a refractor gives less aperture per pound than a reflector, so faint fuzzy targets are usually better served by a reflector of the same price.
How it affects your eyepiece choice
A refractor's focal length, divided by an eyepiece's focal length, sets the magnification, exactly as with any telescope. Most refractors use a 1.25 inch focuser, though larger apochromats take 2 inch eyepieces and diagonals for the widest fields. Confirm your focuser barrel size before buying eyepieces.
Get the exact magnification for your scope
Open the Fit My Telescope toolMatch eyepieces to your refractor's focal lengthFind refractor telescope on eBayaffiliateRelated
Sources: Refracting telescope (Wikipedia, CC BY-SA) ยท Telescope Optics, Rutten and van Venrooij; Sky & Telescope optics primers