A sealed tube, a lens at the front, and colour to account for
A refractor gathers light through a lens at the front of a sealed tube, and that single design choice decides almost everything else about it. The tube needs no collimation and holds its alignment through years of being carried in and out of a car, which is why a refractor is the instrument most often still working ten years after it was bought. The cost of the lens is chromatic aberration: glass bends blue light more sharply than red, so a single lens cannot bring every colour to the same point. Makers answer that in two steps. An achromat pairs two glasses and leaves a violet fringe around bright objects, which grows more visible as the focal ratio gets shorter — an f/5 achromat shows it plainly on the Moon, an f/11 achromat barely at all. An apochromat adds a third element or a low-dispersion glass and suppresses the fringe at a price that rises faster than the aperture. Beyond the optics, read the focuser: a rack-and-pinion focuser under a heavy camera behaves differently from a Crayford, and the drawtube travel decides whether a diagonal and a camera can both reach focus. Then the practical part — a long refractor is a long lever, and the mount it asks for often costs more than the tube. We compare aperture, focal length and ratio, lens design and glass type, focuser and drawtube, tube weight and supplied accessories.
70 mm aperture · 400 mm focal length in the title
A 70/400 refractor sold with two eyepieces, a 5x Barlow, a phone adapter and a stainless-steel tripod — and a listing whose own bullets contradict its title.
Best for: A buyer who will check the tube’s printed figures before ordering anything else for it
≈ £79 / €70
70 mm aperture · 400 mm focal length
A 70/400 refractor sold as a first instrument for children from eight, supplied in a backpack with three eyepieces, a star diagonal and an erecting lens.
Best for: A child from about eight who will carry the telescope themselves
≈ £78 / €63
80 mm aperture · 600 mm focal length
An 80/600 refractor on an alt-azimuth head, supplied with two eyepieces, a 5x24 finder, a phone adapter and a backpack.
Best for: A first 80 mm that has to travel in a rucksack
≈ $100 / £70 / €130
70 mm aperture · 700 mm focal length
An achromatic 70/700 refractor on an alt-azimuth mount with an aluminium tripod, sold as a first adult instrument.
Best for: An adult or older teenager starting with the Moon and planets
≈ £94 / €90
80 mm aperture · 900 mm focal length
An 80/900 refractor on an alt-azimuth head with setting scales, supplied with two wide-angle eyepieces, an erecting diagonal, a phone adapter and a carry bag.
Best for: Somebody who wants the longest focal length in the entry class for lunar and planetary work
≈ $70
70 mm aperture · focal length stated two ways
A 70 mm refractor on an alt-azimuth mount, supplied with two eyepieces, a 2x Barlow, a 6x24 finder and a star diagonal.
Best for: A first refractor where the eyepiece fitting matters more than the badge
≈ €171
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
