Telescopes & Mounts
Refractor Telescope
A lens at the front, an eyepiece at the back — the simplest and most robust optical path.
Specifications
| Typical aperture | 60 – 150 mm |
|---|---|
| Typical focal ratio | f/5 to f/11 |
| Optical design | Achromatic doublet or apochromatic triplet |
| Common focuser | 2″ rack-and-pinion or Crayford |
| Tube | Aluminium or carbon fibre |
| Weight (90 mm) | 2.5 – 4.5 kg (OTA only) |
Observing parameters
| Useful magnification range | 0.5× – 2× aperture in mm (e.g. 90 mm → 45×–180×) |
|---|---|
| Practical low power | 15×–40× — wide fields, star clusters |
| Practical high power | 120×–200× — Moon, planets, double stars |
| Typical limiting magnitude | mag 11.5 (90 mm, dark sky) |
| Dawes limit (90 mm) | 1.29 arcsec |
Overview
A refractor uses a large objective lens at the front of the tube to bend incoming light to a focus at the back. That sealed, permanently aligned optical path is the reason refractors are the low-maintenance choice: there is no secondary mirror to knock out of adjustment and no open tube to collect dust.
The classic drawback is chromatic aberration — a lens brings different colours to slightly different focal points, which shows up as a violet halo on bright edges. Achromatic doublets reduce it; apochromatic designs using extra-low-dispersion glass reduce it to the point where only the most critical planetary observers notice it. Apo glass costs disproportionately more per millimetre of aperture.
In practice, a 70–100 mm refractor is a superb first instrument for the Moon, planets and brighter deep-sky objects, and a 4-inch apo is a lifetime telescope that never needs collimation.
Where it fits
Practical notes
- Let the tube reach ambient temperature for 20–30 minutes before high-power work; a warm lens produces a boiling image.
- Use a dew shield on any refractor — the front lens faces the sky and is the first surface to fog.
- On an achromat, a fringe-killer or minus-violet filter noticeably cleans up the violet halo on Jupiter and the Moon.
- Check focuser sag with a heavy eyepiece: a 2″ diagonal plus a wide-field ocular is enough to tilt the image if the focuser is marginal.
Pairs with
- Star Diagonal
- Plössl eyepiece
- Wide-Field Eyepiece
- Red Flashlight
- Dew Heater
Related tools
Reflector Telescope
Mirrors instead of lenses — the most aperture per unit of money, at the cost of collimation.
Telescopes & Mountssetup
Catadioptric Telescope
Lenses and mirrors folded together — long focal length in a short, portable tube.
Telescopes & Mountssetup
Star Diagonal
A mirror or prism that bends the light path 90° so you are not lying on the ground.
Eyepieces & Opticsobserving
Plössl Eyepiece
The symmetric four-element default — clean, cheap, and honest about its limits.
Eyepieces & Opticsobserving