Telescopes & Mounts
Catadioptric Telescope
Lenses and mirrors folded together — long focal length in a short, portable tube.
Specifications
| Typical aperture | 90 – 356 mm |
|---|---|
| Typical focal ratio | f/10 (f/6.3 with reducer) |
| Optical design | Schmidt- or Maksutov-Cassegrain |
| Central obstruction | 30 – 38% of aperture diameter |
| Focus mechanism | Moving primary mirror |
| Weight (8″ SCT) | 5.5 – 6.5 kg (OTA only) |
Observing parameters
| Useful magnification range | 0.5× – 2× aperture in mm (203 mm → 100×–400×) |
|---|---|
| Native true field (f/10) | ≈ 0.5° with a 25 mm Plössl |
| Reduced field (f/6.3) | ≈ 0.8° with the same eyepiece |
| Typical limiting magnitude | mag 13.4 (203 mm, dark sky) |
| Dawes limit (203 mm) | 0.57 arcsec |
Overview
A catadioptric — most commonly a Schmidt-Cassegrain (SCT) or Maksutov-Cassegrain (Mak) — folds the light path with a corrector plate at the front and mirrors at the back, so a 2000 mm focal length fits in a tube half a metre long. That compactness is the whole point: a lot of reach in something you can lift one-handed.
The cost is a large central obstruction and a narrow native field, and the long focal ratio (typically f/10) means narrow true fields and slower imaging. A focal reducer recovers the field for deep-sky work; for planetary imaging the native f/10 plus a Barlow is often what you want.
These are the default choice for planetary imaging and for observers with limited storage, and the fork-mounted variants are the most portable large-aperture instruments available.
Where it fits
Practical notes
- Mirror flop is real: focusing by moving the primary shifts the image slightly. Always approach final focus from the same direction.
- Budget for a dew shield or heater — the corrector plate is a horizontal cold surface facing the sky.
- A 0.63× reducer turns an f/10 SCT into a genuinely useful deep-sky instrument and speeds imaging up by 2.5×.
- Allow a long thermal settle time; the closed tube cools slowly and a warm SCT shows soft, mushy planets.
Pairs with
Related tools
Reflector Telescope
Mirrors instead of lenses — the most aperture per unit of money, at the cost of collimation.
Telescopes & Mountssetup
Refractor Telescope
A lens at the front, an eyepiece at the back — the simplest and most robust optical path.
Telescopes & Mountssetup
Focal Reducer
Shortens the effective focal length: wider field, faster optics, shorter exposures.
Eyepieces & Opticsobserving
Dew Heater
A resistive band that keeps optics a couple of degrees above the dew point.
Imaging & Astrophotographysetup