Definition
In astronomy, a compound telescope is usually another name for a catadioptric telescope. “Catadioptric” means it uses both mirrors (catoptric elements) and lenses (dioptric elements). Typically a clear corrector plate or lens at the front of the tube works together with a primary and secondary mirror. The light path is folded back on itself, so the tube is much shorter than the focal length would suggest.
The word “compound” is sometimes used more loosely, for example for any instrument with more than one optical element. It is also used in “compound microscope”, which is a different instrument. For backyard astronomy, “compound telescope” almost always points to catadioptrics.
How it works
- Light enters through the corrector plate at the front, which cancels aberrations introduced by the mirrors.
- It travels to the primary mirror at the back of the tube.
- The primary reflects it up to a smaller secondary mirror near the front.
- The secondary reflects it back through a hole in the primary to the eyepiece or camera at the rear.
Folding the path this way gives long focal lengths in a compact package, and it keeps the eyepiece in a comfortable position behind the telescope.
Common types
- Schmidt-Cassegrain (SCT): A thin corrector plate and spherical primary. Widely sold and often paired with computerized mounts. See Schmidt-Cassegrain pros and cons.
- Maksutov-Cassegrain (Mak): A thick curved meniscus corrector. Often has a long focal ratio and a small central obstruction, which many observers like for planets and the Moon. Typically small to medium apertures.
- Other variants: Examples include Schmidt-Newtonians and fast astrographs such as Rowe-Ackermann Schmidt Astrograph (RASA) designs. A RASA is a very fast system intended for wide-field imaging with a camera at the front of the tube rather than an eyepiece, so it is not a visual instrument.
What compound telescopes are used for
- Planets, the Moon and double stars: Long focal lengths give high magnification.
- Smaller deep-sky objects: Planetary nebulae and small galaxies suit the long focal length. A focal reducer gives a wider field.
- Astrophotography: SCTs are common for planetary and small deep-sky imaging. Fast astrographs serve wide-field work.
- Travel and limited storage: The compact tube is a major selling point.
- Terrestrial viewing: Maksutovs in particular are sometimes used as long-range spotting scopes with a correct-image diagonal.
Advantages
- Compact for the focal length.
- Sealed tube with a rear eyepiece.
- Works well with go-to mounts and accessories.
Disadvantages
- Cost more per inch of aperture than Newtonians.
- Cooldown can take a while, especially with thicker glass.
- The corrector plate may dew up.
- Narrower fields of view at typical focal ratios.
How compound telescopes compare
Compared with a refractor vs reflector telescope, a compound design shares traits of both: mirror-based light gathering with a lens corrector. If you are deciding between a compound scope and a large Newtonian, our comparison of Cassegrain vs Dobsonian covers the trade-offs.
Can you use a microscope as a telescope?
Not in a practical sense. Microscopes are designed to magnify very close objects, with short focal lengths, and they are not built to gather light from distant objects. Telescopes use a long-focal-length objective to form an image of distant objects. The two instruments do share eyepiece ideas, which is why the similarity comes up.
Choosing one
Consider how you will use the telescope, how much you can carry, your budget, and what mount you need. If you are still comparing options, see telescope types explained.