By Kimberley Evans, Huw Wells and Katy Langley

Catadioptrics use a combination of mirrors and lenses to fold the optics and form an image. There are two popular designs: the Schmidt- Cassegrain and the Maksutov-Cassegrain. Catadioptrics are the most popular type of instrument.

Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while cancelling their disadvantages Excellent optics with razor sharp images over a wide field Excellent not only for deep sky observing and astro- photography, but also very good for lunar, planetary and binary star observing Image degrading air currents are reduced due to the closed tube design

Most are extremely compact and portable Easy to use Durable and virtually maintenance free Large apertures at reasonable prices and less expensive than equivalent aperture refractors Most versatile type of telescope More accessories available than with other types of telescopes Best near focus capability of any type of telescope

More expensive than Newtonians of equal aperture Like refractors, the lenses cannot be made much bigger than a metre It is not what people expect a telescope to look like Slight light loss due to secondary mirror obstruction compared to refractors

Lowest cost per inch of aperture compared to refractors and Catadioptrics since mirrors can be produced at less cost than lenses in medium to large apertures Reasonably compact and portable up to focal lengths of 1000mm Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters due to the generally fast focal ratios (f/4 to f/8), yet also reasonably good for lunar and planetary work

Good for deep sky astro-photography (but not as convenient and more difficult to use than Catadioptrics). Low in optical aberrations and deliver very bright images

Open optical tube design allows image-degrading air currents and air contaminants, which over a period of time will degrade the mirror coatings and cause telescope performance to suffer More fragile than Refractors or Catadioptrics and thus require more maintenance Suffer from off-axis coma Large apertures (over 8") are bulky, heavy and tend to be expensive Generally not suited for terrestrial applications. Slight light loss due to secondary (diagonal) obstruction when compared with refractors

Easy to use and reliable due to the simplicity of design Little or no maintenance Excellent for lunar, planetary and binary star observing especially in larger apertures and also good for distant terrestrial viewing Inexpensive in smaller diameters High contrast images with no secondary mirror or diagonal obstruction Sealed optical tube reduces image degrading air currents and protects optics

Since the tube is closed off from the outside, air currents and other effects are eliminated. This means that the images are steadier and sharper than those from a reflector telescope of the same size Objective lens is permanently mounted and aligned Colour correction is good in achromatic designs and excellent in apochromatic, fluorite, and ED designs.

More expensive per inch of aperture than Newtonians or Catadioptrics Heavier, longer and bulkier than equivalent aperture Newtonians and Catadioptrics The cost and bulk factors limit the practical useful maximum size objective to small apertures Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae because of practical aperture limitations Focal ratios are usually long (f/11 or slower) making photography of deep sky objects more difficult

Some colour aberration in achromatic designs (doublet) Poor reputation due to low quality imported toy telescopes It is technically difficult to make a glass lens with no imperfections inside it, with a perfect curvature on both sides of the lens How well the light passes through the objective decreases as the thickness of the lens increases. But at the same time, since the objective can only be supported at the ends, the glass lens might sag under its own weight

All refractors suffer from an effect called chromatic aberration (or colour deviation or distortion) that can produce a rainbow of colours around the image. As light passes through the lens, the longer wavelength corresponding to redder colours is bent less than the shorter wavelength light (bluer colours). There are a couple of ways to reduce chromatic aberration. One way uses multiple lenses to compensate the aberration. The other way is to use the longest possible focal length (distance between the focus and the objective) to minimise the effect. This is why the early refracting telescopes were made very long.

Both reflector and refractor telescopes can suffer from a defect called spherical aberration. In this case, if the mirror is not well curved or the lens is badly shaped, not all light is focussed to the same point. To compensate this problem corrective optics can be used to intercept and correct the light beams from the secondary mirror before they reach the cameras and spectrographs. The images obtained with a telescope that suffers from spherical aberration can also be computer-enhanced to produce sharper images.