Refractor is a long, thin telescope. Light passes through the refractor in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. Typical focal ratio is f/5 to f/15.
Advantages:
Refractors are the traditional optical design that the average person instantly visualizes when he or she hears the word "telescope"
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
Good for distant terrestrial viewing
High contrast images with no secondary mirror obstruction
Color correction is good in achromatic designs and excellent in apochromatic, fluorite, and ED designs
Sealed optical tube reduces image degrading air currents and protects optics
Objective lens is permanently mounted and aligned—fixed collimation
Disadvantages:
More expensive per inch of aperture than Newtonians or catadioptrics
Heavier, longer, and bulkier than Newtonians and catadioptrics of equivalent aperture
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 for traditional refractors are usually long (f/11 or slower), making photography of deep sky objects more difficult (short focal length apochromats are now available that change this limitation, but their magnification is limited)
Some color aberration in achromatic designs (doublet)
Newtonian Reflectors, including the Dobsonian (Dob), have a concave spherical or parabolic primary mirror at the back of the telescope that collects and focuses incoming light onto a flat secondary (diagonal) mirror. In turn, the secondary mirror reflects the image out of an opening at the side of the front end of the main tube and into the eyepiece. Typical focal ratio is f/4 to f/8.
Advantages:
Because mirrors are less expensive to produce than lenses, Newtonian telescopes are usually less expensive for any given objective diameter (or aperture) than comparable refractor telescopes
Reasonably compact and portable up to focal lengths of 1000mm.
Short focal ratio (f/4 to f/8) offers a wider field of view
The eyepiece is located at the top end of the telescope allowing for a much more compact mounting system, reducing cost and adding portability
Low in optical chromatic aberrations
Generally very little issue with dew except in very high humidity
When used with a Dobsonian mount, reflectors are very simple to operate; targets can be found quickly by star hopping
Reasonably good for lunar and planetary work, but larger apertures are better for deep sky objects
Disadvantages:
When visual observing with large Dobs, the tube orientation can sometimes put the eyepiece in a poor viewing position, requiring a ladder to access it
The location of a Newtonian’s secondary mirror and struts (the support structure for the mirror) creates a central obstruction and causes the appearance of diffraction spikes
Newtonians with a focal ratio of f/6 or lower suffer from coma, an off-axis aberration that causes images to flare inward and towards the optical axis (stars take on a "comet-like" shape)
Generally not suited for terrestrial observing due to visual images being inverted (upside down and backwards)
Very bulky and heavy in 10" and larger sizes
Fast Netwonians (f/6 and lower) need optics to be collimated regularly
Additional coma corrector optics are needed for best performance for deep sky photography
Planetary imaging may require very high power 3x-5x Barlows to obtain to suitable focal length for many cameras
Dobs and other alt-az mounted Newtonians are more difficult to motorize and many may not track at all
With larger Dobs, the base itself can be heavy and bulky
Maksutov-Cassegrain (Mak) optical system uses a combination of mirrors and lenses and is referred to as a compound or catadioptric telescope. This unique design offers large-diameter optics while maintaining very short tube lengths, making them extremely portable. The Maksutov-Cassegrain system consists of a corrector lens, a spherical primary mirror, and a secondary mirror spot. Once light enters the optical system, it travels the length of the optical tube three times. Typical focal ratio is f/13 to f/15.
Advantages:
Instead of a secondary mirror, most Maks features a “secondary spot” of reflective coating applied directly to the inside of the corrector plate, limiting the system's complexity
Does not require frequent collimation as the optics are factory aligned
Very compact for such a long focal ratio design
Excellent for high-resolution lunar and planetary observation and photography, especially in larger sizes
Moderate costs per inch of aperture compared to other optical designs
Usually a smaller secondary obstruction compared to Newtonians and SCT-variants, meaning higher-contrast views than those designs
Disadvantages:
The larger, heavier Maksutov corrector lens takes much longer to cool down and adjust to ambient air temperature
Like SCTs, mirror flop can sometimes be an issue when reversing focus direction and during long imaging sequences
There are generally no field flatteners or focal reducer/correctors optomized for Maksutovs, meaning that it can be difficult to correct for all aberrations with large camera sensors
With such a slow focal ratio, very long exposures are needed for deep sky photography, and Fields of view are narrow, making it more difficult to image large and/or faint targets
Dew can accumulate on the Maksutov corrector plate and may need to be managed with heaters or dew shields in humid locations
Schmidt-Cassegrain
(SCT) optical system uses a combination of mirrors and lenses and is referred to as a compound or catadioptric telescope. This unique design offers large-diameter optics while maintaining very short tube lengths, making themextremely portable. The Schmidt-Cassegrain system consists of a zero power corrector plate, a spherical primary mirror, and a secondary mirror. Typical focal ratio is f/10, f/2 if using Fastar capability.
Advantages:
Best all-around, all-purpose telescope design—combines the optical advantages of both lenses and mirrors while mitigating their disadvantages
Excellent optics with razor sharp images over a moderately-wide field
Excellent for deep sky observing or astrophotography with film, DSLR, CCD or CMOS cameras
Very good for lunar, planetary and binary star observing or photography due to long native focal lengths and high resolution
Most are extremely compact and portable for their aperture
Durable and virtually maintenance-free except for collimation, which is fairly simple and needs no special tools besides a high power eyepiece
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
C5 optical tube has superior near-focus capability compared to other types of telescope (approximately 20 ft or 6 m)
6" to 14" optical tubes are Fastar compatible
Disadvantages:
More expensive than Newtonians of equal aperture but far less exensive than refractors of similar size
Due to secondary mirror obstruction, significant contrast compared to refractors
Mirror flop can be an issue when changing direction with the focuser knob or during long imaging sequences
Coma and field curvature present in optics are not usually noticeable in visual observing but show up in imaging with larger sensors
Unlike large APO refractors or EdgeHD scopes, SCTs cannot correct a full-frame-size DSLR or CMOS sensors (43mm diagonal) and are better for APS-C (28mm diagonal) and smaller sensors
Subject to dew accumulation on corrector plate, often requiring dew shields or heaters in more humid areas
Larger sizes may need to have collimation checked after transporting the telescope by car to an observing site
(credit: Celestron)
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