Types of telescope

There are three basic types of telescopes -- Refractors, Newtonian reflectors, and Catadioptrics

1.       Refractors

The refractor is the oldest telescope design.  In its basic form, as Galileo used it 400 years ago, it is simply a curved lens which focuses light to a point.  The aberration introduced by using a single lens can be largely eliminated by using a design incorporating two or more lens elements.  This is the design used in modern refractors.

Refracting telescopes use lenses to gather light and focus it to the eyepiece.  The objective lens is located at the front of the telescope and the eyepiece at the rear.

There are two styles of objective lens:

Achromatic refractors use two lens elements to help minimize chromatic aberration, an optical effect which causes differing wavelengths of light to focus at different points. 

Apochromatic refractors (often called "apos") use three or more lens elements, one or more having special properties, to eliminate chromatic aberration entirely.

Advantages

• 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 or diagonal 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.

Disadvantages

• 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 color aberration in achromatic designs (doublet).

• Poor reputation due to low quality imported toy telescopes; a reputation unjustified when dealing with a quality refractor from a reputable manufacturer.
  

2.       Newtonian reflectors

Newtonian telescopes use a curved mirror to focus incoming light to a second, flat mirror which directs the light to a convenient viewing position on the side of the telescope.

Most Newtonian Telescopes have been supplied on equatorial mounts. The last few years have seen a new commercial telescope available on the market: Dobsonians

Dobsonian telescopes are simply Newtonian optical designs on an ingenious type of mount.  The telescope is held in place on the mount by friction.  If the amount of friction is too much the telescope is difficult to move, if it the friction is too low, the telescope will not stay where it is pointed.  This makes the Dobsonian a difficult telescope to make properly. Dobsonians (or Dobs) are popular among amateur tele- scope makers for their simplicity.  The simple design also makes these telescopes very popular for beginners.  A Dob is also the easiest and cheapest way to make a telescope with a large aperture.  Thus the Dobsonian is perfect for the advanced amateur stricken with a case of "aperture fever"!

Advantages

• 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).

• Reasonably good for lunar and planetary work.

• Good for deep sky astrophotography (but not as convenient and more difficult to use than Catadioptrics).

• Low in optical aberrations and deliver very bright images.

Disadvantages

• Generally not suited for terrestrial applications.

• Slight light loss due to secondary (diagonal) obstruction when compared with refractors.
  

3.       Catadioptrics

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. In the Schmidt-Cassegrain the light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube and intercepted by a small secondary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece. Catadioptrics are the most popular type of instrument, with the most modern design, marketed throughout the world in 3 1/2" and larger apertures.

Maksutov-Cassegrain

The Maksutov design is a catadioptric (using both mirrors and lens) design with basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt's giving it slightly better resolution for planetary observing.

The Maksutov is heavier than the Schmidt and because of the thick correcting lens takes a long time to reach thermal stability at night in larger apertures (over 90mm).

The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt-Cassegrain

Schmidt-Cassegrain Disadvantages

• Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while canceling their disadvantages.

• Excellent optics with razor sharp images over a wide field.

• Excellent for deep sky observing or astrophotography with fast films or CCD’s.

• Very good for lunar, planetary and binary star observing or photography.

• Excellent for terrestrial viewing or photography.

• Focal ratio generally around f/10. Useful for all types of photography. Avoid faster f/ratio telescopes (they yield lower contrast and increase aberrations). For faster astrophotography, use a Reducer/Corrector lens.

• Closed tube design reduces image degrading air currents.

• 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 telescope.

Schmidt-Cassegrain Disadvantages

• More expensive than Newtonians of equal aperture.

• It is not what people expect a telescope to look like.

• Slight light loss due to secondary mirror obstruction compared to refractors.
  

Maksutov-Cassegrain

The Maksutov design is a catadioptric (using both mirrors and lens) design with basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt's giving it slightly better resolution for planetary observing.

The Maksutov is heavier than the Schmidt and because of the thick correcting lens takes a long time to reach thermal stability at night in larger apertures (over 90mm).

The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt-Cassegrain