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TYPES OF TELESCOPES

There are three basic types of telescopes -- Refractors, Newtonian reflectors, and Catadioptrics. All these designs have the same purpose, to collect light and bring it to a point of focus so it can be magnified and examined with an eyepiece, but each design does it differently. All designs can perform satisfactorily if properly and responsibly manufactured and all have their own special virtues.

Choosing a particular telescope depends on your individual needs including cost, portability, versatility, usability, appearance, etc. You should also contemplate what you plan to do with the instrument both now and in the future. Many amateurs own two or more telescopes to satisfy their varied interests.

Some amateur astronomers build their own telescopes but this market has rapidly declined due to the abundance of affordable commercial telescopes available and the time, materials and equipment needed to hand-construct an instrument.

We will briefly discuss the most popular types of telescopes and describe advantages and disadvantages of each.

REFRACTOR

Refractors (also known as dioptrics) are what the average person identifies with the word "telescope", a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube.

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.

NEWTONIAN REFLECTOR

Newtonians (also known as catoptrics) usually use a concave parabolic primary mirror to collect and focus incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image out of an opening at the side of the main tube and into the eyepiece.

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

• Open optical tube design allows image-degrading air currents and air contaminants, which over a period of time will dergrade the mirror coatings and cause telescope performance to suffer.
• More fragile than Refractors or Catadioptrics and thus require more maintenance (such as collimation).
• 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.

DOBSONIAN TELESCOPES

Most Newtonian Telescopes have been supplied on equatorial mounts. The last few years have seen a new commercial telescope available on the market - the Dobsonian. A Dobsonian is a simple altazimuth mounted Newtonian telescope which is excellent for beginners and in large sizes is an economical "Light Bucket."

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.

Schmidt-Cassegrain Advantages

• 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.