Types & Parts of Teles copes

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.

Refractors
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.
• Colour 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 colour 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 Reflectors
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 degrade 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."

 

C atadioptrics
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 cancelling 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 (requires optional erecting prism) 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.

 

Types of Telescope Mounts

Altazimuth Mountings

The simplest type of telescope mount allows the telescope to be moved up-and-down (in vertical, or altitude) and left-to-right (in horizontal, or azimuth). The altitude-azimuth (altazimuth) mounting thus permits the observer to follow objects by simple motions of the telescope in vertical and horizontal. Slow-motion controls, sometimes operated through flexible cables, can facilitate these motions. The altazimuth mount, owing to its simplicity and relatively lower cost, is widely used with telescopes in both land-viewing and astronomical applications.

 

Equatorial Mountings

Although celestial objects are essentially fixed in their positions in the sky (on the celestial sphere, the imaginary spherical surface on which all astronomical objects are located), they appear to move in an arc across the sky, as the earth rotates underneath the sky once every 24 hours. From an astronomical point of view, therefore, the task of the telescope mounting is to compensate for the Earth’s rotation and allow the observer to track the Moon, planets, and stars. This task is made vastly easier by the equatorial mounting, the type of mounting incorporated into larger or more advanced telescopes. By aligning one axis of the equatorial mount to the Earth’s rotational axis (a simple process which involves pointing one telescope axis to the South Celestial Pole), the observer can track astronomical objects by turning one control cable, instead of the two simultaneous motions required with the altazimuth mount. If a small motor is attached to the equatorial mount, this tracking can be performed automatically. These motor drives are available for most equatorially mounted telescopes.

The German equatorial mount is most commonly encountered form for refractors and Newtonian reflectors. Very often the declination and polar axes will not only be equipped with slow motion controls, but graduated setting circles as well. These graduated circles act like protractors so that you can effectively ‘dial-in’ the co-ordinates of celestial bodies in a manner analogous to looking up the latitude and longitude of a place on the Earth’s surface. The more upmarket equatorials will also be equipped with an electric motor of some description that will drive the telescope about the polar axis keeping your desired object in the field of view at all times.
 

Eyepieces
Choosing eyepieces
Whichever type and size of telescope that you opt for there are a few essentials that you will need, the first of which are eyepieces. Although you can just about get away with two, a much better proposition would be three: a low power, medium and high power. By low magnification I mean about 30x which will give you fields of view in excess of 1 degree (or two Full Moons side by side) which is ideal for looking at comets, star clusters and nebulae. A medium power eyepiece is about 90x which will show you the Moon nicely cradled in the field of view, while a high power ocular will be around the 150x to 200x mark, capable of showing roughly 2/5ths of the Moon’s disc at once.

This range of magnifications — 30x to 200x — is applicable to our ‘average’ 9cm refractor or 11.5cm Newtonian. Larger instruments can use correspondingly higher powers, but there are practical limits imposed by the steadiness of the atmosphere that set the maximum magnification at 300x or so. At this juncture it is instructive to say that magnification is NOT the be all and end all of owning a telescope — ALWAYS use the minimum magnification that shows you crisply defined detail. If — as is commonly the case — the air is very unsteady making the image of the Moon ripple like looking through running water, then no amount of additional magnification is going to improve the view. Choose a lower power and wait for another night of good seeing.

Types of eyepiece
There are two main things to know about your prospective eyepieces: their design and focal length. The designs to look for are Plossls or Orthoscopics, though the slightly cheaper Kellners are a good standby — particularly if you have a typical Newtonian telescope. My personal preference is for the Plossl design, especially when anti-reflection coated. They possess well-corrected, wide fields of view and are well suited to all types of telescopes and observational subject matter. At roughly $69 to $200 each they are not cheap, but most good instruments are supplied with at least one these days. However, when you consider that an eyepiece is just as important a link in the optical chain as the objective mirror or lens, then the cost is put into perspective.

Calculating your eyepiece’s magnification
Upon close inspection of the eyepiece you will see a number engraved or printed on the barrel close to the name — this is its focal length and is of great importance when calculating the power it will deliver with your telescope. Quite simply, the magnification may be found by dividing the focal length of the telescope by this number. For example, a 9cm refractor with a focal length of 1000mm (100cm) will give 40x when used with an eyepiece of 25mm focal length. It follows that eyepieces of shorter focal length give higher magnifications.

It is also worth mentioning here that most modern eyepieces are supplied with a push-fit chromed barrel 31.75mm (1.25") in diameter and are threaded internally to accept coloured glass filters to enhance the view of certain objects. It is wise to buy a telescope with a focuser of such a size, though adapters can be obtained for second-hand instruments. Older telescopes of smaller aperture tended to have 24.5mm (0.965") push-fit barrels, while for larger instruments you will see some 50.8mm (2.0") push-fits for spectacular low power, wide angle views and astrophotographic use.

Use the Correct Magnification

There are two common magnification myths in astronomy.  The first is that more magnification is better.  The assumption of many beginners is that to see things farther away you need more power.  But the trick isn't seeing farther, it's seeing fainter.  And for this, aperture is the key more than magnification.  In fact, lower power produces a brighter image and almost always produces a better view than high power.  However, this leads to the second myth, an assumption made by more experienced stargazers:  that lower power is always better.  The truth is, there is an ideal magnification depending on the telescope used, the object being viewed, and the observer's visual acuity.  In general, a medium magnification produces the best view.

Low power is great for finding objects and for viewing very large objects such as the Pleiades or Andromeda Galaxy.  High magnification can be good for small targets like planets or double stars, but most observers quickly learn that there is definitely such a thing as too much magnification.  For most deep-sky objects, though, medium power is ideal for bringing out the most detail.  We'll spare you the details here, but suffice to say that it has to do primarily with the resolving power of the human eye.  In general, use the following guidelines for magnification.

Lowest Power	4x per inch of aperture
Best Resolution	12x per inch of aperture
Highest Power	30x per inch of aperture

For example, using an 8" telescope, the minimum magnification would be around 32x, the best resolution for deep sky objects would occur at around 96x, and the highest useable magnification would be around 240x.