The purpose of a telescope is not to magnify, as commonly thought, but to collect light. The larger the telescope’s main light-collecting element, whether lens or mirror, the more light is collected. Importantly, it is the total amount of light collected that ultimately determines the level of detail, in a distant landscape or in the rings of Saturn, visible through the telescope. Although magnification, or power, is useful, it has no inherent effect whatever in determining the level of detail visible through a telescope.
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The magnification, or power, at which a telescope is operating is a function of the focal length of the telescope’s main (objective) lens (or primary mirror) and the focal length of the eyepiece employed.
A telescope which has an objective lens focal length of 600mm; when this telescope is used with a 25mm eyepiece, has a power or magnification of 600/25 = 24 x .
The focal length of the objective lens is given as a specification of the telescope.
The focal length of each eyepiece (which typically ranges from 4mm to about 40mm) is printed on the upper surface of the eyepiece.
To calculate power or magnification, divide the focal length of the eyepiece into the focal length of the objective lens.
A Word about Power:
When buying a telescope one of the least important factors to consider is the power, or magnification, of the instrument.
The key to observing fine detail, whether on the surface of the Moon or on a license plate one mile in the distance, is not power, but APERTURE - i.e., the diameter of the telescope's main (objective) lens or primary mirror.
The power at which a telescope is operating is determined by the eyepiece employed.
Most telescopes include one or more eyepieces as standard equipment, and optional eyepieces are available for higher or lower powers. Within reason power is useful,[but the most common mistake of the beginning observer is to OVERPOWER the telescope and to use magnifications which the telescope’s aperture and typical atmospheric conditions can not reasonably support.
The result is an image which is fuzzy, ill-defined, and poorly resolved, through no fault of the telescope. Keep in mind that a smaller, lower-power, but brighter and well-resolved, image is far superior to a large, high-power, but dim and poorly resolved, one.
Suggested ideal magnifications for a popular range of uses are:
30x Observing the Moon
36x Andromeda Gallery
60x Orion Nebulae
Types of Telescopes
There are three basic types of telescopes:
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.
Refractors (also known as dioptrics) are what the public image is of a “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
- 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.
- More expensive per inch of aperture than Newtonians or Catadioptrics.
- Heavier, longer and bulkier than equivalent aperture reflectors.
- Less suited for viewing small and faint deep sky objects because of practical aperture limitations.
- Not that good for astrophotography of deep sky objects more difficult.
- Some color aberration in achromatic designs.
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.
- Lowest cost per inch of aperture
- Reasonably compact and portable
- Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters.
- Reasonably good for lunar and planetary work
- Good for deep sky
- Low in optical aberrations and provides very bright images
- Open tube allows image degrading air currents.
- Mirror coatings can degrade images
- More fragile than refractors
- More maintenance and *collimation required
- Large apertures are bulk and heavy
- Generally not suited for terrestrial applications
- Slight light loss due to secondary (diagonal) mirror when compared with refractors
*This refers to how correctly the optics are pointing towards each other. If a telescope is out of collimation, you will not get as clear an image as you should.
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.
- 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 and for lunar, planetary and binary star observing or photography
- Extremely compact and portable
- Easy to use
- Excellent for terrestrial viewing or photography
- Durable and virtually maintenance free
- Closed tube design reduces image degrading air currents
- Large apertures at reasonable prices and less expensive than equivalent aperture refractors
- More accessories available than with other types of telescopes
- Best near focus capability of any type telescope
- More expensive than Newtonians of equal aperture.
- Slight light loss due to secondary mirror obstruction compared to refractors
- Short and stubby it is not what people expect a telescope to look like
Types of telescope mountings
A telescope mount has two functions:
provide a system for smooth controlled movement to point and guide the instrument
support the telescope firmly so that you can view and photograph objects without having the image disturbed by movement.
- Tracking is made with two motions, altitude (up and down/vertical) and azimuth (side-to-side/horizontal) Even if it is moved anywhere it can be set up quickly
- Good Alt-Azimuth mounts will have slow-motion knobs to make precise adjustments, which aid in keeping tracking motion smooth
- Good for terrestrial observing and for scanning the sky at lower power but are not for deep sky photography
- Certain Alt-Azimuth mounts are now computer driven and allow a telescope to track the sky accurately enough for visual use but not for long exposure photography
- Designed to track an object in accordance with the diurnal motion
- Absolutely necessary for astrophotography.
- As the earth rotates around its axis, the stationary stars appear to move across the sky.
- If you are observing them using an Alt-Azimuth mount, they will quickly float out of view in both axes. A telescope on an equatorial mount can be aimed at a celestial object and easily guided either by manual slow-motion controls or by an electric clock drive to follow the object easily across the sky and keep it in the view of the telescope.
- By pointing the R.A. (Right Ascension) axis to a celestial pole (in northern latitudes that will be roughly at the star Polaris), you can follow the star by slowly turning the scope along this axis in the opposite direction of the earth’s rotation. For this purpose, most of these mounts are fitted with so called slow motion controls in order to allow you to slowly turn the axes in order to keep the object in view. The DEC (=declination) axis allows you to change the height of the scope above the celestial equator (see also using telescopes).
- By attaching a small motor to the R.A. axis, and if the mount is adequately lined up with the earth’s axis, you will effectively be able to hold the image exactly in place, which does not only make it easier to view at larger magnification, it also allows you to use the scope to make long exposure photographs.
While the stars, Moon, planets and other celestial bodies do not appear to move, in fact all such objects are in constant apparent motion due to the earth’s rotation on its axis once every 24 hours.
The effect of the telescope’s magnification is to speed up this apparent motion, to the point where, without operation of the telescope’s tracking controls, objects, quickly (within 10-30 seconds) move out of the telescopic field of view.
Electric motor drives may be added to most equatorial models resulting in completely automatic tracking of astronomical objects.
A clock drive is essential to do astrophotography. The electrical system can be AC or DC depending on the type of system used. The gearing system is usually a spur gear or a worm gear type.
Depending on how much you intend to use your telescope and for what applications, you may wish to add two or three eyepieces and/or multiplying Barlow lens to the eyepiece(s) included as standard.
On any telescope, eyepieces of 25mm or 40mm focal length are best suited for extended star fields, or nebulosites (such as the Orion nebula) or for terrestrial applications.
These eyepieces result in low-powers, wide fields and bright images.
On the Moon and planets, eyepieces between 4mm (high power) and 18mm (moderate power) are advised, although the highest powers should be employed only under favourable atmospheric conditions.