Celestron C11-SGT User Manual download pdf (Page 40)

38 Mount and Telescope Descriptions

refraCtor advantages

Easy to set-up and use

Simple and reliable design requires little or no maintenance

Excellent for lunar, planetary and binary star observing especially in larger apertures

Good for terrestrial viewing

High contrast images with no secondary mirror or diagonal obstruction

Color correction is good in achromatic designs and excellent in apochromatic and fluorite designs

Sealed optical tube reduces image-degrading air currents and protects optics

Objective lens is permanently mounted and aligned

refraCtor dIsadvantages

More expensive per inch of aperture than Newtonians or Catadioptrics

Heavier, longer and bulkier than equivalent aperture Newtonians and Catadioptrics

The cost and size factors limit the practical maximum size primary to smaller apertures

Some color aberration in achromatic designs (doublet)

newtonIan refleCtor telesCope

A Newtonian reflector uses a single concave mirror as its

primary. Light enters the tube traveling to the mirror at

the back end. Light is then “bent” forward in the tube to a

single point, its focal plane. A flat mirror called a “diagonal”

intercepts the light and points it out the side of the tube at

right angles to the tube through the eyepiece. The eyepiece is

placed there for easy viewing.

Newtonian Reflector telescopes replace heavy lenses with

mirrors to collect and focus the light, providing much more

light gathering power for the money. You can have focal lengths up to 1000 mm and still enjoy a telescope

that is relatively compact and portable. Newtonian Reflector telescopes do require more care and

maintenance because the primary mirror is exposed to air and dust. However, this small drawback does

not hamper this type of telescope’s popularity with those who want an economical telescope that can still

resolve faint, distant objects.

Newtonian reflectors produce a “right-side-up image” but the image will appear rotated based on the

location of the eyepiece holder in relation to the ground. Newtonian reflectors are best for astronomical

use where right-side-up does not matter.

newtonIan 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 1000 mm

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)

Adequate for lunar and planetary work

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

Free of color aberration due to the use of a primary mirror

newtonIan dIsadvantages

Generally not suited for terrestrial applications

Slight light loss due to secondary (diagonal) obstruction when compared with Refractors

CatadIoptrIC telesCope

Catadioptrics use a combination of mirrors and lenses

to “fold” (reflect) the light path and form an image. In a

Schmidt-Cassegrain, the light enters through a thin aspheric

Schmidt correcting lens. It then strikes the spherical primary

mirror. It 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 and most modern type of telescope optical design and are marketed throughout the

world in 3.5” and larger apertures.

Catadioptric telescopes combine the practical advantages of lenses and mirrors while eliminating their

disadvantages. They offer the clarity and contrast of refractors with the low aberration of reflectors. Catadioptrics

have an average focal ratio of f/10, which is wide enough for all types of photography. They are also easier

to maintain because all optical elements are solidly mounted and rigidly collimated. Catadioptric telescopes

provide the best possible combination of light gathering power, long focal length, portability and affordability.

sChmIdt-CassegraIn advantages

Very versatile, best all-purpose telescope design

Combines the optical advantages of both lenses and mirrors while eliminating their disadvantages

Excellent optics and razor sharp images over a wide field

Excellent for deep sky observing and astrophotography as well as terrestrial viewing

Very good for lunar, planetary and binary star observing

Focal ratio generally around f/10, it also has the best near focus capability of any type of telescope

Closed tube design reduces image-degrading air currents

Extremely compact and portable

Easy to use, durable and virtually maintenance free

Large apertures at reasonable cost and less expensive than equivalent aperture refractors

More accessories available than with other types of telescopes

sChmIdt-CassegraIn dIsadvantages

More expensive than Newtonians of equal aperture

Slight light loss due to secondary mirror obstruction

compared to refractors

The Maksutov-Cassegrain is similar to the Schmidt-Cassegrain

with essentially the same advantages and disadvantages.

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 which gives it slightly better resolution for planetary observing.

advantages of maksutov-CassegraIn Compared to sChmIdt-CassegraIn

Smaller secondary obstruction results in a slight increase in planetary detail and contrast

Less expensive to manufacture

Longer focal lengths resulting in higher magnifications for planetary viewing

dIsadvantages of maksutov-CassegraIn Compared to sChmIdt-CassegraIn

Slightly heavier because of the thick meniscus correcting lens

Increased time to reach thermal stability in larger apertures over 90 mm

Longer focal lengths resulting in smaller field of views

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Celestron C11-SGT User Manual Page 40

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