Newtonians 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.

Newtonian reflectors are great all-around scopes, offering generous apertures at affordable prices. They work for both planetary and deep-sky viewing. These scopes are more fragile and require more maintenance than the others. An open optical tube design allows image-degrading air currents and air contaminants, which over time will degrade the mirror coatings and decrease performance. The optical components can go out of alignment, requiring collimating. Of course, the larger the aperture, the more you'll see. Smaller 3" and 4.5" equatorially mounted Newtonians will provide a nice survey of celestial luminaries, and they are easily portable. 6" and 8" Newts have enough aperture to deliver captivating images of fainter fare—clusters, galaxies and nebulas, especially in a reasonably dark sky. The trade off is their bulk and weight—something you should definitely take into consideration. But a 6" Newtonian on a Dobsonian mount is easily manageable by one person and makes a great starter scope. Dobsonian-mounted reflectors are lower in price than their equatorial counterparts and start around $350.

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

REFLECTOR REPORT CARDS
(used in excellent seeing conditions and with no light pollution; adapted from Astronomy Magazine):

E = excellent; VG = very good; G = good; F = fair; P = poor.

Small aperture (3" to 6") reflectors:
Price range: $120-$1000
Portability: E
Ease of setup: VG
Ease of use: VG+
Performance on the Moon: E
Performance on comets: F
Performance on double stars: VG
Performance on galaxies and nebulas: F
Performance on planets: VG

Medium aperture (8" to 12.5") reflectors:
Price range: $400-$14,000
Portability: F
Ease of setup: F
Ease of use: VG+
Performance on the Moon: E
Performance on comets: VG
Performance on double stars: VG
Performance on galaxies and nebulas: VG
Performance on planets: VG

Large aperture (13" and larger) reflectors:
Price range: $1200-$36,000
Portability: F
Ease of setup: P+
Ease of use: G+
Performance on the Moon: VG
Performance on comets: E
Performance on double stars: E
Performance on galaxies and nebulas: E
Performance on planets: E