What are the types of optical systems used for Celestron telescopes?

Here are brief descriptions of the eight major types:

Refractors
• Optical system: 2-element achromatic objective lens
• Aberrations: refractors have some color aberration especially at small f/ratios
• Advantages: easy to use, low maintenance, closed tube, no obstruction
• Disadvantages: chromatic aberration, available in small sizes up to 150 mm aperture only

ED APO Refractors
• Optical system: 2-element low-dispersion ED objective lens
• Aberrations: very little chromatic aberration
• Advantages: easy to use, low maintenance, closed tube, no obstruction
• Disadvantages: available in small sizes up to 120 mm aperture only, relative high cost

Newtonian Reflector
• Optical system: spherical or parabolic primary mirror, flat elliptical-shaped secondary mirror
• Aberrations: coma away from central field of view especially at small f/ratios
• Advantages: relative low cost
• Disadvantages: larger sizes bulky, central obstruction by secondary and support, unsuited for terrestrial viewing, back focus issues

Dobsonian Reflector
• Optical system: parabolic primary mirror, flat elliptical-shaped secondary mirror
• Aberrations: coma away from central field of view especially at small f/ratios
• Advantages: relative low-cost
• Disadvantages: larger sizes bulky, central obstruction by secondary and support, unsuited for terrestrial viewing, back focus issues

Barlowed Newtonian Reflector

• Optical system: as with Newtonian, except Barlow lens added in eyepiece drawtube
• Aberrations: coma away from central field of view especially at small f/ratios
• Advantages: relative low-cost, short tube for long effective focal length
• Disadvantages: central obstruction by secondary and support, unsuited for terrestrial viewing, back focus issues

Maksutov-Cassegrain Catadioptric (MAK)

• Optical system: spherical primary, spherical secondary spot on spherical meniscus corrector
• Aberrations: higher-order spherical aberrations in shorter f/ratios
• Advantages: maintenance free, short closed tube for long effective focal length, relative low-cost
• Disadvantages: only available in sizes up to about 130 mm and high f/ratios, central obstruction

Schmidt-Cassegrain Catadioptric (SCT)

• Optical system: spherical primary, spherical secondary, aspherical corrector plate
• Aberrations: some coma and field curvature in outer field
• Advantages: short closed tube for long effective focal length, versatile set-ups
• Disadvantages: central obstruction, relative high cost, relative high f/ratio

EdgeHD SCT
• Optical system: spherical primary, spherical secondary spot, aspherical corrector plate, two-element lens internal corrector
• Aberrations: essentially none
• Advantages: short tube for long effective focal length, flat coma-free field, versatile set ups
• Disadvantages: central obstruction, relative high cost, relative high f/ratio