Telescope Terminology

Collimation:

This refers to the alignment of the optic mirrors in a Telescope. Collimation brings collected light into perfect focus. All reflector Telescopes, whether Newtonian or Dobsonian, work by collecting light with a primary mirror at one end of the telescope tube.

If a telescope is not properly aligned, then Starlight will be out of focus.

Refractor telescopes are permanently collimated at the factory, therefore should never require collimation.

Air Spaced:

Refers to lens assembly spacing.

Doublet:

This is the simplest telescope lens system. This term refers to the number of glass elements in the main lens (objective). A doublet has two lens elements that are glued together. The term “Achromatic” is often used in reference to a Doublet. This means that the lenses don’t produce Chromatic Aberrations.

Triplet:

This term also refers to the number of glass elements in the main lens (objective). A triplet has three lens elements that are glued together. This system is typically referred to as an "Apochromatic" lens. 

Achromatic Lens:

Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.

Apochromatic Lens or APO:

For advanced color purity a third lens is added, this lens is made from a different type of glass. This type of lens is often referred to as Apochromatic. APO lenses are designed to bring three wavelengths of light (red, green, and blue) into focus in the same plane. 

Chromatic Aberrations or CA:

A color fringe around bright objects.

OTA (Optical Tube Assembly):

Refers to the tubes of a Telescope wherein the Optical system is housed.

ED Glass:

Optical glass with extra low dispersion.

Extra Low Dispersion:

Chromatic aberrations or optical color defects are corrected with the use of Extra-low Dispersion Glass (ED Glass). These aberrations are caused when different light wavelengths  do not converge at the same point after passing through optical glass.