Aperture
Aperture is the diameter of a telescope's primary mirror or lens listed in millimeters or inches. The bigger the aperture of a telescope, the more light it will gather, allowing the observer to see more detail on celestial objects and ascertain finer details that a telescope of lesser aperture may not see.
Dobsonian Telescope
The Dobsonian telescope consists of a Newtonian reflector optical tube assembly mounted on a very simple alt-azimuth box-style mount with a lazy susan base. This base was invented by John Dobson to encourage people to make their own telescopes from start to finish. Up until that time, telescopes required a heavy equatorial mount that produced a financial or weight barrier for some otherwise enthusiastic budding astronomers. John Dobson and his base changed all that, and even today, Dobsonian telescopes still provide the most light-gathering dollar for dollar.
EdgeHD Telescope
EdgeHD is Celestron's top of the line flat field aplanatic Schmidt-Cassegrain telescope series. You can buy a variety of apertures, from 8 to 14 inches, as an OTA or as part of a telescope/mount package. EdgeHD telescopes are great for visual use but they really shine for astrophotography. With a wide field of view that is three times flatter than a standard SCT, you will see beautiful, pinpoint stars across the entire field, even if you have a full frame (35 mm) CMOS or CCD camera sensor to satisfy.
Eyepiece
An eyepiece is a group of lenses housed in a small package that is closest to the eye when used with a telescope, microscope, or spotting scope. The eyepiece provides a particular magnification when paired with a telescope, therefore most amateur astronomers use a variety of eyepieces to change magnification for different types of objects. The eyepiece nomenclature is expressed in its focal length in millimeters. To figure out the magnification, simply divide the focal length of the eyepiece into the focal length of the telescope. The result is the magnification provided in your particular telescope or one with the same focal length.
Finder Scope
A finder scope fits on top of the main telescope and is used to help you find and center objects in your eyepiece. A finder can be as simple as a red dot finder or it can be a high quality small telescope in its own right.
Focal Length
The focal length is the distance, usually measured in millimeters, between the primary mirror or lens and the point at which the image comes to focus. Generally, classic refractors have a longer focal length, Newtonian reflectors tend to have a focal length that is shorter, and Schmidt-Cassegrain fall somewhere in the middle.
Focal Ratio
The focal ratio is calculated by dividing the aperture (mm) of the primary mirror or lens into the focal length. Example: 2500 mm divided by 254 mm (10") equals an f/ratio of 9.84, which is usually rounded off, in this case to f/10. The focal ratio signifies how quickly a telescope gathers light and tells us something about the telescope's field of view, how long exposures will take during astrophotography sessions, and how much magnification the eyepiece will produce for that telescope.
Highest Useful Magnification
The term Highest Useful Magnification is used by telescope manufacturers to describe the most magnification you can typically be expected to use on a normal night and still bring an image to sharp focus. A basic rule of thumb for maximum magnification is 40X-50X per inch of aperture, with max magnifications generally topping out at 500X or so regardless of the aperture. You will find that maximums vary depending on the night. Observing conditions change constantly and will cause a once sharp view to become blurry or allow a blurry view to clear up in seconds. It is the nature of telescope observing on a planet with an atmosphere.
Imaging Newtonian
An Imaging Newtonian is a fast reflector telescope that is normally optimized for astrophotography or astro-imaging. Most Imaging Newtonian telescopes have an f/5 focal ratio or less, and some are not meant for visual use at all but rather, were designed to be dedicated imaging telescopes. It is common to find fast imaging Newtonian reflectors for sale as optical tube assemblies only, allowing the astro-imager to use his choice of equatorial mount.
OTA
The acronym OTA stands for Optical Tube Assembly. An OTA is simply the telescope portion of a telescope/mount/tripod package. Some telescope users prefer to buy the OTA separately so they can create a custom astrophotography set-up or use a mount they already own.
Reflector Telescope
A reflector is a telescope design in which mirrors are used to gather and focus light. Reflector telescopes are commonly called Newtonian Reflectors, or simply a Newtonian in deference to their inventor, Sir Isaac Newton.
Refractor Telescope
A refractor is a telescope design that uses lenses to gather and focus light. While there are some exceptions to the rule, achromatic refractors use two lenses in their design, and while they are economical, achromats are only capable of focusing two out of three wavelengths of light. An APO telescope, or apochromatic refractor, uses 3 lenses to bring all three wavelengths of light to a single focus to produce an image virtually free of extraneous color.
SCT
The acronym SCT stands for Schmidt-Cassegrain Telescope, one of the most popular telescope designs in amateur astronomy today. A Schmidt-Cassegrain, which belongs more broadly to the Catadioptric telescope type, uses a folded optical design incorporating both mirrors and lenses to gather and bring the light to focus. The folded light path allows for a short tube assembly even with relatively large apertures of 8" or more. A shorter tube length makes the SCT far more portable than a classic Newtonian or refractor of the same aperture.
