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#-Element (i.e. 2-element, 3-element, etc.)

When a refractor is described as having a 2-element, 3-element, 4-element, etc. optical design, that dictates the number of lenses within the refracting telescope. The number of elements also dictates how that refractor is classified: 2-element refractors are doublets, 3-element refractors are triplets, 4-element refractors are quadruplets, and so on.

1.25"/2" diameter

Typically these are used to describe eyepieces, referring to the outside diameter of the barrel that is inserted into the focuser or diagonal for visual astronomy. However as some astrophotography setups do use/reuse some of the same connections, this is seen in some astrophotography accessories, most notably nosepieces.

500 Rule

In astrophotography, the 500 rule refers to how long of an exposure you can have when taking a non-tracking photo with a camera on a tripod. Any longer of an exposure and the stars will begin to trail. To find this exposure length, you will need to divide 500 by the focal length of your lens. For example, if you have a 35mm focal length for your wide angle lens, then you would do the following: 500/35 = 14.28 seconds of exposure.