When people think of telescopes, the image that most commonly comes to mind is the refractor type of telescope. They are the earliest types of telescopes having been created in the early 1600s. Refractors consist of a tube into which is placed the "objective" lens at the front. The objective may be a single lens or it may consist of two or three lens bundled together either air or oil spaced. At the rear of the telescope is the focuser into which is placed a diagonal. Into the diagonal goes an eyepiece. The magnification of the telescope is largely depedent on the eyepiece.
Large refractors tend to be the most expensive telescopes due the cost of the glass objectives. There are also a variety of glass types used in the objective. Those glass types and configurations of the objective lens contributes significantly to the cost of the telescope.
My refractor is made by Astro Tech. The objective is 102mm in diameter - about 4 inches. It has extra dispersion glass to help with the view. This particular telescope is the easiest I own to grab and carry outside. It is particularly good for the moon, planets, and double stars. Good refractors can be purchased at a good price, especially from the used market. They are (usually) portable, provide good views, are easy to use, and will last a lifetime - provided you don't drop it or run over it with the car.
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