THE NEWTONIAN DESIGN: THE BASICS
The Newtonian design is named after Sir Isac Newton. It is believed that he was the first to utilize this design in his scopes.
The concave mirror at the back of the scope is the first mirror to reflect the incoming light. It is therefore called the primary mirror. The diameter of this mirror is called the aperture of the scope. Its diameter and focal length determine the most important characteristics of the scope. The primary mirror is mounted in a holder called the cell.
The light cone reflected by the primary is intercepted by a flat secondary mirror which is placed diagonally in the lightpath. The secondary mirror is mounted in a holder which in turn is attached to the tube by a device called the spider. This second reflective surface reflects the light out of the tube into the focuser.
The focuser is used to mount eyepieces, camera, screen etc. Since most items need a different distance to the focal plane, the focuser length is adjustable.
After the scope has been assembled it must be adjusted. All components are aligned and collimated as to minimize image deformations and maximize light gathering.