What is a Finder Scope/ Star Diagonal/ Eyepiece/ Star Pointer?

A small auxiliary telescope attached to the main telescope. A finderscope uses low power with a wide field of view and assists you in locating objects to observe through the main telescope. Crosshairs aid in centering the object you seek. Most finderscopes give an inverted (upside down and left/right reversed) image.

A star diagonal is a prism or mirror set to direct light at a 90 degree angle to the telescope’s optical axis. It is an ergonomic accessory to make viewing the sky more convenient and comfortable for objects nearly overhead or for a less neck-craning angle for objects at other altitudes.

Newtonian-type reflecting telescopes do not require a star diagonal since their design already incorporates a right-angle reflection to the side of the tube and a comfortable eyepiece position.

An eyepiece is simply a precision magnifier. (Another name for an eyepiece is ocular.)

The eyepiece used with a telescope magnifies the image that is formed by the main optical system. There are many different kinds of eyepiece designs for various applications. The design and quality of the eyepiece can drastically affect the overall performance of the telescope system. 

An eyepiece is usually described by barrel diameter and focal length. Common barrel diameters are 0.96", 1.25" and 2". 1.25” barrels are regarded as the standard for telescopes worldwide. Commonly available eyepiece focal lengths range from 2 mm to 40 mm. A shorter focal length eyepiece will be a higher power eyepiece when used with your telescope. 

A telescope provides magnified views of the Earth and the sky. As such, its field of view is very narrow. Pointing the scope at a distant object on the ground or a small, faint object in the sky can be difficult.

This is where a small telescope or another kind of pointing device mounted on the side of a scope can be a great help. Like the riflescope on a rifle, the pointer is aimed parallel to the main telescope and provides a low-power view that is easy to point at the target.

Some models of Celestron telescopes use StarPointers, a type of pointer called a red dot finder. It has a brightness-adjustable red LED or LED fiber-optic light source. It is reflected off a curved transparent surface that allows both a direct view of the sky and the red dot LED to be seen at the same time. It has no magnification and works by superimposing the tiny brilliant red dot on top of your target. 

Alignment of your scope and StarPointer starts with sighting a distant building or object (half-mile away) in the daytime. Once you’ve trained your scope on the distant object and centered it in the scope’s eyepiece, position your eye about 6-8 inches back from the StarPointer’s curved window. Turn the knobs until the red dot is atop the same object you have already centered in your eyepiece.

Now your StarPointer can be used to successfully aim the main scope. Check it right away by pointing to another object using the StarPointer and verify that it is in the center of your scope’s eyepiece.