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TELESCOPES

11

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DIMENSIONS: MAGNIFICATIONS USING FOCAL LENGTH

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Focal Length
When a reflecting mirror is made, the depth of the parabolic curve on it's face determines it's focal length. The light is concentrated to a point and magnified by the eyepiece. The distance between the center of the mirror and the point of it being focused by the eyepiece is the focal length.

A refracting telescope's lens is similar except the light travels through the lens where it is bent by its curvature to the focal length and then magnified by the eyepiece.

Eyepieces also have focal lengths and these are written in millimetres on the side. When you know your telescope's focal length and also your

TELESCOPE LINKS:

Basics

Mounts

Light Gathering Power

Closer Look At Refractors

Closer Look At Reflectors

Balancing

Eyepieces

Cleaning Optics

eyepieces, the

magnifications

of each eyepiece can be worked out.

Tips

Divide the length of the eyepiece into the length of the telescope.

Astrophotography

Build your own telescope

Example telescope Tasco 302012 Newtonian Reflector

Build your own telescope
for schools

Binoculars

eg.Telescope Focal Length 900mm Eyepiece Focal Length 20mm & 4mm

900/20 = 45 The magnification is 45x for the 20mm eyepiece.
900/4=225 The magnification is 225x for the 4mm eyepiece.

Aperture Size
The diameter of the main mirror or lens of a telescope is the aperture (size). This is usually expressed in millimeters, but because of the American influence on telescopes a lot of the sizes are produced in inches. The typical apertures of reflectors are:

Imperial
3"
4.5"
6"
8"
10"
12.5"
16"
18"

Metric
76mm
114mm
150mm
200mm
254mm
317.5mm
406mm
457mm

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