Optical Instruments - the very basics

Optical Telescopes

telescope repairsTelescopes with large apertures can distinguish fainter objects and finer detail than ones with small apertures.

There are two main types of optical telescope, the reflector and refractor. The reflector uses a curved mirror to collect light. This main mirror fixed to one end of a tube, concentrates the light on to a secondary mirror which reflects it into the eyepiece. This is fixed on the tube at right angles to the mirrors. The object is then viewed through the eyepiece by the observer.

The larger the telescope’s mirror, the greater the magnifying power so the fainter the object that it can view. One of the largest reflecting telescopes in the world has a mirror ten metres in diameter.

The refractor uses lenses, and in its simple form, it is a tube with a main lens at one end and an eyepiece with at least two lenses at the other. The main lens collects and focuses light, and with the image being viewed, is formed in front of the eyepiece lens. The image is then magnified by the eyepiece, but gives an upside down view.

Other optical telescopes include the Catadioptic, which combines lenses and mirrors, and the Schmidt telescope, which is used for taking wide-field photographs of the sky.

The majority of modern astronomical telescopes are reflectors.


The standard monocular microscope has at least two magnifying lenses, with a light reflecting mirror fixed below the specimen glass slide. The focusing screws adjust the lenses to give a sharp and clear picture of the specimen being studied.

Just a small microscope can magnify a specimen 100 times, so a hair of 0.1 millimetres thickness would look 1 centimetre wide. A good light microscope can magnify clearly to 2000 times. The specimen being viewed however, must be thin or reasonably flat, as light passes through it from the reflecting mirror below, and on to the objective lens that magnifies the image. This image is then magnified again by the eyepiece lens and viewed by the observer.

Good quality microscopes can have more than one objective lens with varying degrees of magnification. They can be swung into position when required.