Archive for the ‘Instruments’ Category

Chromatic Aberration

A defect of a lens that creates a fringe of colour round an object. It is caused by the fact that different wavelengths of light are refracted by different amounts.


There are several astronomical meanings. It can be used to describe the hazy looking patch that surrounds the nucleus of a comet or the blurred effect surrounding the images of stars on a photographic plate, or in the observers field of view in a telescope (or binoculars) due to defects in the lenses.


A lens that comprises two elements glued together in order to reduce chromatic aberration. One of the elements is made from crown glass and the other from flint glass. They work well if the light is travelling parallel to the lens’s axis but performance is worse if it is not, in which case multi-element lenses should be employed.


An ellipse is the correct name for an oval shape.

The planets do not orbit in perfect circles, as was believed before Kepler’s Laws of planetary motion, but rather in slightly oval (elliptical) orbits. Their eccentricity is low so the orbits are not far off being circles. An ellipse has two foci (plural of focus), in the Solar System, the Sun lies at one of these.

The same applies to binary stars, they have elliptical orbits too.

There are several methods of drawing an ellipse. A simple way is to make a loop of string and firmly fix two pins into a drawing board, through the paper set apart along where you want the axis of the ellipse to be. Place the point of your pencil inside the string loop and use the pencil to pull the string tight. Then try to draw a freehand circle. The string will guide the pencil, producing an ellipse.

A more accurate way is to use the trammel method. Naturally, this takes longer. If you use an accurate method of drawing the planetary orbits to scale, you will probably be disappointed – they will look like circles as their eccentricity is small.

Exit Pupil

The exit pupil is the image formed by the eyepiece of an optical instrument.

At low magnifications, the exit pupil is wider than at higher magnifications.


An eyepiece is the combination of lenses that magnifies the image formed by the objective (main mirror or lens) in an optical instrument.

There are different types of eyepiece, each has its merits. The most common type seems to be the Plössl, which is a good general eyepiece. Huygens and Kellner are the names of two other types of eyepiece.

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