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5.3.7 Ramsden Eyepiece

With your telescope still set up as before, take lens C and mount it at the position of the primary image; i.e. at 36.5 cm from the object glass and 6.3 cm from the eye lens. You now have a two-component eyepiece, consisting of a field lens and an eye lens. The plane surface of the field lens should face the object glass, and the plane surface of the eye lens should face the eye, so that the two convex surfaces are facing each other. This type of eyepiece is called a Ramsden eyepiece.

Now look through the telescope. The field of view is very much larger, and you almost have a satisfactory telescope.

Can we improve on it further? Locate the exit pupil in the same manner as you did before. It is now uncomfortably close to the eye lens, and there will be difficulty in getting your eye there.

Also, look through the telescope, and see all those greasy fingerprints on the field lens. Are they yours or your partner's? Shame on you! Regardless of whose they are, they, and any dust, are clearly visible because the field lens is in the focal plane of the eye lens.

Keep looking through the telescope and move the field lens slowly towards you, just enough that the dust, etc., becomes sufficiently out of focus that it is not visible. Do not go any further than this, for you are introducing lateral chromatic aberration the more you move the field lens.

Locate the exit pupil again. Now you have a much more comfortable eye relief, although the diameter of the exit pupil is still larger than the entrance pupil of your eye.

Look through the eyepiece and hold a pencil point just outside the field lens, so that you are using the two-component eyepiece as a magnifying glass. You will note that the pencil does appear magnified. You could replace the pencil point with a pair of crosswires if you wanted to use your telescope as a measuring instrument.

For further reading on eyepiece design we suggest you read Miczaika and Sinton, Tools of the Astronomer , or Dall, J.B.A.A., 79, No. 5, p. 349 (1969).

next up previous contents
Next: 5.3.8 Resolving Power Up: 5.3 PROCEDURE Previous: 5.3.6 Use of a   Contents
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