Ramsden

TM 9-258

g. Ramsden. The Ramsden eyepiece consists

h. Huygenian.

This eyepiece employs a

of two planoconvex lenses made of ordinary crown glass

collective or field lens moved away from the eyelens so

of equal focal lengths separated by a distance equal to

that the real image lies between the two lenses. The

two-thirds of the numerical value of the focal length of

Huygenian eyepiece (A, fig 4-6) is not suitable for use

either. The reticle is located in front of the field lens at

with reticles in telescopes as distortion of the reticle

one-quarter of its focal length as illustrated in B, figure 4-

would result since the individual components are not

5. The field lens thus contributes to the magnification

corrected for aberration. It is sometimes used however

and quality of the image. Dirt on its principal plane is not

with a small reticle in the center of the field in some

in focus and, therefore, not visible. The convex surface

microscopes and it is useful in observation instruments.

of each lens faces the other. This eyepiece has the

Eyepiece focal length usually used is over 1 inch for

disadvantage of considerable lateral color which can be

sufficient eye relief. Both elements are normally of the

defined as a difference in image size for each color. It is

same type of crown glass with convex surfaces facing

widely used with reticles, however. This type of ocular

forward. This type of ocular is of negative type in

can be used as a magnifier and it is therefore called a

contradistinction to the Ramsden or positive type (g

positive ocular.

above).

4-7