Figure 5-10. Symmetrical erectors.

TM 9-258

b. Symmetrical. A true symmetrical erecting

separation between the elements in the erecting system

(A, B, and C fig 5-12).

system is composed of symmetrical lenses so positioned

that the object (real image formed by the objective) and

image distances are symmetrical or equal (fig 5-10). In

other words, the image formed by the objective is at the

focal point of the front erector and the rays between the

two erectors are parallel. Therefore, spacing between

erectors is not critical and does not affect magnification

as the magnification in such a system (always-1)

indicates an inverted image of same size as object

regardless of lens separation.

c. Variable

Magnification.

Variable

magnification erecting systems used in variable power

telescopes may provide (in the high-power position) from

two to three times the power produced in the low-power

position depending on the design. In the simplest lens

erecting system (a single lens), magnification can be

shown to be a function of object distance and image

distance (A and B, fig 5-11, and para 2-24e). The same

is true if a combination of lenses (usually two) is used.

Figure 5-12. Asymmetrical erectors.

(2) When magnification is increased by

shifting erectors forward, the image position shifts toward

the eyepiece as in B, fig 5-12. A change in distance

then, between the asymmetrical elements in this forward

position, further affects magnification and image

distance. A decrease in the distance between the

elements, i.e., moving the second lens nearer the first,

lessens the increase in magnification slightly but more

important decreases the shift in the resulting image

position as in C, figure 5-12. By design, the image

position can be the same for any of the possible

Figure 5-11. Simple erector.

magnifications. In this case, the separation between

erector elements is always such that the image position

(1) Magnification of an erecting system

remains fixed for any magnification and the eyepiece can

composed of a combination of asymmetrical lenses

be fixed (C, fig 5-12).

(lenses of different focal lengths) can be varied either by

(3) If magnification is increased, either by

varying the distance of the erecting system from its

shifting the erector system forward as a complete

object (image formed by objective) or by varying the

5-5