Diaphragms or Stops.

TM 9-258

or ghost images caused by internal reflections from

4-10. Diaphragms or Stops.

curved lens surfaces.

a. General. Diaphragms are rings of opaque

b. Antiglare Stops.

material placed in an optical system so that light passes

(1) Antiglare stops improve contrast by

through their centers. As their apertures limit the field of

preventing rays exterior to the field from bouncing off the

view they are sometimes called field stops. When

interior of the instrument and fogging the field or causing

placed around the edges of lenses, they prevent rays

glare ray (2, fig 4-26). Nonreflecting paint and baffle

from passing through the margins of the lenses and

finish on inner wall of the tube likewise eliminate most of

causing aberrations.

When placed between the

this undesired light. Light from a billiant source may still

objective and the erecting system or between the

be reflected off a dark wall and cause serious trouble

erecting system and the eyepiece, or in both or these

especially with a glass reticle. This is particularly true in

positions or between parts of the erecting system,

wide-angle telescopes and antiaircraft instruments which

they eliminate marginal rays which would otherwise

may be pointed toward the sun.

cause glare and haze by reflecting from the inside walls,

Figure 4-26. Diaphragm (stop) location.

(2) In straight-tube telescopes, the stops are

image of the first at that point so that slight inaccuracy in

merely washers or disks with a hole in the center. The

size or positioning will not conflict with the sharply

prism shelf in a binocular is designed to act as a stop.

defined image of the first.

d. Aperture Stops. In telescopes, this stop (fig 4-26)

The groove in the face, the rounded corners, and

flattened apex of Porro prisms are designed to function

usually is the objective lens cell or retaining ring as there

as stops.

is no reason for reducing the size of the aperture of the

(3) Antiglare stops usually are located between

single compound objective lens usually used in such an

the objective and the erectors or wherever an image of

instrument (as contrasted with the need for such stops in

the aperture stop is formed (fig 4-26). They are known

wide-angle camera lenses to reduce aberrations). A

as erector stops if they are located between erectors.

stop in close proximity to such a single compound lens

Such stops provide balance illumination by limiting the

objective would only reduce the illumination and exit pupil

rays from the center of the field to the same area as

size (the same as using a smaller lens) without reduction

those from the edge of the field.

in aberrations from the lens area used. In a complex

c. Field Stops. The field stop (fig 4-26) limits the

objective (two or more separate lenses), however, a stop

field to that area which is fully illuminated and sharply

located between the lenses may reduce aberrations.

focused by eliminating the peripheral region of poor

Also, in a complex eyepiece of several elements, a stop

imagery (caused by aberrations) and also prevents the

between the elements may serve to reduce aberrations.

e. Field Effects. All stops, excepting an objective

observer from viewing the inside of the instrument. It is

located in an image plane providing a sharply defined

aperture stop, can be considered to limit the field since

limit to the field. It is designed to admit all the light or

they may prevent rays exterior to the field from reaching

limiting rays needed by the next element whether

the eye. In designing a system, however, this field limit

erectors or eyepiece. The field stop also may function as

is determined from the apparent field of the eyepiece so

an antiglare stop preventing rays exterior to the field from

that in the final analysis the true field of the instrument

being reflected off inner surfaces back into the field (ray

must be governed by the eyepiece.

1, fig 4-26). If a field stop is used in each image plane,

the second such stop is slightly larger than the

4-28