Dove rotating prism.

TM 9-258

(4) Dove rotating prism.

aiming stake viewed through a horizontal Dove prism

positioned with the reflecting face at 45ƒ angle to the

(a) The Dove rotating prism (A, fig 4-13) is the

means by which the image in some panoramic

horizontal (fig 4-23). Since the image is reverted in the

telescopes is caused to remain erect as the head of the

plane of reflection, it can be considered to revolve about

instrument is rotated. It usually is placed either outside

the axis AB, which is parallel to the reflecting surface. In

this case, angle "a" is 45ƒ; therefore, angle "b" is 45 and

the telescope system proper or between symmetrical

their sum is 90ƒ or twice the angular rotation of the prism

erectors. The line of sight of the rotating head can be

revolved in azimuth 360ƒ (a complete circle) while the

itself. When the 90ƒ prism (in the panoramic head) is

turned 180ƒ, it will transmit an upright but horizontally-

eyepiece is fixed so that the observer does not move.

Some panoramic telescopes (fig 8-16) employ the

reverted image to the Dove prism. Because the field is

objective 90ƒ prism in the head, and the Dove (rotating)

behind the observer, the effect is the same as in viewing

prism is caused to revolve in the same direction and

the user in a mirror where the user's image appears

through one-half the arc of the 90ƒ prism in the head.

reverted horizontally. In order to provide a normal

The field, inverted by the objective 90ƒ prism, and again

image, the Dove prism is so positioned by the 90ƒ

inverted by the Dove prism, also is rotated through twice

rotation of the plane of reflection that horizontal reversion

the arc of travel of the Dove prism itself as illustrated in

occurs.

figure 4-23.

(d) As both incident and emergent rays are

refracted by a Dove prism, it must be fed by parallel light,

otherwise an aberration is introduced. For this reason it

is usually placed outside (in front of) the telescope

system proper. If it is desired to place a rotating prism

within a telescope system where rays are not parallel, the

Pechan prism is used as its incident and emergent faces

are perpendicular to its surfaces and it is, therefore, not a

refracting prism and will not cause aberrations. If it is

desired to introduce a Dove prism within a telescopic

system, it must be placed between erectors so

positioned that principal rays between erectors are

parallel.

(5) Pechan or Z rotating prism. The Pechan or

Z rotating prism (B, fig 4-13) can be used, similar to a

Dove rotating prism, as a rotating prism in panoramic

telescopes to maintain an erect field by neutralizing the

Figure 4-23. Image rotation in Dove prism (end view).

reverting effect of the objective prism and when the

rotating head is turned ( (4) above). When this prism is

(b) An object such as a vertical aiming stake

rotated about its axis, the image rotates in the same

will appear upside down when viewed through a Dove

directions with double angular speed. By rotating the

prism positioned with its long face horizontal. When

prism, erection of the image is accomplished.

looking forward through the panoramic telescope this

4-8. Mirrors. The mirrors used to reflect the paths of

inversion corrects that initially introduced by the 90ƒ

light in fire-control instruments are not the common

prism in the head. As the panoramic head prism is

household type made of plate glass. The reflecting

revolved from this forward looking position, it introduces

surfaces on household mirrors produce a faint reflection

angular tilt equal to its angular rotation so that when it is

from the front glass surface in addition to that produced

turned to look to the user's left (for example) a vertical

by the silvered back. To eliminate this "ghost image," the

aiming stake will become horizontal in the image. Since

reflecting surfaces of optical mirrors are placed on their

the rotation, or tilt, of the field image produced by rotation

front faces. Mirrors of this type are termed front surface

of the Dove prism is twice the rotation of the plane of the

mirrors. The reflecting surface is a thin film of metal

reflecting surface (in accord with the law of reflection),

chosen for its resistance to tarnish. Such mirrors must

this 90ƒ tilt will be eliminated by a 45ƒ rotation of the

be handled carefully as they can easily be scratched and

Dove prism.

become useless until they can be resilvered or

realuminized.

4-25