Small angle generating apparatus

A small angle generating apparatus is described for deviating optical beams y accurately measurable small angles to calibrate autocollimators or the like. It includes a double mirror combination which is mounted on an elongated mechanical beam which is rotatable about a pivot at one end of the beam by means of a micrometer coupled at the opposite end thereof. Rotation of the double mirror combination deviates a beam of light from a test autocollimator from its original position either horizontally or vertically through a small angle which is linearly proportional to the angular rotation of the mechanical beam.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
Subject invention is related to calibration of optical systems such as 
autocollimators and more particularly to a small angle generating system 
for deviating or deflecting light beams. 
(2) Description of the Prior Art 
The angular deflections of light beams have been accomplished in the past 
by converting large mechanical movements into corresponding small 
movements of reflecting or refracting surfaces in order to obtain a high 
optical advantage. However, such methods have been expensive in order to 
obtain accurate mechanical movements which are repeatable. Furthermore, 
any nonlinearity in the mechanical movements makes the calibration of the 
devices difficult. One novel technique to accomplish this is described and 
claimed in our copending application Ser. No. 07/215,200 filed July 5, 
1988. That technique makes use of an optical wedge of small apex angle 
.alpha. which is rotated to produce a small deviation of a collimated 
optical beam. However, in case of a rotating optical wedge there are 
spurious images due to back and forth reflections from the faces of the 
wedge which give rise to noisy signals. Furthermore, an extended spectral 
range for a small angle generating system will require several optical 
wedges with transmission for each wedge in a particular portion of the 
spectrum. Additionally, there is chromatic dispersion and variable 
attenuation with wavelength of the wedge. It is thus desirable to have a 
small angle generating system which is free of the above-mentioned 
disadvantages. 
SUMMARY OF THE INVENTION 
A small angle generating system according to the teachings of subject 
invention includes a double mirror arrangement instead of using an optical 
wedge. The double mirror combination is rotated on a flexural pivot. 
Besides, the two mirrors of the combination can be rotated about an axes 
through their respective centers. Besides, the double mirror arrangement 
can be used to induce either vertical deviation or horizontal deviation of 
the optical beam and thus increase its versatility. 
An object of subject invention is to have a small angle generating system 
which does not produce spurious images at the optical surfaces due to back 
and forth reflections. 
Another object of subject invention is to have a small angle generating 
system which has a wide spectral range from ultraviolet to infrared 
regions. 
Still another object of subject invention is to have a small angle 
generating system which has no spectral dispersion of an optical beam 
passing therethrough. 
Further object of subject invention is to have a small angle generating 
system which has no variable attenuation of the optical beam. 
Still another object of the invention is to have a small angle generating 
system wherein the input mirror rotates about it own center. 
An additional object of subject invention is to have a small angle 
generating system wherein the double mirror assembly can be removed, 
turned 90 degrees and remounted for generating angular deviations in 
zenith rather than azimuth and thus making the angle generating usable 
along two axes. 
Other objects, advantages and novel features of the invention may become 
apparent from the following detailed description of the invention when 
considered in conjunction with the accompanying drawings wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like parts correspond to the like 
numbers throughout, FIG. 1 is a top view of the small angle generator 10 
using a double mirror combination. Here X- & Y-axes are in the plane of 
the paper and Z-axis is pointing perpendicularly out of the plane of the 
paper. The small angle generator includes a rigid, optically stiff and 
elongated mechanical beam 12 having its one end mounted on support 14 
using a flexural pivot 16. This enables beam 12 to rotate either 
counterclockwise or clockwise about a horizontal axis through pivot 16. A 
flat mirror 18 is mounted near the flexural pivot which forms a part of 
auxiliary autocollimator 20 for determining accurately the angular 
position of mechanical beam 12. The rotation of beam 12 is accomplished by 
using a micrometer 22 engaged at the other end of the elongated beam 12. 
End 24 of the micrometer 22 rests on support 26 as shown in FIGS. 1 and 2. 
Additionally, a double mirror combination 30 including prism-like mirrors 
32 and 34 are mounted on mechanical beam 12 with the respective reflecting 
surfaces 36 and 38 positioned as shown in FIG. 1. Mirror 34 is the input 
mirror which is rotatable about a horizontal axis through center point 40 
of the mirror. Dynamic autocollimator (DYNAC) 42 is used to test its 
calibration using small angular deviations generated in its line of sight 
by the device of subject invention. DYNAC 42 includes a rotatably mounted 
reflecting cube 50 and a source of light and associated optical hardware 
54 to obtain a collimated light beam 60. Beam 60 gets reflected at 
reflecting surfaces 38, 36, 64, 36 and 38 respectively to trace paths 
indicated by numerals 62, 70 and 72 as shown in FIG. 1. Mirrors 32 and 34 
are rotatable about horizontal axes through their respective centers so as 
to change their orientation by misalignment from the position of 
parallelism of 36 and 38 to cover a wide range of small angular deviations 
of the collimated light beam 60. Rotation of beam 12 is achieved by the 
movement of micrometer 22 with its lower end 24 resting on support 26. The 
micrometer is coupled to beam 12 and causes it to move either 
counterclockwise or clockwise. Mirrors 18, 32 and 34 rotate with the 
rotation of beam 12. The angle .phi. through which beam 12 is rotated is 
measured by using mirror 18 and auxiliary autocollimator 20 whose line of 
sight is deviated because of the rotation mirror 18 or that of beam 12. 
When mechanical beam is in horizontal position, collimated beam 60 is made 
to experience reflections at reflecting surfaces 38, 36, 64, 36 and 38 
with a preset misalignment from their parallel position for surfaces 36 
and 38. Light beam 72 returning to the DYNAC 42 deviates by an angle 
.beta. which is measured by the DYNAC 42. If mirrors 32 and 34 have their 
reflecting surfaces 36 and 38 exactly parallel, the collimated beam 60 
will not be deviated at all despite the rotation of the reflecting cube 
50. The beam 12 is made to rotate counterclockwise (downward mostly 
vertical) which in turn causes mirrors 18, 32 and 34 rotate by the same 
amount in the vertically downward direction. Besides the same (vertically 
downward) motion of beam 12, mirror 18, 32 and 34, there is a slight 
horizontal movement of the collimated beams 60 and 72 if the reflecting 
surfaces 36 and 38 are slightly misaligned from parallel orientation. If 
the angular distance traveled by beam 12, mirror 18 and mirrors 32 and 34 
is .phi., and the horizontal leftward deflection is .beta., the vertical 
motion of the collimated beam 60 is given by .beta. Sin .phi.. 
Consequently, the optical advantage which is defined as the ratio of the 
horizontal angular distance traveled by the light beam and the angular 
distance .phi. through which mirror combination 30 has moved is given by 
##EQU1## 
Thus, the line sight of DYNAC 42 experiences a very small angular 
deviation resulting from a relatively large angular distance through which 
the mechanical beam has rotated. The deviations caused in the direction of 
the collimated beam 60 due to the rotation of beam 12 are the horizontal 
direction. This means that we are in position A of the circle 100 which 
has radius .beta. equal to the vertical deviation of the collimated beam 
60. This is shown in FIG. 3. An azimuth motion can be obtained by rotating 
the entire double mirror assembly 30 in its original configuration through 
90 degrees about an axis parallel to the DYNAC 42 collimated beam axis and 
centered on mirror 34. In that case, we would be at area B on the locus 
circle 100 and motion would be zenith. This increases the versatility of 
the small angle generator by obtaining small angular deviations along two 
orthogonal axes. 
In operation, mechanical beam 12 is placed in horizontal position by means 
of micrometer 22 and the double mirror combination is adjusted so that 
reflecting surfaces 36 and 38 are slightly misaligned from their being 
parallel to one another. The deviation of the collimated beam 60 is 
measured and found to be angle .beta.. The beam 12 is rotated through 
.phi. in the counterclockwise direction and the horizontal component to 
the right is then measured which gives the optical advantage to be 
##EQU2## 
By rotating mirrors 32 and 34 through 90 degrees, we can generate small 
angular motions of the collimated beam to find the vertical component. 
Many modifications and variations of the present disclosed invention are 
possible in the light of above teachings. As an example, a rhombic prism 
can be used in lieu of the two mirror assembly. Besides, the rotational 
motion of the mechanical beam can be measured using other techniques. It 
is therefore understood that within the scope of the appended claims, the 
invention may be practiced otherwise than as specifically described.