Autocollimators

Introduction:

An autocollimator is an optical instrument that is used to measure small angles with very high sensitivity. As such, the autocollimator has a wide variety of applications including precision alignment, detection of angular movement, verification of angle standards, and angular monitoring over long periods.

Objectives:

1. To measure straightness of a beam with the use of Auto-Collimator.
   
2. To identify the principle of Auto-collimator device.
   
3. To be able to draw conclusions about straightness error using graphical methods and least square method.
   

Apparatus:

1. Auto collimator.
   
2. Straight edge with 100mm marked intervals.
   

Theory:

Autocollimators are sensitive and inherently very accurate optical instruments for the measurement of small angular deviations of a light reflecting flat surface. The autocollimator has its own target which is projected by collimated light beams on a remotely placed surface and the reflected target image is observed in the ocular of the instrument.

The autocollimator is stationed at the end of the bed with a rigid support base. The movement of the reflector along the bed will cause the reflected image of the target to deflect according to the angular error of the bed,

The autocollimator is a flat mirror mounted in a short tube made to fit a Newtonian telescope focuser, and set accurately perpendicular to the tube’s axis. Centered in it is a small peephole or pupil that you look through.

The autocollimator projects a beam of collimated “parallel” light. An external reflector reflects all or part of the beam back into the instrument where the beam is focused and detected by a photo detector.

The autocollimator measures the deviation between the emitted beam and the reflected beam. Because the autocollimator uses the light to measure angles, it never comes into contact with the test surface.

How to calculate tilt of 1 sec. of arc of the reflector:

 θ is the angle taken after adjusting the micrometer properly (sec)

 h is the vertical distance (to be found)

 L is the length of the reflector carriage is 0.1m in our case. 

 (These parameters are shown in the below figure)

tanθ = h/L

Assume θ = 1sec

tanθ = θ for small values.

1sec = (1/60*60)(π/180) = 4.8*10^-6rad

h = 4.8-10^-6m

i.e. in each 1sec indicates a vertical distance of approximately 0.5 micron “since L = 0.1m”

As we can see the relation is nearly linear. We connected the best fit line by connecting the first and last point by means of a straight line.

The vertical rise h as we discussed in the theory section was:

H = 4.8*10^-6 m = 0.5 μm

This rise is due to 1sec of reflection angle so 1 sec = 0.5 μm

As we can see from our results that the time needed for reflection don’t depend on the position of the carriage, this is shown by the curve above.

 

We have some human errors like the readings.

Also the beam o light may not been reflected one hundred percent.