An auto-collimator is an optical instrument that uses a collimator and telescope combined to measure small angular differences very sensitively and accurately. It works by projecting parallel light using a collimator and measuring the linear displacement of the reflected light when the reflector is tilted, which corresponds to twice the angle of tilt. The auto-collimator responds only to tilts of the reflector, and its focal length and aperture determine its measurement range and sensitivity. It can be used to measure straightness, flatness, angles, squareness, and parallelism.

2. Auto-collimator is an optical instrument used
for the measurement of small angular
differences.
For small angular measurements it provides a
very sensitive and accurate approach.
It is essentially an infinity telescope and a
collimator combined into one instrument.

4. ….
 When a beam of rays coming out from a monochromatic
source of light are made parallel by placing a collimating
lens(source should be at focus point of lens), and if these
parallel rays are made to fall on reflector ,the rays reflect
back and travels in the same path and converges at the
source through the lens (fig A).
 If the reflector is tilted by an angle “theta” as shown (fig B)
, then the reflected rays makes an angle “2*theta”(twice
the angle theta) and converges at point O’.
 OO’ = (2*theta)*(f)…….where f is focal length(distance
between source and lens).

6. ….
• A cross line target graticule is positioned at the focal plane
of telescope objective. when the target graticule is
illuminated rays of light diverging from the intersection
point reach the objective via beam splitter and are
projected from objective as parallel . In this mode the
optical system works as collimator.
• A flat reflector placed in front of objective and normal to
the optical axis reflects parallel light back along their
original paths. They are then brought to focus in the plane
of target graticule and exactly coincident with its
intersection. A proportion of the returned light passes
straight through the beam splitter and the return image of
the target crossline is therefore visible through the
eyepiece thus operating as telescope focussed at infinity.

7. …
• If the reflector is tilted for an angle, the reflected rays
reflect twice the angle of tilt and are brought to focus in the
plane of the target graticule but linearly displaced from the
actual target by 2*(angle of tilt)*focal length.
• The linear displacement of the graticule image is measured
by an eyepiece graticule , optical micrometer or electronic
detector system , scaled directly in angular units.
• Auto-collimator responds only to reflector tilt.
• Focal length and the effective aperture are the factors
determines the basic sensitivity and angular measuring
range.

8. I. Photoelectric microptic auto-collimator
Micrometer adjustment is provided for setting ,but
coincidence of setting graticule and target image is
detected photo-electrically and shows null reading.
ii. Automatic position sensing auto-collimator
Angular displacement of the reflector is displayed on a
digital readout, eliminating any micrometer reading for
measuring . It is used for cramped positions.
iii. Microptic auto-collimator
In this a pair of target wires take place of the point light
source as it is not convenient to visualise the reflected
image of a point and then to measure the distance
precisely

9. Measurement of straightness and flatness.
Precise angular indexing in conjuction with
polygons.
Comparative measurement using master
angles.
Assessment of squareness and parallelism of
components.
Measurement of small linear dimensions.

12.  Levelling base
It supports the autocollimator and enables it to be levelled to bring its
axis parallel to the surface being measured.
 Surface plate stand
A multi purpose stand of heavy duty construction for general bench use,
comprising ground cast iron surface plate , column and bracket.
 Steel reflector
A reflector must be regraded as an integral part of any auto-collimator
system.
 Mounted glass corner reflector
This is used in conjuction with a reflector carriage and mounted reflector
for calibrating a surface plate.
 Steel cube reflector
It can be used as a general purpose reflector and for providing a 90
degrees angle standard in three planes and for checking perpendiculars.