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Auto-collimators
The Auto-collimator is a single instrument combining the functions of a collimator and a telescope to detect small angular displacements of a mirror by means of its own collimated light.
The two reticles are positioned in the focal plane of the corrected objective lens, so that the emerging beam is parallel. This usual configuration is known as infinity setting, i.e the auto-collimators are focused at infinity.
When moving the reticles out of the focal plane of the objective lens, the auto-collimator can be focused at finite distances, and the beam becomes divergent (producing a virtual image) or convergent (real image). This results in a focusing auto-collimator. The shape of the beam -convergent or divergent- depend on the direction in which the reticles are moved
The main components of a standard auto-collimator i.e. focused at infinity are:
- Tube mounted objective lens
- Beam splitter mount which contains two reticles
- Eyepiece
- Illumination device
The illuminated reticle projected over the beam splitter towards the lens is known as collimator reticle. The second reticle placed iin the focus of the eyepiece is the eyepiece reticle.
The beamsplitter mount together with the eyepiece and the illumination device form a main unit called: Auto-collimator head.
A focusing auto-collimator (finite distance setting) is similary built. The auto-collimator head containing the two reticles is now mounted on a draw out tube for focusing adjustment.
Auto-collimation is an optical technique of projecting an illuminated reticle to infinity and receiving the reticle image after reflection on a flat mirror. The reflected image is brought to the focus of the objective lens in which the eyepiece reticle is located. Thus the reflected image of the collimator (illuminated) reticle and the eyepiece reticle can be simultaneously observed.
When the collimated beam falls on a mirror which is perpendicular to beam axis, the light is reflected along the same path. Between the reflected image and the eyepiece reticle -which are seen superimposed- no displacement occures.
If the reflector is tilted by an angle (in radians), the reflected beam is deflected by twice that angle i.e. 2. The reflected image is now laterally displaced with respect to the eyepiece reticle. The amount of this displacement "d" is a function of the focal length of the auto-collimator and the tilt angle of the reflector: d = 2 f.
The tilt angle can be ascertained with the formula:
= d / 2f
where f is the effective focal length EFL of the auto-collimator.
Since the f is a constant of the auto-collimator, the eyepiece reticle can be graduated in angle units and the tilt angle can be directly read off.
The Autocollimator and its reflections
I have long had a feeling that the existing descriptions of the autocollimator and its mode of operation have not been based on a clear understanding. This analysis attempts to describe the reflections seen and their positions as functions of miscollimation and builds on a discussion in the Yahoo Collimate_Your_Telescope forum.
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. If the primary mirror’s center is marked with a bright or reflective spot, you can see the spot (reflected in the secondary) and a few more reflections of the spot after several reflections back and forth. This picture shows a 2” autocollimator (INFINITY, ™ by Jim Fly).
To use it, you first do a fairly close collimation with a sight tube and a Cheshire collimator (or, if you prefer, with a laser with a Barlow attachment), then insert the autocollimator and fine-adjust the collimation. When collimation is ideal, the reflections will appear “stacked” or coincident – but if they are not, how do you proceed? To answer this vital question, I believe you need a clear understanding of how the reflections are generated.
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