How to Collimate your Telescope

Adapted from "UNIVERSE" April 1993

In order for you to gain optimum performance from your Telescope, it must first be properly "collimated".

Collimation is the precise alignment of all optical components in a telescope. (ie. the mirrors, the eyepiece and the focusser are all in the exact same line of sight. This may be achieved by following these basic guidelines:

Firstly, you must have access to a set of Tectron (or similar) Collimating tools, consisting of:

  • Sight Tube
  • Cheshire Eyepiece
  • Auto Collimator
    1. Insert the Sight tube into the focusser, and tighten the screw.
    2. Aim the telescope at a bright background (NOT THE SUN!) at a suitable level for your eye.
    3. Take a sheet of white paper and place it behind the secondary (diagonal) mirror, holding it so that it obscures the view of the Primary mirror.(Be careful never to touch the mirror with your fingers, clothes, or the paper.
    4. Looking through the Sight Tube, you should see the following:
      1. The bright image of the secondary
      2. A dark ring around the Secondary Mirror
      3. The white paper
    5. If the central image (i) is slightly oval rather than round, turn one of the three secondary screws, found on the top of the secondary holder. Continue this with the other two screws if necessary, until the image of the secondary is exactly circular
    6. If the dark ring (ii) around the secondary is not of the same thickness all the way around, then the secondary must be moved up or down the tube. If it is slightly thicker toward the bottom, then the secondary is too low, and must be moved up the tube slightly (away from the primary). Conversely, if the ring is slightly thicker toward the top, then it must be moved down the tube slightly. These adjustments are accomplished by adjusting the secondary nuts/bolt. If the dark ring is slightly thicker on one side, then the secondary must be rotated (by grasping the secondary holder and rotating very slightly)
    7. Ensuring that the Primary mirror is firmly seated on all flotation points of the mirror cell, remove the sheet of paper and look through the Sight Tube, where you will now see the Primary Mirror, and the central dot, which will probably not be central. Identify the components of this multiple reflection image, and become comfortable with them.
    8. Insert the Cheshire Eyepiece into the focusser, and tighten the screw with the opening pointed towards a bright part of the sky.
    9. Looking through the Cheshire Eyepiece, you will see a bright ring with the image of the centre dot on your primary, plus another black dot (which is created by the Cheshire Eyepiece & represents the exact centre of the focusser)
    10. With the aid of an assistant at the back of the telescope, rotate each of the three bolts behind the primary, until both dots accurately co-incide. The Telescope is now almost collimated.
    11. Insert the Auto Collimator into the focusser, and tighten the screw.
    12. Looking through the Auto Collimator, you should see:
      1. The centre dot on your primary mirror, surrounded by a small circle of the inner primary mirror surface,
      2. An annulus or ring created by the Auto Collimator, which may be light or dark,
      3. Reflections of the secondary mirror.
    13. Concentrating on the annulus (ii), rotate the secondary VERY SLIGHTLY until the annulus is dark. This movement is so fine, that it is recommended that you rock the Auto Collimator in the focusser first, as this may change the annulus from light to dark. If this is so, then it is useless to try to make any further adjustments.
    14. The last step to achieving exact collimation, is to be done after dark, using a star of about 3rd magnitude, with an eyepiece inserted in the focusser, and racked right out towards your eye (beyond focus). The star image will be like a disk, with a dark spot hopefully right in the centre. If the dark spot is not right ion the centre, then make fine adjustments to the primary mirror bolts until it is centred.
    15. Rack the focusser in a little, and repeat the last step if necessary, and continue until you have the star in focus.