The Astronomical Unit


This page last modified 2002 October 31

Collimating a Refractor

Many amateur refractors are assumed by the manufacturer to be permanently collimated when they are shipped to the distributor, and thus they may not have facilities for recollimation. Therefore, some or all of the following may be inapplicable to your telescope and you would be wise to establish just what is possible before you begin. You should also check with the vendor and/or manufacturer that any actions you propose to take would neither invalidate any warranty nor be irreversible.

There are only four steps in the collimation of a refractor:

  1. Centre and square the focuser. This is not usually possible with commercial amateur instruments, and should anyway be unnecessary in a well-made instrument. If it is both necessary and possible, the simplest way is to remove the objective lens with its cell and place a pair of cross-hairs over the aperture of the telescope tube so that they cross in the centre. Centre the focuser and use a peephole sight tube to align it on the cross-hairs. You can make simple sighting tube for a 1.25" focuser with a 35mm film can with 1mm holes in the centre of the base and the cap.
  2. Centre the objective lens. If this is possible, it is usually achieved by rotating an eccentric ring in the cell. This may require a special tool (e.g. a peg spanner) and you may have to slacken grub-screws before the ring will rotate. You can mark the centre of the lens with a small disc, such as is produced by a hole-punch, of acid-free tissue which is dampened with clean water to enable it to be stuck to the glass. The tissue disc should be aligned with the peephole sight tube.
  3. Square on the objective lens. There are often either push-pull screw pairs or single screws with springs for this purpose. Fix a small light source (e.g. penlight or LED) such that it is a few centimetres outside the focuser and exactly along its axis. The light will be reflected off the surfaces of the lens elements. Introduce a thin piece of glass, such as a microscope slide cover slip, at 45º between the light and the focuser, so that you can see the reflections off the objective. When the objective is squared, the reflections will be coincident
  4. Star test the telescope.