Actual Step

Collimation Tools and Conventions

updated: 2024-05-15


Understand the mechanics of the mirror adjustments and prepare all necessary tools for the collimation

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Step 1: Correct the Focuser Tilt

Secondary Mirror Collimation Screws

reference: Orion Ritchey-Crétien Astrographs

NOTE: Only adjust the three screws X, Y and Z around the perimeter of the holder—do not adjust the center screw! (Figure 10). Adjusting the center screw can cause the secondary mirror to fall off!


  • it is highly recommended never to touch the central locking screw on the second mirror, as this may cause the mirror to become completely misaligned. 
  • It is recommended that the "Y" screw be maintained in its current state, thus maintaining the mirror in its current position and changes in mirror spacing are avoided. 

Always start by loosening one screw at a time. Adjust the screws very slightly - no more than 1/10 turn at a time, one screw at a time, before checking the view through the Collimator to see how things have changed. Only small adjustments should be necessary to achieve collimation. 

This will also help to prevent the telescope from being accidentally grossly out of collimation. With each small adjustment of a screw, make a mental note of which way and how far the center dot has moved, as this will tell you which screw to turn next and by how much. Experiment with different combinations of loosening/tightening the three screws, one at a time, until the black dot of the collimator eyepiece is centered within the dark ring of the secondary mirror. Proper alignment of the secondary mirror is critical in determining whether the optical axis needs to be aligned. Be sure the secondary mirror is properly aligned before proceeding to the next step.

reference: Orion Ritchey-Crétien Astrographs

Primary Mirror Collimation Screws

To tip/tilt either the primary or secondary mirror, one or more of the sets of collimation screws A, B and C may be adjusted. To move one side of a mirror cell closer to a set of screws, the push screw(s) is loosened and the pull screw is tightened in that order. To move one side of a mirror cell farther from a set of screws, the pull screw is loosened and the push screw(s) is tightened. Only light to moderate force should be required to securely hold the mirror cells but if collimation instability is observed, a bit more force may be required.

reference: Orion Ritchey-Crétien Astrographs

Note that there are three pairs of screws on the rear cell of the optical tube where the focuser attaches (Figure 11). 

Each pair consists of a small black screw and a larger chrome screw

The small black screws are merely locking screws or pull screws, which you should loosen before adjusting the mirror’s tilt. 

The larger chrome screws or push screws are spring-loaded collimation screws that actually adjust the tilt of the primary mirror. 

Turn these collimation screws only a fraction of a turn at a time. Turn one and check the view through the Collimator to see if it improved the white optical axis ring. Keep tweaking the collimation screws, each time checking the optical axis ring, until it is concentric and uniform in width. Then tighten the three lock setscrews to fix the mirror in that position.

  • It is recommended that the "B" screw be maintained in its current state, thus maintaining the mirror in its current position and changes in mirror spacing are avoided.

  • Push screw pushes plates apart
  • Pull scres pulls plates together
  • To move plates apart:
    • Loosen pull screw then
    • Thighten push screw
  • To move plates together
    • Lossen push screw then
    • Thighten pull screw

reference: A Procedure for Collimating Ritchey‐Chrétien and Other Cassegrain Telescopes by Rich Simons – Deep Sky Instruments

Required Equipment 

  • Mechanical tools:
    • 3 metric Allen or hex keys: 
      • a 4 mm (for the secondary mirror),
      • a 3 mm  and an 2.5 mm key for the primary mirror
    • 1 Phillips screwdriver
    • A Collimation Tilting Ring (to compensate for focuser tilt)

Collimation Handbooks and Manuals: please see here

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