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Telescope Collimation for Reflector Type Telescopes

updated: 2024-05-08


For a perfect alignment of your optical train (secondary and primary mirrors)  you should check the collimation of your telescope  from time to time, especially for reflectors, such as Newtonian, Schmidt-Cassegrain (SCTs) or Ritchey-Chrétien (SCTs) type telescopes.

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The following article(s) discusses the collimation of Ritchey-Chrétien telescopes, which are the most difficult telescopes to collimate due to their hyperbolic primary and secondary mirrors. 

Possible collimation problems

  1. Tilted focuser: this means that the optical axes of the focuser and the telescope do not match
  2. A shifted secondary mirror resulting in incorrect mirror distance between primary and secondary mirror and wrong focal length and back focus
  3. Tilted secondary mirror
  4. Tilted primary mirror

Collimation of Ritchey-Chrétien telescopes and similar systems

It is well known that collimating an RC is not an easy task. Usually it is necessary to use a star at night. With the TS collimator it is possible to achieve an ideal collimation result in daylight, comparable to the Takahashi collimator. This means that you always have an optimally collimated RC telescope at night. At the same time, collimation is simple and can be done by anyone. But it is important that the secondary mirror of your telescope has a central mark. Just follow the steps below.

There is a general problem with collimation: the equipment required for very good collimation is usually very expensive, and the price of the equipment sometimes exceeds the value of the telescope to be collimated. So compromises have to be made.

Collimation Status

General Picture

TS RCKOLLI Collimator

Ideal Collimation

When the image looks like this, collimation is almost complete and can be tested. 

If necessary, the distance between the mirrors must be checked (only visible on the measured focal length using a current image with this optic) and the primary mirror must be fine-tuned using stars and a Ronchi eyepiece.


Bad Collimation

If the image does not look centered, you should begin with the collimation for achieving the optimal result with the telescope.

The picture shows 2 collimation errors:

  1. the black spot in the red center is not centered: this is an alignment error of the secondary mirror
  2. the black outer ring is also not centered showing different white fringes: this is an misalignment of the primary mirror or the optical axis is out of alignment

General Collimation Steps

Collimating an Cassegrain or RC telescope is usually done in 3 steps:

  1. Step 1: Correct the Focuser Tilt (typically done with a tilting adapter flange)
  2. Step 2: Check and correct Mirror Spacing
  3. Step 3: Collimation of the Secodary Mirror
  4. Step 4: Adjusting the Primary Mirror
  5. Step 5: Final Collimation 



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