<p>No, the fundamental designs of the two types of telescope are in this respect quite different.</p>

<p>The Newtonian laser collimator works by shining a laser through the eyepiece holder to the secondary mirror which reflects the beam onto the primary. The primary, usually a parabolic mirror but always concave, is adjusted so that the beam is reflected back to the secondary mirror such that the reflected beam returns to its point of origin.</p>

<p>A RCT, or other Cassegrain-like telescopes have a convex, possibly hyperbolic, mirror as the secondary. While it is possible to shine the laser through the eyepiece holder to the secondary mirror, and the mirror can be adjusted to reflect the laser back to the point of origin, it cannot be reflected onto the primary and then back to the secondary*. Any adjustment of the secondary to reflect the laser back to the origin only ensures that the incident surface of the secondary mirror is perpendicular to the line-of-sight through the eyepiece holder. It <strong><em>does not</em></strong> ensure that the secondary mirror is aligned correctly with respect to line-of-sight through the eyepiece, and neither with respect to the primary mirror.</p>

<p><a href="https://i.sstatic.net/nTG9z.png" rel="noreferrer"><img src="https://i.sstatic.net/nTG9z.png" alt="Cassegrain"></a> <a href="https://i.sstatic.net/ridPU.png" rel="noreferrer"><img src="https://i.sstatic.net/ridPU.png" alt="Newtonian"></a></p>

<p>* Notwithstanding that the two mirrors could be misaligned so badly that this could occur at some particular point.</p>

CC BY-SA 4.0
2018-06-19T07:20:23.947Z
2018-06-19T08:52:52.800Z
2018-06-19T08:52:52.800Z
Mick
<p>Is it possible to use a standard Newtonian laser collimator, <a href="https://rads.stackoverflow.com/amzn/click/B01D8HMBCQ" rel="nofollow noreferrer">such as this one</a> to collimate an RCT?</p>

<p>If so, is the procedure any different than collimating a Newtonian (i.e. make the laser go into the hole by adjusting the mirrors)?</p>

CC BY-SA 4.0
2018-06-19T17:33:59.793Z
Possibly helpful alternative at  http://garyseronik.com/no-tools-telescope-collimation/  , or this not-cheap gadget:  https://optcorp.com/products/takahashi-collimating-telescope-for-refractors-cassegrains-ritchey-chretiens
Carl Witthoft
<collimation><rc-telescope>
Can a Ritchey–Chrétien telescope be collimated using a standard Newtonian laser collimator?
stix
# Can a Ritchey–Chrétien telescope be collimated using a standard Newtonian laser collimator?

(Asked by: username_0 on 2018-06-18)

Is it possible to use a standard Newtonian laser collimator, [such as this one](https://rads.stackoverflow.com/amzn/click/B01D8HMBCQ) to collimate an RCT?

If so, is the procedure any different than collimating a Newtonian (i.e. make the laser go into the hole by adjusting the mirrors)?

**Tags:** collimation, rc-telescope

> **Comments:**
> **username_1**: Possibly helpful alternative at  http://garyseronik.com/no-tools-telescope-collimation/  , or this not-cheap gadget:  https://optcorp.com/products/takahashi-collimating-telescope-for-refractors-cassegrains-ritchey-chretiens  

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## 1 answers

### Answer by username_2 on 2018-06-19. Score: 6 | Accepted answer

No, the fundamental designs of the two types of telescope are in this respect quite different.

The Newtonian laser collimator works by shining a laser through the eyepiece holder to the secondary mirror which reflects the beam onto the primary. The primary, usually a parabolic mirror but always concave, is adjusted so that the beam is reflected back to the secondary mirror such that the reflected beam returns to its point of origin.

A RCT, or other Cassegrain-like telescopes have a convex, possibly hyperbolic, mirror as the secondary. While it is possible to shine the laser through the eyepiece holder to the secondary mirror, and the mirror can be adjusted to reflect the laser back to the point of origin, it cannot be reflected onto the primary and then back to the secondary\*. Any adjustment of the secondary to reflect the laser back to the origin only ensures that the incident surface of the secondary mirror is perpendicular to the line-of-sight through the eyepiece holder. It ***does not*** ensure that the secondary mirror is aligned correctly with respect to line-of-sight through the eyepiece, and neither with respect to the primary mirror.

[![Cassegrain](https://i.sstatic.net/nTG9z.png)](https://i.sstatic.net/nTG9z.png) [![Newtonian](https://i.sstatic.net/ridPU.png)](https://i.sstatic.net/ridPU.png)

\* Notwithstanding that the two mirrors could be misaligned so badly that this could occur at some particular point.


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