Kitchen table collimation of a Cassegrain-type reflector

2015 September 29

Russell Slater

Introduction

The holy grail of collimation for any reflecting telescope is the perfect alignment of the optical axes of the primary and secondary mirrors onto a single path, to minimise aberrations. It goes without saying that with a properly collimated telescope, a suitable target placed squarely and accurately on the optical axis of the primary will appear centred and distortion-free when viewed through the eyepiece. Putting these two statements together raises the interesting prospect of being able to use an on-axis target as an alignment aid for daytime collimation of a Cassegrain-type telescope.

A precise position on the optical axis of the primary mirror can be determined easily by setting a point source of light at the centre of curvature of the mirror, defined as the point where a spot of light will reflect back to the same point. The fact that the mirror may be spherical, parabolic or hyperbolic is immaterial for this purpose, since any aberration of the returned beam is very small and in any case will be symmetrical about the optical axis and will not affect the ability to bring the image to an adequate focus overlying the point source. A target centred on this point is the basis of this approach to optical alignment.

Current daytime collimation techniques
I have been unable to find an accepted ‘standard’ daytime collimation routine for an SCT. Manufacturers’ instruction manuals and other respected sources invariably describe only star collimation, typically adjusting first for symmetry of the shadow of the secondary mirror when off-focus at moderate magnification, followed by symmetry of on-focus Airy rings at high magnification. Several collimation aids exist in the market place that attempt to bridge the gap that exists between what can be achieved with daylight techniques and the need to make final adjustments using a suitable star, with examples ranging from the artificial star to complex (and expensive) laser-based products.

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