Collimating a Newtonian Telescope - A simple guide for the everyday amateur astronomer

Sep 15, 2026

Upload by Lute

A simple guide to collimating a Newtonian telescope, focused on aligning the telescope optics so incoming light is properly routed into the eyepiece and the observer’s eye. It explains why small optical shifts occur as the telescope is moved and affected by gravity on the primary and secondary mirrors. The guide emphasizes periodic checking, especially after transport, and details the physics behind star images using Airy disks and diffraction concepts, leading to achieving a correct star diffraction pattern.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages37

Reading Time1 h 33 min

AccessFree to download

Contents

Definition – what is it?

Technical considerations – the why

Available Adjustments

The stages of collimation

Pre-requisites

Preparation

The flat

The primary mirror

Star testing

Acknowledgements

Collimating a Newtonian Telescope A simple guide for the every day amateur astronomer. Contents • Definition – what is it? • Technical considerations – the why. • Available Adjustments • The stages of collimation 􀁸 Pre-requisites 􀁸 Preparation 􀁸 The flat 􀁸 The primary mirror 􀁸 Star testing • Acknowledgements Definition – what is it? So, collimation has everything to do with making your telescope optics line up with eachother and also making sure that all of the light that enters into the aperture of your telescope also enters the eyepiece and your own eye. Technical considerations– the why. • What should a star look like? • Diffraction • What do your stars look like? This is the point where we have a little look at some physics that help us to understand what a star (which is a point source of light) looks like in a telescope. But before we start this the optical alignment will alter by small amounts as the telescope is moved and gravity has an effect on the primary and secondary mirrors. So getting things aligned well in the first place will ensure that during normal usage the optics stay very well aligned. It is worth checking your alignment every now and again though, a star test will show you if anything has moved. Certainly you should re-check if you’ve transported the telescope for observing at a remote location or if you’ve moved house. The vibrations from the vehicle will contribute to the movement of the optics far more than every day use. But bear in mind that trying to collimate in the dark is not easy. Technical considerations– the why. What should a star look like? So you probably know that a star is a point source of light – it’s so far away that it does not present a disk, so it’s just going to look like a (maybe white, maybe coloured) dot in my field of view. In reality it presents what is known as an Airy Disk, which is the result of light diffraction through the aperture of your telescope. So you can see that there is a bright central “point” of light surrounded by rings of light that get fainter as they recede from the centre – this is especially apparent if you defocus the view of the star slightly. Why does it appear like this? Technical considerations– the why. Diffraction? At this point in the presentation it is useful to take a step back and say to ourselves“what does my telescope look like to the star?” because this helps to explain the view that you get when you look at the star in your telescope. Well of course because your telescope is as far from the star as the star is from your telescope the aperture of your telescope just looks like a tiny hole. Technical considerations– the why. Diffraction? So how does light behave when it goes through a tiny hole? We’ve probably all seen pictures of the effect when light passes through a pair of narrow slits, the light is diffracted at the edges of the slits and interferes producing a series of bright and dark lines. Technical considerations– the why. Diffraction? If we reduce the slit to a circular hole then we get bright and dark circles instead. Technical considerations– the why. Diffraction? So if we now introduce a focal plane to this image, as we move from the axis towards the edge, we see bright, dark, bright, dark etc. so we see concentric bright and dark circles – the Airy Disk. Technical considerations– the why. Diffraction? So we want to achieve a perfect Airy pattern for a star when we look through a telescope.

FAQs

What does collimation in a Newtonian telescope accomplish?

Collimation aligns the telescope optics and ensures light entering the telescope aperture reaches the eyepiece and the observer’s eye.

Why does telescope collimation change when the telescope is moved?

As the telescope is moved, gravity and handling cause small alignment shifts in the primary and secondary mirrors, so transport vibrations can significantly disturb optics.

What physical idea explains how stars appear in a telescope?

Star images form an Airy disk pattern due to diffraction through the telescope aperture, producing a bright central region surrounded by fainter rings.

Sep 15, 2026

Upload by Lute