Collimating a Newtonian - Part 1

For any reflecting telescope, collimation of the optics is one of the most important and regular tasks. Collimation simply refers to aligning the primary mirror and the secondary mirror along with the eye-piece. However, the job it self is not that simple.

If a telescope is not collimated, the image it creates would not be in the same plane as the eye or the projection plane (in case of a camera/ccd/projector is used). This could lead to various ring patterns, distortion of the image to appear when viewed. This means that even when you have focused the image properly (or brought the viewing plane up to the focal point), the image is still not clear/smeared. Fortunately, this it self provides an easy way of identifying whether or not a telescope is properly aligned.

Ray diagram of an uncollimated Newtonian - Red line depicts the plane where the image is created. The green line depicts the viewing plane.

To try to understand what happens in an uncollimated telescope, let's take a very simple example. Shine a torch on to a wall keeping the torch perpendicular to the wall. What do you see? You would see a circular spot of light. Now slant the torch so that the light falls on to the wall at an angle. What do you see now? The spot of light will now be an ellipse as opposed to a circle. This is what exactly happens to an image when it is focused through the telescope. The viewing plane is an angle to the projected plane, and the image appears distorted. When you point the telescope at a star and slowly move the eye-piece from unfocused to focused to unfocused again, you will see a circular disc -> point light -> circular disc if the telescope is collimated. If not, you will see an elliptic disc -> possible point light -> elliptic disc. You would also see "double" when viewing larger objects such as the moon or sun (as shown below).

Collimation errors seen on an image of the sun.

An uncollimated telescope would have disastrous effects on astrophotography. My earliest shots of Saturn were completely ruined by an uncollimated telescope. Some of my earlier lunar photos too were in a very bad shape due to the same reason.

A Newtonian reflector has two mirrors and therefore requires a two step alignment (first for the secondary mirror and then for the primary mirror). It is possible that in most of the cases, the primary

mirror might not be in need of any alignment.

Moon through an uncollimated telescope.

Moon through a partially collimated telescope
(secondary mirror is aligned) .

In the two photos above, you can see the improvement of the photo quality with the collimation. The photo on left was taken when both of the mirrors were slightly misaligned. The mirror on the right is through the same telescope with the secondary mirror properly aligned. In both of the photographs, the actual center of the view were the craters which appear towards the bottom of the image. Note that in the second image, although the top half of the image appear to be out of focus, the bottom half of the image is in almost perfect focus. Although I haven't really tried this out, I think for a point object like a star or a planet, even this amount of collimation should work well.

So now we know all the problems with an uncollimated telescope. Fortunately, it is not that hard to collimate a Newtonian by one's self. There are various methods of doing this from absolutely cheap DIY collimator caps to laser guided collimation tools. In the second part of this post, let's go through the various methods and how to collimate your telescope.