Collimation for Dobsonian telescopes

Accurately adjusting the line of sight of the telescope

Once we buy or finish making a Dobsonian telescope, most of us think that we are ready to start exploring the night sky using it. But may I have your attention please, the job is not yet done, it is yet to be completed. The process of collimation is the last process, but it is very important and some stages of it have to be done repeatedly also.

So first let’s understand what is collimation.

Collimation simply means aligning all the optical instruments in a telescope to optimize the optical performance of the telescope.

Why collimate?

Collimation is very much necessary in Dobsonian and Newtonian telescopes. If not done properly it may lead to problems like flaring of the image of an in-focus star, loss of contrast between different celestial bodies(precise features of outer space objects are indistinguishable), and loss of uniform focus(and sometimes no focus at all).

Collimation is even more essential in faster telescopes, i.e., the telescopes which give a brighter image. These are called fast because when images have to be clicked from these telescopes it is a fast process; in slower ones, the camera has to be set up for a prolonged period to get a perfect image. Let’s understand the process of collimation easily and simply.

Steps of collimation

1. Centring the primary mirror

This step is not as simple as it sounds. First, we start by marking the centre of the primary mirror. I did this by cutting out a semi-circle having the radius of the mirror. This has a very small semi-circular hollow at the centre, just beside this hollow, I stuck a circular sticker marking the centre of the mirror. Now centre the primary mirror with the telescope’s centreline.

2. Aligning the focuser

The focuser is the instrument that is adjusted to focus on an object in the night sky(this is the one on which we fix the eyepiece while looking through the telescope).

The focuser always has to be perpendicular to the tube of the telescope in two axes.

The red lines denote the axes.

How to align the focuser?

1. Mark a point on the opposite end of the telescope tube.
2. Insert a laser in the focuser.
3. Adjust the focuser so that the laser light falls exactly on the point marked.

Note- Please note that the secondary mirror has to be removed before this process or it will come in line with the laser.

3. Centring the secondary mirror

In this process, we just have to bring the secondary mirror to the centre of the telescope tube.

4. Aligning the secondary mirror

Try looking through the focuser of your telescope, what do think is the shape of the secondary mirror(or diagonal) it holds? Yes, it appears to be circular, but it is oval. The mirror is built at an angle of 45 degrees and so when seen from the telescope it appears to be circular. Our job is to make this circle concentric with the primary mirror. This is done by adjusting the ends of the spider until the mirror is concentric with the primary mirror.

Spider

This process is required rarely if the mechanism of the telescope is disturbed while travelling or observing.

5. Aligning the primary mirror

This is the most important process of the whole collimation drill. Using the collimation screws of the primary mirror, you need to align it such that the shadow of the secondary mirror falls on the centre of the primary mirror.

Before starting this process touch the base of the secondary mirror to judge where it is exactly placed. Very often people assume some other part of the telescope as the secondary mirror and do the collimation incorrectly because of which they have to start this whole process of collimation again.

This process is almost always required before starting the observation.

Processes 1,2 and 3 come already done by the manufacturer if you have bought a telescope and have to be done only once if you are making one.