Collimation

If you use a Newtonian reflector then you will probably have come across this term. So what is collimation? Collimation is the process whereby the telescope optics are aligned so as to ensure the light travels almost exactly parallel to its inward path as it travels back up the tube having been reflected by the primary. I say almost, as each photon of light after hitting the primary mirror is in fact travelling in a cone of light which is converging in order to hit the secondary mirror before being bounced into the eyepiece to come to focus at the back of your eye. This cone of light should be exactly symmetrical and this is what collimation is about; putting every element in the optical path into perfect alignment with each other. There are a number of tools for doing this but the one I favour is the laser collimator.

Laser collimators are like Marmite for those who understand what I mean. You either love them or hate them. Personally I like them, but they are not straightforward to use.

I had to work with and in the end I calculated the light had travelled forty feet. This means small errors in the collimation of the laser are magnified and this makes it easier to see the effect of the small adjustments you will need to make.

I made a Vee frame by knocking four nails in two pairs into a piece of wood. Each pair crossing at a 45 degree angle to support the collimator. I clamped the frame to a workbench to keep it still whilst I carried out the collimation.

Having set the collimator in the Vee frame shine the light onto a target which can be any white surface which can be marked; I use a sheet of paper. Mark where the light is shining then turn the collimator through 180 degrees and mark it again. This will indicate how far out the collimator is. In an ideal world the marks should be one on top of the other. Then turn the collimator through 90 degrees and mark and then again another 180 degrees and mark the spot. This now gives me four marks at 90 degrees to each other which can be used to create a circle which will describe the extremities of the accuracy or otherwise of the collimator.

Each collimator will be different but on the Revelation collimator I use, there are three small set screws which can be loosened or tightened to align the laser. This should be done carefully; avoid turning the screws by more than a quarter turn as small movements will produce big changes. Ideally you should seek to move the spot by half the distance to the opposite extreme. By systematically adjusting these screws it should be possible to arrive at the position where turning the laser through 180 degrees keeps the spot of light in one place.

Now you can use it to collimate your 'scope.

Collimating a Newtonian

The first step in collimating a Newtonian telescope is to check the secondary is correctly positioned to send the beam of light from the collimator down the telescope tube to the main mirror. To do this we need to ensure the secondary is positioned precisely on the centreline of the main OTA. On a Sky-watcher telescope this is easy enough to achieve as all we need is a twelve inch rule to measure the distance from the tube edge to the centre of the secondary holder. By adjusting the knurled knobs at the end of the spider vanes the position of the secondary holder can be varied until it coincides with the centre of the OTA.

Place the collimator in the eyepiece holder of the scope and switch it on. Now we need to ensure the secondary is aligned so that the light from the laser goes down the tube to the centre of the primary mirror. You will see there are three small screws to adjust the aim with. Again I would advise small movements. The object of this exercise is to place the beam of light from the sensor into the middle of the target on the primary.

Once this has been achieved we now turn our attention to the secondary mirror. Again there are three screws which provide the necessary facility to aim the mirror. This time watch the laser carefully. Most collimators have a window cut in the side and a target placed at 45 degrees with a hole in the middle through which the laser shines. By watching this target as you adjust the mirrors aim, you will see the point of light move around and once the aim is correct then the laser point should disappear back into the hole it originates from. Once this has been achieved your 'scope is collimated. Sounds simple but like everything else in life it requires practice.