Collimating Newtonian & SCT telescopes

To ensure the  best possible performance of a telescope the optics need to be properly aligned or collimated. Adjusting the main and secondary mirrors is a simple operation that should be checked and carried out regularly.
The adjustment is usually carried out in two steps.
1. Preliminary adjustment is completed during day light.
2. Precision adjustment, is completed at night on a star.


1. Preliminary adjustment

looking through a telescope focuser with the eyepiece removed the secondary mirror can be seen reflected in the main mirror at the bottom of the tube telescope tube fig 1. With the eye centred in the focuser this is how a correctly collimated telescope looks 

Fig 1

The main and secondary mirrors require adjustment
The secondary mirror is moved using the adjusting screws so that it is centred  as in figure 2. A 35 mm film canister with a small hole in the centre placed into focuser will help the operation.

Fig 2


Small mirror adjusted. primary mirror requires adjustment.

The small silhouetted reflection of the primary mirror must be bought into the center as shown in fig 3. this is achieved by positioning  main mirror using the three adjusting screws on the rear of the mirror. Adjustment of these screws to center the mirror is best carried out by trial and error, rather than trying to work it out!

Fig 3