Collimating Newtonian and 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 fig 2.
The secondary mirror is moved using the adjusting screws so that it is centred  as in figure 3.  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 fig 3.
The small silhouetted reflection of the primary mirror must be bought into the centre as shown in fig 1. this is achieved by positioning  main mirror using the three adjusting screws on the rear of the mirror. Adjustment of these screws to centre the mirror is best carried out by trial and error, rather than trying to work it out!

Fig 3

Fig 4. secondary and main mirrors aligned.

When the image in fig 3 is seen the next step is to precision align the telescope

Fig 4

2. Precision adjustment

The telescope is pointed at a bright Star high in the sky and observed with with a medium power eyepiece in the centre of the field of view. The defocused image seen may look like fig 5 or 6

fig 5. Small centring error.

The star image is moved in the direction of the arrow using the main mirror adjustment screws. Turning them a small amount at a time. When the image looks like fig 5 the medium power eyepiece is replaced with a high power and the final adjustments made.

Fig5

Fig 6. Perfect alignment.

  inside focus     at focus      outside focus

Fig 6

SCT Telescopes

A 35mm film canister with a small hole drilled in the center is placed in to the focussing unit.when looked through a image similar to fig 2 or 3 may be seen.

Fig 7

If the image looks like fig3  the three screws in the centre of secondary mirror shown in fig 7 can be adjusted. There is no adjustment on the main mirror. The screws are adjusted  using trial and error to give a similar image as shown in fig 4. Only make small adjustments  to the screws do not use excessive force.

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Now complete step 2 precision adjustment as above using the same adjusting screws.