10
Fine Collimation
NOTE: Please take care that the telescope has time to adapt to the ambient temperature before starting
collimation – this will need up to 30 minutes (otherwise you will not get a good star image). You may use the
fans to speed up this process.
During fine collimation, locate Polaris and view it with high magnification.
If the telescope is collimated well, you will see a system of dim rings of light surrounding a central bright spot —
the so-called airy disc. You will also notice a dim cross of light coming from this airy disc. This is the diffraction that
is caused by the secondary spider vanes. We have left this cross out of the next figures for clarity. What we want to
see is shown in Figure 12 – a central airy disc that is surrounded by concentric rings. However, it is much more likely
that the picture will be different – more like Figure 10. It is essential that you always center the star because outside
of the optical axis all stars do show some distorted images. Note: You will get those images only during moments
of perfect steady air because air turbulence will distort this image. However the collimation goal remains the same
– getting a concentric star image. So let us assume you have good seeing and your eyepiece shows you something
like Figure 10:
Try turning the main mirror collimation screws – when you are turning the correct screw in the proper direction,
you will notice that the distracting “tail” is getting shorter (Figure 11):
Remember to recenter the star after you have turned a screw. When the telescope is perfectly aligned, you will see
the picture of Figure 12 (when the air is perfect). During the use of your telescope you will get a feeling for this – it
does not make sense to spend a lot of time on collimation when the air is moving too much.
Important: Outside the optical axis (the middle of the field of view) all stars will be distorted and show tails
directed to the edge of the field of view. If you are using cheap eyepieces those star images will be even more
distorted because the off-axis aberrations of the telescope and the eyepiece will add up. For this reason it is
essential to recenter Polaris after every turn of a collimation screw because the turning of the screw will also
shift the star from its centered position. As mentioned before it may not be possible to get a sharp star image
during periods of turbulent air. In this case try to achieve a symmetrical image during collimation. If you don’t see
any more improvement abort the process. During nights of unsteady seeing it is not recommended to use high
magnification. Try to enjoy low magnification objects like nebulae, clusters and galaxies on those nights instead of
fretting over the bad conditions.
Collimation
Figure 10
Figure 11
Figure 12
