one of the collimation screws is doing. When you have centered all images as in figure 7 the rough collimation of your telescope
is finished.
Fine Collimation
During fine collimation we watch Polaris with high magnification. 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. 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 off the next figures for clarity.
What we want to see is shown in figure 11
– 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 9. Please notice that it is essential that you always center the star –
outside the optical axis all stars do show some distorted images. Please also notice that you get those images only during
moments of perfect steady air
– 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 9:
Figure 10
Figure 11
Figure 9
Try turning the main mirror collimation screws
– when you are turning the right screw in the right direction, you will notice that
the comatic tail is getting shorter (figure 10):
Remember to recenter the star after you have turned a screw. When the telescope is perfectly aligned, you will see the picture
of figure 11 (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. During those nights it makes no sense in watching the
planets with high magnification
– you will not get a sharp image even with perfect collimation.:
Figure 10
Important:
Outside the optical axis (the middle oft he field of view) all stars will be distorted and show comatic tails with the
tails directed tot he edge oft he 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
symmetric image during co
llimation and abort the process, if you don’t see any more improvement. During those nights it is not
recommended to use high magnification. Try to enjoy low magnification objects like nebulae, clusters and galaxies in those
nights instead of fretting over the bad conditions.
