This picture is not an actual collimation step, but it will get some definitions out of the way and will make the next steps clearer.
First note that we have 4 collimation knobs on the back of the secondary holder. It would seem that a 3 knob unit would make more sense (as 3 points define a plane), but once you have owned both styles of holders, you will never go back to a 3 knob - the 4 knob models are much easier to control. A famous author from a national astronomy magazine used to downgrade scopes with 4 knob secondary holders. When questioned, he would always say he "had tried them both, and 3 knobs are clearly better". One day he bought a scope with a 4 knob unit and it forced him to really "try them both". He now only recommends 4 knob units. I'm not going to call him out in public, but I have a few of his old books as proof.
Next, note that the secondary holder is not centered under the spider. This is called Offset. When you read books about collimation, they always say "You can offset your secondary, but you don't really have to". These books must have been written before the fast (f/3.6 or faster) optics revolution hit the Dobsonian market, because if you don't offset the secondary, the light cone can collide with the inside of the UTA. On slower telescopes, the offset might be 4mm and not concern anybody. But on a large fast telescope the offset might be .6 inches! If you want a rule of thumb to live by we can say that if a scope is slower than f/4 it probably does not need the offset holder.
We get emails asking why we don't just make the spider itself offset, but if you do that, you end up with thicker diffraction spikes. The spikes in a 4 vane Newtonian end up reinforcing themselves. If you can't picture it in your head, think how a 3 vane spider gives SIX diffraction spikes at the eyepiece.
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