Collimation

Collimation is the technique to align the lenses and/or mirrors in your tube to produce a pinpoint focus at 120 degrees apart over the the whole disc of the image. If one or two are out of line you'll get elongated stars. Telescopes with lenses have different ways of collimating than the mirror types. The one ill 'focus' on here is for the Schmidt-Cassegrains with the Bahtinov and Tri-Bahtinov masks. Well to be honest I won't do that myself as there is this more than excellent video from Matt's Colorado Astro Adventures explaining this whole process for crude to good to excellent.

So when do you have to perform this operation?
Well actually anytime when you have been messing around with, in the case of my SCT, the secondary mirror. In my case that comes down to each time I exchange the secondary mirror with my Hyperstar to reduce the focal length, widen the field of view and speed up the scope from f10 to f2. I have not yet done this but to my understanding even the Hyperstar needs collimation each time it is mounted.

Some really good advice? If you purchased the scope new from the factory... stay away from that secondary mirror if you have no accessories to place there. The manufacturer has collimated the scope and unless you drop the scope or bump it hard you won't need collimation for a long time.