Crikey! I collimate before every imaging session - if there is considerable shift from one planet to another when we image multiple planets, I'll re-collimate again for the 2nd etc...despite pretty effective primary mirror locks, seeing conditions are always dynamic regardless of elevation - & the seeing might present a better chance to fine tune the collimation later in the session...star-collimating on a nearby star wherever possible.
I made a good artificial star years ago set up on an old EQ mount for fine adjustments for targeting - SCT's need a heck of a lot of distance (in a park etc) to get a decent result...worked much better spacing it just down the driveway for the Newt.
Seems like I know diddly-squat about collimating our C14...better delete that page on our website on some of the finer aspects* of collimation of an SCT: http://momilika.net/...3Processing.htm - must be doing things completely wrong by the contents of some of the posts in this thread...I wondered about some of the points I make there about various helpful aspects to look out for - such as the inner rings becoming more illuminated around the Poisson Point with a star defocused to a dozen or more rings...& that bit about a "strobing" effect (Pat calls that "helicoptering!") around the image - another confirmation that you're getting close!
They never seem to be mentioned in any of the "guides" I've ever looked at: then again, most of what you reference are C-G images etc...but taking some of the posts here seriously, I think we must be delusional!
*Ok, a little bit of an admission - our C14 spends about 1/2 its' time in the back of our 4WD vehicle so it does get a bit of "rock & roll"...but we do exactly what I said in my first paragraph here regardless, even if it is used continuously over time when set up on the mount in the backyard: we want maximum results dependant upon whatever seeing we encounter. (below a certain quality we simply pack it in)
I intend to expand our website guide to fully encompass collimating an SCT from "go to whoa" which essentially means the earlier stages & a simple means of working out what to do from the very start...note that this is all an "in camera" approach which does make it very easy for a single person to collimate & know what they are doing throughout - simply turning the laptop screen to face you as you work on the collimation. But its' essence can easily be translated to an ep collimation.
But those little "pointers" I mentioned in para.3 here are real & do assist in knowing you're well down the correct path...some folks who have permanent setups might well find that their collimation holds reasonably well over extended periods of time: but that sort of level of collimation would never satisfy us quite frankly...if you consider any combination of metal & glass etc componentry, it will inevitably experience at the very least mechanical expansion & contraction over time due to temperature, let alone if you utilise primary mirror focusing (which we do not) as well as (possibly) other types of stress so that in any circumstances the fine alignments you are enabling when collimating is never going to remain static or "set & forget" - well, certainly nowhere near well enough for our own demands!