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The frequency of collimation checks is very dependant upon the type of scope. A solid tube newt is going to be more stable than a truss tube dob naturally. I always check my dob after assembly. As 90% of the time I am assembling in twilight or daylight, the couple of minutes needed is not taking away from observing time so why not check. It always needs a tweak but it is nice knowing that it is ready to go.
One of the first lessons I learnt with collimation was in relation to the locking screws found on many Newtonians. I always struck me as odd that we had an adjustment knob, a spring and a locking knob. If you tightened the locking knob, why was the spring there? And if felt like I had to do the primary collimation twice, once with the adjustment knobs and then again with the locking knobs.
Then at one of the early Snake Valley camps I attended, another user of the same scope advised my that the best thing to do with the locking knobs was remove them from the scope, walk down to the lake and pitch them as far into the water as possible. Al you need is adjustment knobs and good strong springs. To check the springs are strong enough t hold collimation, just move the scope to a variety of positions while checking.
Some scopes will have adequate springs, certainly the early GSO dobs had springs that were utterly inadequate.
The other thing to remember as far as primary mirror alignment is concerned is that the purpose of collimating the primary is to ensure that the coma free area of the image created by the scope is centred in the focuser (here I am assuming that the secondary and focuser have been setup correctly) and so get the best possible image. In a fast scope (<f5) the coma free area starts to get quite small so accurately centreing it is the way to go. Slower scopes have a larger coma free area so if it not precisely centred, the image will be OK as the stuff you are looking at is still within the coma free zone. That is why I regularly advise beginners who often have f6 or even f8 dobs and are getting stressed about collimation to relax and make sure they get it as close as they can and just go out and enjoy using their scopes.
Malcolm
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