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Orestis.
Mirror cooldown is mainly to avoid tube currents. If you have a chunk of glass sitting at the boittom of a tube sitting in a nice warm room at 15-20 deg then move it outside at night at 0-5 deg, the air around the glass warms up and moves and this shows up in the images especially at high power as stars and especially planets almost seem to "boil". On smaller scopes current really are the only issue, on larger mirrors thermal stability is more of an issue IMHO. I use a fan on my 12" and it definaitely allows faster cooldown. Once I am happy it is cool enough I turn it off eliminating vibrations, once the mirror is at ambient it isn't going to warm up again!
Truss tubes without a shroud abviously suffer less from tube currents, butr a shroud is still very useful if there are any lights from neighbours, streetlights etc nearby.
Not sure about collimation on the SW, I replaced the knobs on my GSO with Bobs Knobs and think they are the Bees Knees!
Haven't really got the hang of the cheshire yet, I just use a simple collimating cap, dust cap with a hole in it and a laser, seems to work fine for me!
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