First I centre the secondary under the focuser so that I can see all three primary mirror clips. Then I use my cheshre/sight tube to adjust the primary so that the black dot is within the annulus in the centre of the primary mirror. The I tighten the locking screws and check it again. All good.
Then I collapse the scope (8" skywatcher).
Then I move it. View for a while, everything looks great, stars are nice pinpoints, plenty of detail in nebulae, moon and planets where possible.
Move scope back inside. I leave the mirror horizontal to let it dry for a few hours before capping and covering. Check collimation, guess what, it's out by as much as 1cm sometimes.
Locking screws and adjusting screws are tight, as are the screws for the spider vanes. Sometimes the mirror moves out of collimation when I tilt it to the zenith.
What is going on? I have been trying to get this right for almost a year now and it will not hold collimation. I read about 8" scopes holding collimation for long periods of time, so I can't understand why i have to collimate every time I use the scope (can sometimes take close to an hour because it's extremely difficult to line up the black dot with the annulus on the primary - the adjusting and locking screws don't seem to be doing their job properly).
I am about to order primary and secondary Bob's Knobs and new springs because I originally thought the movement was due to weak springs (read a few threads here and on CN on this) but now I don't know if the problem goes further. Any advice/comments greatly appreciated before I'm committed. It's driving me nuts.
Cheers
Wade