The question remains as to whether the accurately measured geometric centre of the mirror is indeed the optical centre - lowest point in the paraboloid - if I have my nomenclature right (sorry mirror-makers). But I guess it won't be much out? Or could it be?
But, before that, just do it by eye. You can estimate when you have the secondary tilted so the spot is close to centre of the primary. Sure, it's not as accurate as if it were spotted, but it's better than nothing, right, especially if the collimation is a long way out. Worth a try to see how the image improves, before you go pulling the primary mirror out and risk dropping it etc.?
Ash, re your description of collimation:-
"After putting the laser collimator into the focuser and making sure its flush i align the secondary mirror so the laser beam dot is in the little "donut" in the middle of the primary mirror. I check the collimator and make sure the laser beam dot is hitting the hole in the collimator so the laser beam dot seems to disappear.
( by hole what i mean is where the laser beam comes out from) "
I am a little confused. If you achieve this (laser spot is hitting centre of primary and reflecting back down the hole in the collimator), I would stop there! That's it!
Normally when you have the secondary tilted correctly so that the laser is hitting the centre of the Primary, the Primary tilt is likely to be a little out so the return spot is not going down the hole. Then, as you describe, you adjust the Primary tilt knobs until the return beam goes down the hole.
The additional suggestions are:-
1) As Brendan says - check if the laser collimator itself is well collimated. If it is a snug fit in the focusser, try to rotate it without rocking it and see what the spot does on the primary. If it turns a large circle - say more than a few mms, probably the size of the spot - then the laser collimator needs a bit of adjustment - come back to us. Alternatively make up some form of V-block or tool, or put it in a lathe and watch the spot on a wall about 5m away as it is rotated.
2) Barlow that laser! Read:-
www.smartavtweaks.com/RVBL.html and also search for other articles on "barlowed laser".
Keep searching the threads here - there are pages of arguments on collimation for your enjoyment!
One further comment - those "lock" screws on the primary. Experiment a little. Once you have the laser disappearing back down the hole, move the scope in altitude from horizon to zenith and see what happens. If the return spot moves a lot on the angled face plate (say right out of the hole) then your mirror cell is moving. Usual culprit is weak support springs. Two solutions - replace them with stronger springs, or pull the primary down with all three screws to put the springs under more compression. Here is where doing up the lock crews may help retain collimation. However, most people have noted that nipping them up on these Chinese scopes usually pushes the collimation back out again!
I only do mine up for transport. Once set up, I wind them back and they stay that way until packup time.
Collimation Assurance!
