Collimation problem

[shaneaust]

I've attached a drawing of what I see in my scope (10" reflector/dob) when I look thru my home-made collimation tool, a 35mm film canister.

The reflection of the "centreing" mark on my primary is a bit off-centre. My question is: what needs to be adjusted - the secondary mirror's placement in its holder, the arms of the spider on the secondary, or the primary mirror?

I've looked at a number of collimation-related sites but havent yet found the answer to this one!

Help?:)

[janoskiss]

That is not really enough to show what is going on. The position of the spider and secondary need not be centred (and won't be for a properly collimated 10" dob because of 2ndary offset).

Assuming your secondary is good enough, what you need to do is place something highly reflective on the back of the film canister and then illuminate it so that you can see its reflection through the the hole in the film canister in the focuser. Then adjust the primary tilt until reflection of the back of the film canister is centred on the centre spot of the primary.

PS. I've attached a more browser-friendly version of Shane's drawing.

[shaneaust]

When you say "place something highly reflective on the back of the film canister" are you actually saying somethng around the "inside" back of the canister, surrounding the hole?

[Starkler]

Yep thats what he means, or should mean ;)

[janoskiss]

Yes, the side facing the secondary mirror.

[ving]

assuming that the offset dot would be similar to say the crosshair on a cheshire i'd say the secondary needs to come dow the tube a bit.... or am i totally off base?