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Collimation on 12" Dobsonian

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Please can someone tell me which screws on the base of a Dobsonian are used for collimation?

There are 6 on the base, 3 with the springs and 3 which are flat screws in contact with the mirror housing.

Also should the screws with the springs be tight or just firm?

Any guidance much appreciated.

Many thanks


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The screws with the springs are the collimation adjustment, the other 3 are basically 'locking nuts'. I find that tightening the 'locking srews' can throw out the initial collimation and so end up collimating the scope twice!

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Yeah, you don't really need the locking screws. I never used them on my old 12". Just unscrew so they don't contact the mirror cell and get to it. The purpose of the locking screws is to support the mirror across a range of elevations and maintain primary collimation should the springs sag a little. If the springs sag then the best thing to do is replace the springs rather than mess about with the locking screws. Springs should be good, though.

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