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collaminate 8" sky watcher Dobsonian ?

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Hi i am in learning how to collaminate my scope and have been watching videos on how to do it but am a bit stuck with adjusting the primary mirror - in the picture i took do i adjust the 3 screws to adjust the mirror ie turn clockwise of anti clockwise or do i have to unscrew them all and take that outer plate off - the reason i ask this is because in the videos i have been watching the screws are not screws but chrome knobs ?

many thanks


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Hi....the 3 screws are your adjusters, the 3 Alan bolts are the locks............Unlock the 3 locks slightly, (Anti-clockwise) then adjust the screws ( Both Anti and Clockwise to make the adjustments. Note when you tighten the Alan bolts, do it gently, or you'll misalign the mirror again?

Doesn't matter which screw you start with, just make small adjustments and look at the result on the laser or through the tool in the focuser. Its great if someone else can adjust while you look through the focuser!

The chrome knobs you refer to are Bobs Knobs?  Custom built to allow manual adjustments without tools! their not cheap either, but handy.

Note: if you manage to unscrew one completely, don't worry. Just ensure the rubber 'o' ring is still in place, and screw it back in. Its like a milking stool, adjust one leg/screw at a time.

Some folk don't even bother with the locks, but their there for security. 

When you get very confident, and you ever need a Mirror Clean, that whole bottom end comes off by unscrewing the screws around the side of the telescope. Then the whole mirror cell will drop out, literally if your not careful so cushion its release? mark an external side  screw  position to a mark on the rim, for accurate reassembly.

Edited by Charic
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Hi Dazraz,

As you look at the picture, the cross-head screws at 1, 5 and 8 o'clock are used to adjust the primary. The small holes next to each of these screws have Allen head bolts in them, think they are 2.5mm Alllen key. These are the locking screws. You need to undo the locking screws i.e. turn anti-clockwise, in excess of 10 or 14 turns. next turn the cross-head screws clockwise until some resistance is felt, so as pull the primary mirror all the way down the tube.

Now start to collimate the primary by adjusting the cross-head screws, turning them anti-clockwise (has to be anti-clockwise as there should be not travel left as you have tightened the primary all the way down the tube).

Plenty of collimation guides on SGl and interweb.

Hope that helps,


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.........I generally adjust both ways, but as Uplooker correctly points out, its best to tighten first, then loosen to correct the tilt of the Primary mirror, but at a later date, you may just need a tweak either side of its set position!

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When I did my first collimation attempt I got it the wrong way round and spent ages fiddling with the Allen bolts thinking they were the adjusters rather than using the Philips head screws to collimate. You can probably imagine the frustration! :-) It's a good idea having a second person to do the adjusting while you check through the focuser.

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