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Dear Peter
Lay the tube horizontal, reach through the spider with one hand and grip the secondary mirror holder, then undo the central screw (between the three collimating screws) with the other hand. The holder will come off and can be eased out past the spider vanes. A spring and washer may fall off the screw. Just retrieve them for when you replace the secondary.
Only remove the spider if you need to get into the tube with a drill or similar (yes, I've done that!)
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