Polishing The Mirror


Making a 4.25 Inch Dobsonian Reflector Telescope




          At this stage, you see some magic starting to happen.  You secure the mirror blank on the cell, put a small dab of polishing compound, usually cerium oxide, add a small amount of water, and mix it gently with your finger.   Then you carefully place the pitch lap on top and rub back and forth.   Like the grinding stage, you turn the tool often, and then less often turn the mirror underneath.   By stroking across the center, you are digging slightly more toward the center.   This is what you want as you are going for a paraboloid.   You want to do this until the entire surface is very glassy looking.   Do not be in a hurry.   Make sure the entire surface is polished out before preceding.  Unlike the grinding stage where you can let the tool provide most of the weight, in this step you do want to put down a fair amount of pressure.   This requires additional care as you don't want to overlap the disk more than a third.  Do so puts you at risk of chipping the pitch.   If you do hear or feel the pitch chip, stop immediately.   Remove the tool and clean everything off with water.   If you continuing polishing over a pitch fragment, you will get an almost un-removable scratch.   You might need to go back to the grinding stage to remove it.   Now a certain amount of scratching is not really a big problem, it's mostly an esthetic one.