In order to slump or fuse glass for a telescope mirror one needs a slumping convex mold over which the glass is placed and then heated to slump/fuse points.Making a convex mold requires fist a concave mold into which the mold investment is poured.To make that mold takes more time plus if one follows the usual route of casting a solid cylinder one then needs to form it into a concave bowl.Lots of time to dry,lots of material needed to make it and this adds up in cost and frustration.Especially if one wonts to make several different focal lenght mirrors.
Well there is an easier,faster way that I've been using from the very start of my glass work back in '83.I realized I didn't wont to take all the extra time to make two molds for each focal lenght ,nor pay the cost of material.
It occurred to me that one could use a variation of the technique one uses to make corrector plates,but without the vaccumn part.Any material will deform if you apply enough pressure on it.Wood and any plastic sheeting works fine for our purpose.You make a accurate ring support for the outside edge slightly larger than the diameter of the needed mold.You then place a bolt or screw in the dead center of the form material (be it wood or better yet plastic sheets).You then screw/bolt down the centre of the form material into the base until,while using a long straight edge and vernier micrometer you reach the needed sag depth for your specific focal lenght and mirror diameter.
Once the mold investment is cast ( the edge of the ring/form needs to have a collar and be sealed of course) and dried enough you can slip it off the edge of the form.I generally apply several coats of car wax to the surface of the plastic sheet form (plexiglass or lexan and the like) before casting the investment.Now the form is useable again for any other sag depth.This approach is also good for making grinding tools and polishing tools at the same time.
For really fast mirrors such as a f/2.8 what I've noticed is that the central 2 to 3 inches around the bolt/screw deforms into a slight cone (about 1/32 inch deep),not a smooth sphere so you have to glue an extra layer of plastic underneath the central area to take the extra pressure that is needed to get to the deeper sag depth.The screw head needs to be counter sunk into the first plastic layer (or wood).
Well there is an easier,faster way that I've been using from the very start of my glass work back in '83.I realized I didn't wont to take all the extra time to make two molds for each focal lenght ,nor pay the cost of material.
It occurred to me that one could use a variation of the technique one uses to make corrector plates,but without the vaccumn part.Any material will deform if you apply enough pressure on it.Wood and any plastic sheeting works fine for our purpose.You make a accurate ring support for the outside edge slightly larger than the diameter of the needed mold.You then place a bolt or screw in the dead center of the form material (be it wood or better yet plastic sheets).You then screw/bolt down the centre of the form material into the base until,while using a long straight edge and vernier micrometer you reach the needed sag depth for your specific focal lenght and mirror diameter.
Once the mold investment is cast ( the edge of the ring/form needs to have a collar and be sealed of course) and dried enough you can slip it off the edge of the form.I generally apply several coats of car wax to the surface of the plastic sheet form (plexiglass or lexan and the like) before casting the investment.Now the form is useable again for any other sag depth.This approach is also good for making grinding tools and polishing tools at the same time.
For really fast mirrors such as a f/2.8 what I've noticed is that the central 2 to 3 inches around the bolt/screw deforms into a slight cone (about 1/32 inch deep),not a smooth sphere so you have to glue an extra layer of plastic underneath the central area to take the extra pressure that is needed to get to the deeper sag depth.The screw head needs to be counter sunk into the first plastic layer (or wood).
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