Our 4 1/2 year wait for the 42-inch Pyrex mirror blank came to an end in August 2002. Wangsness Optics, in Tucson, fabricated it for us. Here is a picture of the ceramic molds after they were glued and machined. These triangular pieces will become hollow spaces in the final mirror blank.
Here is a picture of the same ceramic molds but with their corners rounded. Notice the curve in the top surface. That will match our final intended curve on the front surface. The curve that you see here will thus ensure that the face plate is of uniform thickness when we are all done.
Here is a picture of the first core as it is bolted down in the furnace. The core is the top piece, and it rests on a base plate that sits on top of firebrick. There is a specially fabricated ceramic bolt that has been inserted from the bottom into the triangular core piece that will hold it down and keep it from floating when it is surrounded by molten glass in the furnace.
Here are all of the cores bolted down and the surrounding firebrick ready to go.
And now 295lbs of Pyrex glass is added on top of the furnace. Notice edge of the turntable at the base of the firebrick. The entire setup can be rotated at 1100oC!
The furnace is turned on, it is rotated (in our case at about 10 RPM). The glass melts and flows down around the ceramic cores, it is slowly cooled and annealed, and if all goes well, we have a mirror blank! Since it was rotating while the glass was molten, it formed a roughly parabolic curve in the top surface. So we did not have to grind the curve from a piece that started out flat. The curve in the top surface will ultimately match that curve that you saw above in the ceramic cores so the top plate will be of uniform thickness. What a sight!
Here is a side view once the firebrick is removed. You can see the ceramic cores, now surrounded by glass inside.
Gorgeous, eh? The face plate came out a little under 1.1 inches thick. The diameter is 42.4 inches. There were 50-100 small bubbles in the top 2mm of glass, and there was a thin oxidation layer on top. We have removed this during our rough grind where we ground down through 3-4 mm of glass.
Let's get this baby home at El Camino College from Tucson and get going...