So it begins…For my first homemade mirror i’ve chosen to start with a relatively slow (f/7) 8 inch Newtonian mirror, which should be great for a starter mirror and eventual planetary scope. From 2013 to 2018 I worked as an optical engineer with optical coating and component fabrication equipment. Our company’s specialty was lenses and objective assemblies under 1″ in diameter. Despite the focus on much smaller optics, luckily much of what I learned is still relevant for larger optics. For everything else, I have Texereau.
The mirror blank is made from Schott Supremax 33 borosilicate glass purchased from United Lens Co. I explored the option of having UL grind the appropriate radius concave spherical surface to avoid the monotonous rough grinding stage, but this option was prohibitively expensive. Grinding and polishing work will be done in my basement, which has steady air and a yearly temperature fluctuation of only about 10° to 15° F. Once I get to the polishing stage I may have to wait a few months depending on the basement temperature at the time. I’m currently planning to use Gugolz 64 or 73 (possibly a blend of the two) depending on the basement temperature once it’s time for polishing. The tile tool for grinding was made by following the technique outlined by Stellafane. First, a plaster disc is cast from Plaster of Paris using the mirror blank as a template (with layer of saran wrap between to prevent the plaster from bonding with the mirror blank). The plaster tool was then sealed with epoxy, and 1″ unglazed ceramic tiles were applied to one surface with more epoxy.
Rough grinding work was done using #80 Silicon Carbide. The “ASGH Wooden Barrel” grinding stand is also of Stellafane design, and has about 150 lbs of concrete ballast in the base for stability. A couple 5 lb weights were used to provide additional pressure between the tool and blank. The Chordal stroke with mirror blank on top was used for the majority of rough grinding to hog out material. As my curve progressed towards the edge of the mirror blank I slowly transitioned to the Normal stroke until finally reaching my target sagitta of about 0.070″. In total, I spent about 12 hours in the rough grinding stage. This definitely took longer than it should have since it was my first attempt. It took some time to dial in my grit/water slurry ratio to maximize cutting efficiency and grinding time between wets. Once this was dialed in it was just a matter of putting in the time and elbow grease to reach my target depth and nearly spherical shape. My 1.4″ spherometer indicates that i’m spherical within about 0.0005″, which should be more than sufficient as I transition into the fine grinding stage where the spherical shape will be further improved.