26 April – I ground too far
Last night I put in another hour with the #320 grit, smoothing away the pits and subsurface fracturing from the previous grit levels. It has not taken long to get to a nice spherical shape again, and after about 5 or 6 wets, I decided to prop up the mirror for one more rough optical test. Unfortunately, the new home has no garage, and it’s getting a bit chilly to work out on the balcony, so I’m doing my grinding inside. I’ve had to modify the spit test to avoid soiling our pretty clean floors, but the new method has turned out to be a little more accurate. Our dining room table has a seam down its middle (it’s a six-seater which folds and collapses down into a 4 seater – very space efficient!), which has proven handy. I wedge the end of my tape measure in that seam, and then prop up the mirror on a cushion of towels. The surface is lined up exactly with the seam, so it’s now a simple matter to measure the distance from my eye to the mirror. Result: Radius of Curvature = 194 cm, for a Focal Length (F) of 97cm, for a focal ratio (based on the measured diameter of 155mm) of f=6.26
This is too short, but only by a few centimeters – my target is F = 100 cm. For the next while, I’ll be grinding Tool-on-Top which will cause the curve to slowly grow more shallow and increase the focal length. I worked for about a half hour in this fashion.
One interesting bit of news: The bubble which I discovered last weekend at the ATM class has now ground out a little bit, and turned out to not be a bubble at all! It is clearly visible in the image to the left, and the shape is definitely jagged and irregular – it is a very large pit. Current strategy: Continue with the #320 until all large pits are gone, and no fresh ones appear. Then work a bit longer until I feel confident that I’ve ground down past the deepest subsurface fractures.