"Red for Fred"
Background
Recently, I volunteered to assist another amateur astronomer whose particular interest is solar observing. He needed a very long focal length lens, both to get a good image scale, and also to keep the convergence angle of the rays coming to a focus very small. Fred will be using an interference filter to isolate the hydrogen alpha wavelength, and interference filters expect parallel or very near parallel light.
One of the side effects of using only one wavelength and a very long focus is that a simple lens will do very well. For Fred's f/30 requirement, a 62" ROC plus a flat back side will yield the required 120" focal length. Some design work showed a K of -0.5 (0 is a sphere, -1 a parabola) would yield near-zero spherical aberration.
I had some red and yellow filter glass in storage, purchased some time back for no particular reason, other than a low price. I thought they were a bit thin, but might make good Cassegrain secondary mirrors, as the red, high quality glass would facilitate testing through the back. When the request went out for a volunteer to make a 4" lens, preferably plano-convex, preferably out of red glass, it seemed like events had converged, and who was I to stand in the way of fate?
Making the lens - First steps
The glass I had was 5.8" in diameter, but after trading a few emails, we decided to go ahead and make it for 120", as f/30 was the recommended focal ratio from the filter manufacturer. Fred will stop the lens down to 4" in use, but have the full aperture available for unfiltered viewing if he so chooses. The filter blanks were polished out, and the faces were flat to a few fringes, and very smooth, so I decided to try and protect the best face of the disk during grinding, and touch it up when polishing.
I used my diamond generation setup to cut the convex on one of the filter blanks. Here's a "before and after" picture....
It looks like the generated blank is smaller and much thinner, but that's just an illusion. As you can see in the next photo, I didn't have the diamond wheel perfectly centered, so there is a small (0.1") bump in the center that was untouched.
As I still don't have all the kinks worked out in my generation setup, the radius came out too short, more like 46" than 62". So, I had to regenerate the curve, this time setting the tilt of the cutter well under what the calculated value. I ended up about 20" long on the curve, and decided to just grind that out, rather than fiddle with the generator any more. I also cut a matching concave on another piece of filter glass to use for the tool.
I decided to grind out the generation marks with 220 carbo, and that went fairly quickly, taking a bit less than an hour by hand. My plan was to set this up on my new grinding machine, but I couldn't get the stroke short enough to suit me, so I did it the old fashioned way, using a pizza and iced tea powered grinder, instead of an electric one.
Here you see the lens after about 30 minutes of 220. The generation marks are still visible at the edge of the lens....
Generating left some wedge in the disk. Wedge in a lens (uneven thickness around the edge, due to the faces not being parallel to each other) is a Bad Thing, as it makes the lens a prism as well, so I spent some time at the 220 stage to get it to around 0.001" As anyone who has ground a lens can tell you, you often end up "chasing" the wedge around the disk, and I did a little of that. Finally, some differential pressure, and an "arc" stroke, with the low side of the disk as the pivot, ground it to an acceptable level. The wedge should be reduced as low as possible during each stage of grinding, with less than .0005, and preferably zero remaining before polishing.
Here is the lens at the completion of 320 carbo. The wedge is gone, and the surface is very smooth and right on radius.
[Photo to follow soon]
[More to come Real Soon Now....]
Current Status
The lens has been fine ground through #320 carbo and 25 micron and 12 micron Microgrit. I'm going to try a few very careful wets of 5 micron, as I'm concerned with scratches with this very fine grit on this glass. I've also returned to the machine, and done a few wets with it. It's not really any faster, but I'd prefer to do the required hours of polishing on the machine....