I am attempting to repair a chipped 2.25-inch E & W Optical Cer-Vit diagonal that I want to use in a new 8-inch F/5 telescope. The diagonal was in a 10-inch F/5 telescope that fell over in a high wind event at a star party. The diagonal was ejected from the front of the telescope tube and suffered a scratch and a small clam like chip at the tip. The chip measured about 0.018 inches deep. The diagonal is 0.50 inches thick.
The first photo shows a 4-inch Zerodur reference optical flat on the diagonal after I removed the coating. You can see the area where the chip is located at far tip of the diagonal by the missing portions of the fringes. I used a small monochromatic light and was viewing the diagonal at an oblique angle, which explains the curved fringes. I need to build a suitable light fixture for fringe testing.
I tried to find a similar size diagonal to replace it. There is a substantial jump in standard diagonal sizes going from 2.14 inches to 2.60 inches. I could not find one available near my desired 2.25-inch size.
To rework the diagonal, I needed to affix it in the middle of a larger round disk and fill in the area around the diagonal with small pieces of a Cer-Vit so that what is being worked is more or less circular in shape. It is important to use the same type of glass as the diagonal for the fill-in pieces so that their wear rate is the same as the diagonal. It would be practically impossible to figure the diagonal if the wear rate of the fill in pieces is different from the diagonal.
Fortunately, I had a 0.50-inch thick slab of Cer-Vit laying around and I used a tile saw to cut it up to create the fill in pieces. This pushed the tile saw to its limits. I purchased a flat electric griddle to heat up the disk, diagonal and fill in pieces.
I used a 7-inch Cer-Vit disk as the base. The disk could have been made from Pyrex or similar material but not plate glass.
Late last fall I attempted to affix the diagonal and fill in pieces to the disk out in the garage. I sprinkled small chunks of Gugolz 73 pitch, which is on the hard side, onto the 7-inch disk. I did not check the level of the griddle before starting. After the pitch melted, I placed the diagonal and fill in pieces on the disk. After that the diagonal and all the fill in pieces slid right off the disk. It made quite a mess. Cold weather was setting in, so I waited until this spring to try again. In the meantime, I made an 8-inch F/ 5 mirror.
I was successful this spring in affixing the diagonal and fill in pieces to the disk after leveling the griddle. After the assembly cooled, I cleaned off the pitch that was on the surface of the diagonal and fill-in pieces with a razor blade.
About 20 years ago I bought some Pyrex blanks from Newport Glass Works for the purpose of using them as grinding tools for mirrors and optical flats. I asked Newport to use a diamond saw to cut groves in a rectangular pattern into the surfaces to better distribute the grit.
I put a 10-inch Pyrex tool face up on my grinding and polishing machine and started grinding with #80 grit. My initial goal is to have the entire faces of the fill in pieces and the diagonal have a ground surface.
After that I will switch to #120 grit and will start to control the curvature and make it flat. I will use a spherometer to measure the curvature.
Mark
Attached Thumbnails
Edited by PeteDCard81, 21 May 2025 - 10:13 AM.
