30+inch+F3+Grinding+the+front+surface

Grinding the front surface was done with a glass tile tool. This had been used on the 30 inch TRO mirror in the past and needed to be re-generated to F3. The tool is 24 inch in diameter.

This was done by replication using resin poured into the protected surface of the 30 inch F3 blank. The surface was protected with a think layer of grease and a thin layer of plastic with all bubbles pressed out.

The tool was then laid onto the resin and the resin flowing out to the edges. Care is required to ensure not too much resin is used and that it does not get onto the blank. The excess resin is carefully cut away while in the 'jelly form'. when the resin is hard (about an hour) the tool is carefully slide off to the side.





When the plastic is removed the surface matches the blank generated curve. The surface is the 'painted' with more resin and the tiles laid on and pressed in firmly.

More resin is then painted into the gaps of the tiles, but not so much as to fill the gaps to the top of the tile but just so much as to seal in the lower edges of the tiles together. Cellulose thinners is then wiped across the tiles to remove the excess on the surface. The mirror blank is the again greased and covered with think plastic to protect the surface. The mirror blank is then flipped and put done onto the tile tool to ensure the tile surface conforms the generated surface.











After 15 minutes of fine grinding with 30 micron microgrit and light strokes, the tiles are starting to bed in. There was a large central region that for some reason was lower than the rest? This is probably due to variance in thickness of the tile sheets to each other.

It is getting smaller quickly and the surface of the mirror looks fine. Pencil testing will follow when the tiles are 90% in conformity to the surface.

Grinding continued and after less than an hour the tool had bedded in.







After 4 hours the whole surface was of the same grade out to the edge. We moved onto 15 micro microgrit. This took then less than an hour to bring the whole surface to a fine finish ready for polishing. The pencil test again was used to watch the surface form. The edge seemed to lag a small amount but initial polishing will show how far out this will or will not be. The mirror is now boxed up ready for polishing (24/4/2015).







Grit and water slurry was 'painted' onto the surface. Geoff and Bren are doing some of the final fine grinding before starting polishing. Some test equipment will be made and the area cleaned out before polishing starts. We hope to complete polishing (and maybe figuring) by the end of May 2015.



As can be seen by the image below, the surface is similar to the edge bevel. The surface quality is consistent across the whole surface. A USB microscope was used to take images of the surface at different intervals to keep track of progress. The bevel is actually the larger lower surface. The smaller surface close to the mirror surface is only about 0.25mm wide and is were the generation tool has not quite met the edge of the bevel. 0.25mm loss of aperture is not worth worrying about!