TRO+-+Fine+grinding

=__Fine grinding the front of the mirror__=

Now the back was flat we could re-grind the front of the mirror.



We made a tool 75% the size of the primary. The was made from two layers of 3/4 inch ply separated by a 3 inch inch layer of dense pink foam.

The tool was glued together with epoxy, sealed with fibre glass resin and then the front face pressed into the shap of the mirror with car body filler to get the approximate shape.

The mirror was then placed face up on the table and covered in two thin layers of plastic sheeting. These were held in place and smoothed out with a water mist.

Fibre glass resin was then poured into the mirror and the tool placed down on top. This gave a good surface of the correct smooth shape in which to bond the glass tile sheets.

The resin was left to harden. The tool was then placed face up and painted thickly with resin by brush. The glass sheets were then pressed in.

When dry the tiles then had another coat of resin to fill in the gaps between the tiles. Solvent was used to wipe up the excess off the front surface of the tiles.

The resin between the tile was only enough to cover and seal the cardboard matrix that the tiles were fixed to. Water getting in during fine grinding may have loosened some of the small tile squares.





Again, astigmatism was present early on. The primary having an uneven grinding pattern in the front. We ground away the old coating rather than remove it before grinding. This clearly showed inaccuracies in the mirror. The tool would skip and swirl as it tried to conform.

A few hours of grinding were done using 400 grit down to BM303 to finally smooth the surface.

The pencil test was extensively used to check conformance.