The 14 inch lap was too large to introduce enough correction.
The solution was hand working with a 8.75 inch diameter lap on a glass base.
The 8.75 inch lap used to parabolise the mirror.
At the conclusion of polishing the mirror was tested and found to be in an excellent state. It was showing a generally smooth surface with some correction in evidence. Much to my relief there were no signs of a turned edge. It is most important when working with plate glass to allow the mirror to cool before testing. With this mirror I saw quite dramatic changes occurring as the mirror cooled. At one point the ronchi bands appeared slightly 's' shaped suggesting the presence of astigmatism, as the mirror cooled I was pleased to see the bands take on a more friendly appearance.
The 14 inch lap was too large to introduce the amount of correction needed by this mirror, so a new lap was made up on a piece of plate glass 8.75 inches in diameter and 3/4 inch thick. In order to have more control over the figuring job I decided to work by hand, with the mirror rotating on the machine table under the lap. The stroke I used was the classical 'w' stroke working across the mirror in slow deliberate strokes with a slight overhang. After 17 minutes work testing revealed the introduction of more correction, however the mirror still needed more work to push it towards the required paraboloid. Initial testing was carried out using the matching ronchi test. Working in spells of about 17mins each, the required correction was introduced in 1hour 45 mins.
At this point the predicted and actual ronchi patterns were showing an excellent match, next a foucault test was carried out using a 5 zone couder mask. Several sets of readings were taken using a slitless tester with a red led light source. The results are outlined in the next section on testing.
The picture to the right shows the 8.75 inch lap used to complete the figuring of the 19 inch mirror.