Well when I got back to Australia it was time to start work on the first of the secondary mirrors for the 8" DK bino-scope that I had decided to build.

To make making the the secondaries easier I went with 108mm blanks, I know that is quite large considering that the finished mirrors would only end up being 67mm in diameter, but this would remove the danger of a turned edge in the finished mirror and besides a 4" disk is easier to hold onto than a sub 3" one is. Anyway after the usual hogging and grindine etc. with one departure to aquire a large chip in the "tool" which was to become my referance plate, I eventually got to the polishing and figuring phase of the undertaking.

In this picture you can see the chip in the glass.



The chip is about the size of your thunb nail and this photo was taken during fine grinding. Needless to say it persisted right through to polishing and beyond. One effect that wasn't evident, but that I should have expected was that the cracks in the glass would have an effect on it's usefullness as a referance. These effects weren't evident during testing, but I noticed a distinct astigmatism when I began to use it as the referance for the convex secondary.

Here is a picture of the secondary being fringe tested.



This is actually the "last" test I performed on it the error, if I have it right is, 1.5 fringes or 0.75 wave at approximately 530nm. As the primary is already coated I will star test the system and correct any system errors by figuring the secondary.....

The next step is to, once everything tests out OK, is to core out the secondary from it's blank and then have it coated.

Back.