Another One-of-a-kind Telescope Mirror!

While I've posted about the Large Synoptic Survey Telescope (LSST) by both describing the project and updating recent work, this week we started something never done before with any other large optic. We started hogging out another curvature into the substrate - for another mirror! It was decided early in the project that instead of making each of the 3 mirrors separate substrates, the primary and tertiary (1st and 3rd reflection) would be built into a single piece of glass. As far as I know, this has never been done on anything larger than meter-sized optics. As shown in these images from the LSST website, the inner (M3) surface has a considerable shorter radius of curvature, allowing it to be built into the M1 surface.

Even with the gentle looking curves of the above figures, given the huge 8.4 meter diameter of the mirror, something in excess of 5 tons of glass needs to be removed. We are taking off about 8 cubic inches per minute (about 2 cc/sec) so we will be doing this for a couple weeks of extended shifts. There haven't been any issues yet with the 2 curvatures, and we hope there won't be any. After the diamond generating, our polishing machine has 2 polishing heads, so each surface will have a dedicated tool to allow work at the same time (though likely not simultaneously).

The images of the diamond wheel above show the view down the ventilation system (to remove coolant mist)showing the M3 curve, and the shot with the flash freezes the wheel rotation to show the pellets and coolant droplets. The safety roller is seen at right which provides real-time feedback to the computer of the glass position. The third shot at left shows the reflection of Kirk, the machine operator, in the reflection of M3.

LSST is a seriously cool project - stay tuned!