Possible Minor Degradation in the SolGel coating on Element 4

We recently removed the 4m prime focus corrector to install a "fix" for the scattered light situation noted below (April 5, 2000 tests). During our examination of the corrector, a "mottling" of the SolGel coating on the inside surface of the last element was seen. This may always have been present since it was applied in the summer of 1998.

The mottling can be seen in the purplish region of the reflected light as tiny white spots that form crude hexagonal patterns. The pictures below illustrates these. A report describing them was written by David Vaughnn.

Spots in the SolGel (1):

Close up of the spots:


4m tests April 5, 2000

The following image showing classical scattering was taken in twilight through an H-alpha filter of Zeta Pup (V=3.3, 120s) offset the correct amount to scatter (move telescope west 21', north 24.2').

The next image was taken after rotating the camera ~10 degrees (6 turns of the crank). If you work at it, you can identify stars in this field relative to the one above. Stars in this frame have moved up and to the right relative to those in the normal frame.

There is no evidence for scattering at all in the frame where the camera has been rotated.