Mirror Removal and Initial Inspection

Here are some pictures from our "mirror removal party", on the evening of September 23, 2004.

Sunset
A nice sunset, as we get ready to disassemble the scope.  Those present were Guy Hampel (who brought lights and extension cords), Phil Wall, and myself.
We fired up the generator for lighting and took quite a few measurements of the scope and the mirror positions.  Next, I did a quick star test of the optics, and Guy tried to snap some photos of the patterns through the eyepiece with his digital cameral.  With the documentation finished, we proceeded to remove the primary mirror and cell.

TubeMirror
A view down the tube, looking at the primary mirror and baffle.  The secondary mirror is above, a bit out of focus.  The secondary mirror was not centered in the tube and could rattle around in its holder, and neither mirror appeared to be in proper collimation based on a visual inspection of the secondary tilt and the star test.  The second picture is the mirror and cell, after removal from the tube assembly.  Phil Wall is standing behind it.  The mirror is 3.5" thick, about 16 3/8" in diameter, and weighs about 50 lbs.  It's one beast of a mirror.

focault1focault 2
I took the mirror and cell home, and washed off as much of the dirt as I could.  I placed the mirror on my large test stand to take a look at the figure, but the shadows weren't right - there was a huge amount of scatterred light.  Immediately I knew the coating, 6 years after Clausing did it, was finished.  Above are two views of the mirror under the focault test.  Illumination source is a blue LED.  For the image at left, the light spots are reflective portions of the coating that are scattering light out past the knife edge.  Dark portions are either failed coating areas or areas that are not scattering light.  The image at right has the knife edge removed from the beam more, so that the left edge of the mirror is more intensely illuminated.  This mirror has obviously experienced a severe coating failure.


In the interest of documentation, I wanted to illustrate just how bad it was.  So, I came up with a quick way to illustrate the failure.  Here are some pictures of the coating inspection, using a lamp to illuminate the mirror from behind  The whole-mirror view shows my reflection.  The closeup shows the true extent of the coating loss.

setup

Whole mirror

Closeup

Some have told me that Clausing coatings (at least the Beral variety) fail when exposed to salt air, and when exposed to temperatures above 100 degrees F.  Moisture could also be a culprit.  In any event, I am quite sure the coating has been in bad shape for some time now, likely at least a year or longer.  So, I'll rate the 5-year lifespan of this coating as poor, and the coating itself as unacceptable.  I will recomment overcoated aluminum for the next coating, from L&L Optical Services in CA, Spectrum Coatings in FL, etc.

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