Crowsfeet!

There are lots of little details that need attention when making one of the big mirrors at the University of Arizona's Steward Observatory Mirror Lab.  One of the details I'm currently monitoring is called "crowsfooting".  No, it has nothing to do with local wildlife, but rather, is a defect that sometimes takes on the appearance of a bird's foot.  Check out the image at left and I'll tell you what you are seeing.  Shown are Newton's fringes that result when a "test plate"(TP) is placed on the mirror surface.  The surface of the TP (convex)nearly exactly matches the concave surface of the mirror, and the monochromatic light makes fringes (the light and dark lines) that show the errors between the surfaces.  The feature in the center that generally looks like a fork is the "crowsfoot", and the fringes provide sort of a contour map that illustrates the height differences between the mirror surface that contains the crowsfoot error and the smooth TP. 

What causes a crowsfoot?  If you look closely where the fringes are most dense in the above photo, you will see a little circular feature, which is actually a bubble in the glass substrate that happens to lie on the polished surface of the mirror.  What happens in the early phases of polishing with a pitch lap is that if a bubble has sharp edges, it will scrape the pitch off the lap and fill the bubble with the gummy polishing material.  When the bubble fills, it will scrape the lap, but as it scrapes, it leaves a scrape or gouge with a raised edge (illustrated at left).  These raised edges vigorously polishes the mirror "downstream" of the bubble as the mirror and lap move.  Since the lap generally moves in and out relative to the rotating mirror, the resultant crowsfoot is generally forked reflecting the in-out motion of the lap going past the bubble.

Since the defect is many times the area of the little bubble, you want to "fix" them to minimize their effect on the mirror shape.  Generally that is done by beveling any sharp edges the bubble has so that the lap will pass over it without scraping the pitch lap.  It also helps to use a synthetic polishing material like polyurethane pads that don't promote crowsfooting.  Occasionally one needs to inspect the mirror surface foot by foot (nearly 600 square feet!) to look out for these and other defects and treat them.  Hopefully they will become smaller as material is polished off.  Shown here are comparisons of the same crowsfeet before and after the last polishing run.  As you can see, the affected area has been reduced dramatically, so a couple more polishing runs and they will be minimized...  Just one of the many details that need to be kept track of on a large mirror...