POLISHING, LAPS, FIGURING, AND TESTING

a few comments for the

International Optics Workshop

Bellingham, Washington, USA, March, 1999

by Mel Bartels
 
 

Most Common Polishing Mistakes Amateurs Make

  • not understanding that polishing is all about removing pits left from fine grinding
  • not judging that precise moment when the temperature of the pitch is best for channeling and pressing
  • not enough pressure
  • stroking too fast
  • adding polishing compound at precisely the wrong time
  • not maintaining a consistent but slightly varying pattern of rotation of both tool and mirror
  • glancing at the figure before polishing is done - forgetting that the goal is solely to remove fine grinding pits

    • Simple Fast Tool and Lap Making

  • make tool from layers of plywood (8 inch tool = 1 layer of 3/4 inch plywood, 12 inch tool = 3 layers)
  • make top of tool convex with layers of doorskin so that the pitch has constant thickness
  • use aluminum channel piece to make a couple of channels to aid in pressing pitch
  • do not make traditional channels, instead -
  • scratch surface under running cold water with X-Acto knife for microfaceting
  • use of hot to soften pitch for pressing
  • use of cold water to freeze pitch for polishing

    • Lap Patterns for Figuring

  • very slightly softer pitch on full sized tools for large fast mirrors
  • scrape away thin layer of pitch with X-Acto knife to vary percentage of pitch in contact with mirror
  • combine action of small parabolizing tool with large smoothing lap by full sized lap with gradually less pitch in contact as the pitchlap edge is approached
  • can also parabolize with full sized lap and drastically tapering pitch contact coming in from the edge, such that the edge has 100% pitch in contact tapering to 50% contact at the 70% zone to almost no contact in center

    • Oversized Laps to Control the Edge

  • oversized laps used almost from the start of glass mirror and pitch tools in the later 1800s
  • Strong's Procedures in Experimental Physics from mid 1900's makes a big deal of oversized laps
  • it really does work!

    • Rectangular Lap to Fix Turned Edge

    small lap of 2" x 4" for a 16" mirror used parallel to edge with elliptical strokes

    When is a Mirror Finished?

  • ignore temptation to calculate numbers, instead concentrate on appearance in Foucault, Caustic, and Ronchi tests,
  • and always star test with the final tests over a couple of evenings