This is a basic topic for light weight mirrors that needs to be addressed and is often misunderstood and used to deride light weight sandwich type mirrors. Fundamentally, before considering print-through, the first things to worry about are the overall stiffness of a mirror and proper mounting. The Hextek design offers the best overall stiffness to weight ratio of any light weight concept, a proven fact. If the entire glass structure “flops” around in different orientations due to gravity, print-through is a minor concern. If a mirror is held improperly or with insufficient supports these effects on light loss too far out weight print-through. If we decide to ignore light losses related to a lack of overall mirror stiffness and a poor mount we can address print -through.
What exactly is print-through? Print-through or quilting on a mirror surface is a localized slope error with the peak of the slope centered over cells that make up the light weight core structure between face plates (Fig. 1). Print-through is introduced during the optical fabrication process of grinding and/or polishing. As the lap moves across the substrate surface, downward pressure is applied by the lap to that surface (Fig. 2). The ribs provide a rigid support for this action but the unsupported cell center of the face plate bends (the degree of bending is dependent on face plate thickness) reducing the material removal over that area. The result is a raised area on the face plate centered on a cell (Fig. 3). The implications on degraded mirror performance can be significant in the form of light scatter. When viewed with an optical test, an obvious pattern of bumps can be seen centered on each cell.
 Fig. 1 Basic Light Weight Configuration |
 Fig. 2 Face Plate Deflection From Lap |
 Resultant Print-Through |
The effects of print-through can be significant so minimizing it is very important. The are a variety of methods to reduce print-through but by far the most practical and affective is to reduce polishing pressures. Other methods include; pressure compensation to offset lap pressures add complexity and fabricating sub-ribs into blank design is an ineffective measure to stiffen the unsupported face plate span. The classic rule-of-thumb is to keep the lap pressures limited to 0.2 psi. To be continued…
