Print-Through and Quilting
What is it and how does it affect optical performance? This is typically associated with light weight mirrors. More significant than print-through is the overall stiffness of a mirror and a proper holding fixture. These two factors influence a mirrors optical performance to a much higher degree that surface print-through. The Hextek design offers the best overall stiffness to weight ratio of any lightweight concept. If the entire glass structure "flops" around in different orientations due to gravity, or is supported improperly, there is far more of an effect on light loss compared to 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 the unsupported cell area of a lightweight structure. The feature is introduced during optical fabrication i.e. grinding and polishing. The basic configuration of a cellular structure is shown below (left figure). As the lap moves across the substrate surface, downward pressure is applied by the lap to optical surface (center figure). The ribs provide a rigid support for the pressure, but the unsupported cell center of the faceplate bends or flexes (the degree of bending is dependent on faceplate thickness and pressure), reducing the material removal over that area. The result is a hill on the faceplate centered on a cell (right diagram). The implications on mirror performance can be significant in the form of light scatter and degraded image quality. When viewed interferometrically or even a star test, an obvious pattern of bumps can be seen centered on each cell.
There are a variety of methods to reduce print-through, reducing the polishing pressure is an easy and affective solution but does add time. Other methods include pressure compensation to offset lap pressures by pressurizing the cells and adding sub-ribs into blank design to stiffen the unsupported faceplate span. We typically use low-polishing pressures to avoid the potential problem. We have found that for 1.5 meter class mirrors and under, this method is uncomplicated and effective.