The diagram shows the drop in temperature of one of our light weighted mirror subtrates. A warm (~30°C) mirror was placed in a cooler environment (~20°C) . A thermocouple inside the central support element and in contact to the backside of the face plate was read every 10min. The mirror was a 10" F5 mirror with an areal density of 22,5 Kg/m2. The thickness of the face plate was 4.3 mm. After 50 minutes the temperature differential was <1 C°. In this experiment no fan was used. Please mark that the temperature inside of the mirror blank.
In order to determine the bending characteristics (astigmatic mode) a three point bending test was conducted. The test piece was a 10" F5 mirror with an edge thickness of 43 mm and a total weight of 1.2 Kg. The lightweighted structure consists of 61 support elements in an FEM optimized distribution for minimal wavefront abberation due to gravity loads. The mirror was supported on two points separated by an angle of 180° on a 90% radius. Increasing Forces in the centre were applied by weights with a mass of 1.0 Kg, 5.0 Kg, 10.0 Kg, 15.0 Kg, 20.0 Kg, 25.0 Kg. For reasons of comparison a disc of solid glass with an diameter of 10" and a thickness of 8 mm was used. The flexture of the disc was measured to be 0.61 mm when a force of 243 N (25 Kg) was applied were as the flexture of the mirror was <0.02 mm and beyond reasonable measuring with this kind of test.
Print through or quilting are mid spatial frequencies produced by different bending characteristics of supported and unsupported areas of the optical surface during polishing. The most effective way to reduce print through is a reduction in polishing pressure and friction. Application of computer controlled polishing technologies in the final stages of figuring like ion beam figuring (IBF), fluid jet polishing (FJP), small tool polishing (STP) or magneto rheological figuring (MRF) are also applicable.
The picture shows an interferometric contour phase map of a 10" F5 mirror with an areal density of 22.5 Kg/m2 and 61 support elements (standard mirror). Thickness of the face plate was 4.3 mm. The mirror was polished on a polishing machine to a sphere using conventional pitch polishing with a sub aperture tool of 75% diameter. Polishing pressure was 0.2 psi. Polishing pitch was Gugolz #64, Polishing temperature was 22,5°. Long frequencies aberrations were removed in order to isolate mid spatial frequencies. The residual wave front aberration due to print through is ~0.03 wave RMS (532nm) this translates into a strehl ratio of >95%. Please mark that this result was achieeved with traditional pitch polishing. No sophisticated modern technics were used.