Print through is the displacement (movement) of the unsupported face regions of a lightweight mirror as the polishing tool passes over them. As these areas bend, less polishing action is occuring. Conversely the area that has a rib behind it will bend less and will experience more polishing action. The resulting pattern, which mimics some or all of the rib patterns, is print through.

Print through should not be mistaken with common polishing artifacts. Annular ring zone(s) are most commonly caused from fixed-post polishing and/or sub-diameter tool work, where the tool's edges are allowed to repetitively hit the same radial position on the optical surface. These types of artifacts will be present in a solid mirror also. Such errors are not print through.

The left image shows a different Dream mirror that the Zygo color map (below the mirror) belongs to. The color map shows rib correlation with some, but not all, of the artifacts seen in the interferometry map.

The level of print through in the Zygo data was substantially higher than Dream's engineering showed. It was later learned that the outside vendor was using 45-90 times the polishing pressure the mirror was designed for. The vendor's pressures are extremely high, even for solid mirrors. The outside vendor was told what polishing pressures the Dream mirror was designed for before the first mirror was cast. The mirrors came within specifications for the project but it was a valuable lesson for Dream.

This is a great example because it not only illustrates what actual print through looks like but it also illustrates how Dream's design & engineering can be disconnected from outside vendor processing. Examples like this, as well as many others over the years, have pushed Dream to take over more and more over. Dream specializes in processing lightweight mirrors.

The vast majority of Dream's mirrors are designed for processing using conventional machines and polishing pressures. The mirror to the left included. It was designed based on 0.25psi of polishing pressure, which equates to a 50 pound, full diameter tool on a 16" mirror. Not exactly low pressure. This should put the high pressures used by the outside vendor into perspective.

Click here to see interferometry examples of Dream mirrors that were processed correctly.

Simple designs, with little to no (real) mechanical engineering behind them, using simplistic repeating rib patterns, with fixed height & thickness ribs, often reduce design-related print through by using a thicker face. The main reason this and other simplistic, repeating patterns are so common is due to the fact that they are so quick, easy and cheap to produce, as well as making a traditional optician feel more at ease. Quick, easy and cheap are three adjectives that are rarely associated with a high performance product.

The face of a lightweight mirror is one of the largest drivers for the mass of the mirror. For a typical astronomical application, which is some upward-pointing angle for the primary mirror, the added mass of the thicker face displaces (sags) more during the gravity load case (real-world use of the mirror), compared to a thinner-faced mirror. So although a thicker face can reduce print through during polishing (all things being equal), the mechanical performance of the whole optical surface in final use, where it will spend 99.9% of its life, is made worse.

Dream's internal polishing has proven that print through can be caused by a dozen or more factors related to the polishing machine and tool, not the mirror. Our thinnest face mirror to date (3mm thick for a 424mm physical OD mirror) has proven that we can provide our clients with the best of both words; a thinner face and a lack of print through.

A thicker face is detrimental to the mirrors Thermal Time Constant (TTC). In the final instrument such a basic design using a thick face will weigh more, force other components to be heavier, will not be able to hold the figure as well (glass) and it will take longer to cool down. These are the same negative attributes of solid glass mirror technology, which has changed very little over its more than 165 year history.

Traditional shops rarely test mirrors vertically, or in the actual mirror mount. Horizontally testing an optic puts the face (mostly) in shear, which means the face itself will displace less than the zenith-angle (vertical) case. When horizontal testing is combined with the lower polishing displacements of a thicker face, it gives the false impression that the optic is better than it really is. This is one of the drawbacks of viewing the product from such a narrow viewpoint. Dream is an engineering firm that has never been bound by this type of narrow viewpoint. Dream pursues the performance of the mirror in final use, not the performance of the mirror on a stand.