Lowell Observatory

Overview of Secondary Mirror

The DCT 1.4-meter secondary mirror (M2) allows astronomers to conduct research utilizing a suite of instrumentation in the Ritchey-Chrétien mode. The original M2 blank, fabricated by Corning, Inc., was made of fused silica, a specialized glass characterized by a low coefficient of thermal expansion and therefore a preferred material for telescope optics. Prior to polishing and figuring, the blank must undergo a special machining process called lightweighting. Lightweighting involves grinding out 85 variably-sized pockets of glass in a honeycomb pattern from the backside of the mirror blank thus rendering it 50% lighter -- a finished weight of approximately 500 pounds. Less weight makes the mirror stiffer and more stable, which in turn reduces the complexity and cost of its mounting mechanisms. Lightweighting also reduces the mirror’s thermal mass which mitigates seeing problems and improves optical performance.

Pictured here is the 1.4-meter lightweighted DCT secondary mirror (M2) blank.

The M2 prefabrication and lightweighting contract was awarded to Optical Surface Technologies, LLC (OST) in Albuquerque, NM, who began work on the original Corning fused silica blank in July 2008. In mid-May 2009, OST's lightweighting subcontractor, Mindrum Precision, was nearly 90% complete with lightweight machining when disaster struck. Excessive tooling force on the backside of the M2 caused a fracture on the face sheet under the pocket being machined. The damage, unfortunately, was irreparable. As such, Lowell was obligated to procure a replacement M2 blank, this one a fused quartz piece from Momentive Performance Materials in Ohio. Momentive delivered the new M2 blank to OST in June 2009 for initial curve generation and acid etching. Meanwhile, engineers from OST, Mindrum, and Lowell collaboratively designed an improved support system for the M2 to mitigate the risk of another breakage. On August 26, OST shipped the M2 to Mindrum who began machining the first pocket on September 2, 2009.

By the end of April 2010, M2 lightweighting was successfully completed. The lightweighted M2 is now at L3-Brashear in Pittsburgh, PA for final polishing and figuring. The DCT project team expects the completed M2 will be delivered to the Happy Jack site late summer 2011 for integration into the telescope.

Visit our public photo tour site to view more pictures of the secondary mirror lightweighting process.

Discovery Channel Telescope – Technical Information

Overview of Secondary Mirror