Lowell Observatory

Overview of Optical Coating System

Once the DCT is operational, it will be necessary to strip and recoat the primary and secondary mirrors periodically due to natural accumulation of contaminants on the mirror surfaces which degrades reflectivity and optical performance. Aluminum is the deposition material of choice given the scientific requirements and expected instrumentation covering wavelengths from 300nm ultraviolet through the near infrared.

Graph comparing reflectivity vs. wavelength of aluminum, gold, and silver

To facilitate cleaning and recoating, a custom designed Optical Coating System will be located inside the DCT auxiliary building at the Happy Jack site.

The Optical Coating System includes the hardware, controls, and support equipment required to deposit a controlled-thickness, highly reflective coating of aluminum to the DCT primary and secondary mirrors. It is comprised of a chamber with vacuum and deposition equipment to enable application of the reflective material to the mirror via an evaporative process.

The contract for design, construction, delivery, and commissioning of the DCT Optical Coating System was awarded in Spring 2008 to DynaVac, Inc. located in Hingham, Massachusetts.

A rendering of the DCT optical coating system

DynaVac has extensive experience designing and building vacuum equipment including optical coating systems and large thermal vacuum chambers for the testing of space systems. Recently DynaVac commissioned the Maui Space Surveillance System Optical Coating System.

DynaVac worked with DCT engineers to complete the final design of the Optical Coating System in late 2008 with fabrication starting immediately thereafter. The system underwent acceptance testing in May 2009 and was successfully delivered and installed at the Happy Jack site in July 2009.

Click here to view photos of the fabrication process.

Optical Cleaning and Re-aluminizing Procedure

Cleaning and re-aluminizing the 4.2m primary mirror begins with the removal of the primary mirror cell from the telescope and lowering it into a specially-designed cart, which is then rolled along two rail tracks into the detached auxiliary building.

Rendering of DCT main facility and auxiliary building which houses the optical coating chamber

The mirror is then transferred from the cell to the wash cart, where the optical surface is carefully cleaned and the aluminum layer removed.

After the mirror surface has been cleaned, the mirror is then lifted using the auxiliary building crane and the wash cart is rolled away from the wash area and the bottom half of the vacuum chamber is rolled into position under the mirror. Next, the mirror is lowered onto appropriate supports on the lower half of the vacuum chamber. The chamber is then sealed and the deposition process begins. After the re-coating is complete, the transfer process is reversed to return the mirror to the telescope mount.

Discovery Channel Telescope – Technical Information

Overview of Optical Coating System

Optical Cleaning and Re-aluminizing Procedure