MANUFACTURERS OF CUSTOM, HIGH PRECISION INSTRUMENTATION AND SUPPORT EQUIPMENT

Projects

Roof Periscope

Time Period

Spring 2002 to Present

Project Description

A Lidar research laboratory at the NASA Goddard Space Flight Center has a hatch in the ceiling. That hatch opens at the rooftop to provide a clear view of the sky for instruments in the laboratory below. The top of the hatch is approximately 10 feet (3.05 meters) above the roof and is approximately 3 feet (91.4 centimeters) x 3 feet inside. Since the entire hatch structure resembles a chimney, it is generally referred to as "the chimney."

Typically an instrument in the laboratory can only view directly upward. Welch Mechanical Designs, LLC (WMD, LLC) was asked to design a periscope system that would allow instruments in the laboratory to point in other directions without moving the instrument. The job required mounting a two-mirror, elevation-over-azimuth scanner at the top of the chimney. The aperture needed to be large enough to pass an 11.8-inch (300-millimeter) diameter beam. The entire system needed to rise out of the chimney high enough for the system to view over the hatch lid when pointed horizontally. When not in use, the system had to be lowered into the chimney, allowing the existing hatch to close and seal. No modifications could be made to the chimney structure.

The periscope system was designed to meet these customer requirements: Two 12-inch (30.5-centimeter) x 17-inch (43.2-centimeter) x 2-inch (5.1-centimeter) mirrors were mounted in identical welded aluminum frames. The frame with the elevation mirror was held to the azimuth mirror frame with a large diameter, low-profile, custom ball bearing. The two mirrors were attached to the end of a 9-foot (2.7-meter) long tube with another of the custom bearings to allow the two mirrors to rotate relative to the tube. The tube glided up and down relative to a stationary frame attached to the inside of the chimney. This gliding motion allowed the periscope to rise high enough to view over the hatch and drop low enough to allow the hatch to close.

This version was designed with mirrors that are rotated by hand and has detents to lock the angle every 15 degrees. Design provisions allow the mirror motions to be motorized in future versions.

WMD, LLC specializes in custom tailored solutions. These mirror mounts can easily be adapted to the scanning requirements of other systems. The system can also be increased in size to act as a scanner for larger systems. Slew rates and pointing accuracy can be tailored to match most requirements. Please call us to discuss how we might adapt this system to meet your specific requirements.

Challenges and Lessons Learned

This project presented some tight budgetary constraints. Since quotations to fabricate an early version of the design exceeded the budget, we completely redesigned the system to make it simpler and reduce fabrication costs. We kept the entire project cost within budget. The lessons we learned on simplifying design and reducing fabrication costs will guide us in many future projects.