Optical indexing wheels, sometimes called “filter wheels,” are used to locate various optics or filters in a beam path to change the observational mode. Large wheels a half-meter in size or larger typically operate slowly, although the loss of a few seconds of time moving from one observation mode to another is not an issue. In certain rare applications, it is desirable to index between a large selection of optics at a faster rate. If the device is intended for space use, the power requirement becomes critical as well. Current indexing wheel designs typically use a central direct-drive motor to avoid the complication of gears, but these designs will draw too much power and work too slowly for certain applications. They also may require an ultrahigh bit encoder to achieve required positioning accuracy. Designers of such devices would readily admit that speed and power efficiency are both increased through the use of a gear reduction, but generally see the use of gears as problematic.

A cross-section of the optic indexing wheel.
In this new system, a large ring gear on the optic wheel is driven by small gears attached to multiple motors. Common 14- to 15-bit encoders are integral with the motors. Multiple motors provide both spaceflight redundancy and the ability to control backlash by underdriving or backdriving one motor via a small reverse torque. Multiple encoders can be used to maintain or even increase positional accuracy by averaging output signals. A mechanical labyrinth system and anti-migration coatings are employed in the design to eliminate contamination of the optics from either dust or lubricants. If the system must operate in changing temperature environments, the thermal expansion coefficients of the gear materials and the housing should be more or less closely matched, depending on the thermal excursions anticipated. A titanium housing would closely match a number of ferrous materials used for gears and bearings, whereas an aluminum housing would not.

The present invention takes full advantage of several modern, proven manufacturing and control techniques to create a faster, more rugged optic indexing system. In space applications, the use of lower-bit-count motors is of particular value, as high-bit, space-qualified electronics and encoders tend to be costly.

A high-speed optic indexing wheel would enable more observational modes to be done in a given time, effectively increasing the useful life of a telescope. In satellite Earth observing systems, a sub-one-second indexing capability means less missed data during mapping where various light filters are employed.

This work was done by James D. Moore of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO-49429

Photonics & Imaging Technology Magazine

This article first appeared in the January, 2016 issue of Photonics & Imaging Technology Magazine.

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