The Canada-France-Hawaii hosts a world-class, 3.6-meter optical/infrared telescope. The observatory is located at the 4,200 meter summit of Mauna Kea, a dormant volcano located on the island of Hawaii. The mission of the CFH Telescope (CFHT) is to provide its user community with a versatile and state-of-the-art astronomical observing facility.

The CFHT’s primary mirror is positioned on a set of pneumatic puck assemblies, and three hardpoints for stability. This design maintains the mirror figure throughout varying gravity vectors by modulating the pneumatic pressure that follows a cosine curve from zenith to horizon. Twenty-four pneumatic puck assemblies are placed around the mirror cell in two concentric circles, with fifteen pads forming the outside ring and nine on the inner ring. Load cells are mounted on each of the hardpoints to monitor the pneumatic support performance.

A geometry constraint, bending of mirror cell, even with compliant pads, will cause uneven pressure support on the mirror causing unintended bending and stress concentrations. Incorporating a pressure constraint allows the mirror support to be independent of mirror cell bending.

The original cosine regulator incorporated a passive, mechanically based, open loop system, using mass and gravity to regulate the pressures driving the inner and outer rings of the support system’s pneumatic mirror pucks. Although this had been a reliable method for years, the mechanical regulator required constant air bleed to maintain the proper pressures. This resulted in a very high air compressor duty cycle, which meant excessive electrical costs and air compressor usage.

A full evaluation of the system indicated that incoming pressure to each regulator needed to be supplied by at least 0.0353 CFPS at 72 psi. That is 3049 cu-ft at 72 psi/day/regulator. Pressure to each controller had to be regulated to 35 psi +/- 2.85 psi. Controllers (regulators) were required to be aligned in two planes to within 2 arc-min. An alignment error of 3 minutes would cause a system error of 0.05% (0.005 psi) at a zenith angle of 90 degrees.

Each of the nine inner pads provided 1086 lbf at zenith, and each of the fifteen outer pads provide 1034 lbf at zenith—totaling of 25,284 lbs (12.64 tons). Total load for the nine inner pads, fifteen outer pads, and three defining points needed to be 28,433 lbs.

Design Objectives

In addition to meeting the necessary specifications for fault tolerances and fail-safes to reduce repair time in the case of damage, the objectives for the new regulator upgrade included minimizing component count, using altitude-rated components when possible and minimizing the use of electrolytic capacitors. The new design also had to prevent control from faulty Raspberry Pi. Plus, the plan included using automotive-qualified parts and low-temperature dependent parts wherever possible. The new design would provide a high level of reliability. 

A secondary, but equally important aspect of the design was meant to achieve a high level of maintainability. This included having tested spare parts on-hand, minimizing the number of custom parts required, and using legacy CFHT components wherever possible. The design was also to use MIL-SPEC bulkhead connectors to the outside world. Finally, it was important to have online parameter tuning for the system. 

Proportion-Air provided the CFHT with a new highly accurate, closed-loop system that incorporated a microprocessor to control the company’s QBX electronic variable pressure regulators, while strategically placed pressure sensors provided the required feedback. Most importantly, the system does not require constant air bleed.

The new system was installed and tested extensively prior to being released for operations. It was calibrated and tuned to match the performance of the previously existing cosine regulator. Since the mirror support system was critical to providing image quality and damage to the mirror pucks could incur many man-hours of work, the new system was designed with multiple redundant features that allowed for either false-safe or fault-tolerant, depending on the failure mode. While all of the components were chosen to ensure high reliability and safety, the circuit board was specifically designed with hardware limitations that intentionally prevented damage to the system. 

Programming philosophies were adopted from the space industry to ensure robust, standalone operation. Proven command lockout schemes and control philosophies ensured safe command and setting of the new system gains. By design, the system prevents the pneumatic pads from over-pressurizing.

Cost Savings and Improved Accuracy

The original system cost was around $65,000, which today translates to roughly $400,000, including non-recurring engineering (NRE) costs. The pneumatic replacement system—including the necessary regulators—cost only $5,000, primarily thanks to its simplicity and not having to start from scratch. An estimated $17,000 in annual energy savings is expected to be realized, and other optimizations to the dry air system could increase the savings even further. Ultimately, the customer reports that the new system, using Proportion-Air regulators, is much more accurate and does not require constant bleed. This reduces the air compressor’s duty cycle, electrical cost, and usage.

For more information: 

Proportion Air

CFHT

QBX Pressure Regulator

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