Gainesville, FL

GEC Instruments in Gainesville, Florida was awarded a contract from LSST Corporation in Tucson, Arizona, to build four ultra high accuracy, high resolution temperature measurement instruments, each with 94 thermocouple input channels, accompanied by software for display, plotting, logging, and retrieval of temperature data.

This instrumentation will be used to measure small temperature variations at a multitude of positions on the primary/tertiary mirror for the Large Synoptic Survey Telescope www.lsst.org, a ground-based facility that will be located atop Cerro Pachón in the Chilean Andes. The LSST system will combine a wide field of view, rapid scans of the sky, and deep-imaging capability to map billions of objects and monitor changes in brightness and position.

The GEC Instruments units will go in the space right below the mirror, which is 27 feet in diameter and 2 feet thick at the edges, represented by the white disk at the upper right of the image. The cost of producing the mirror is $20 million, and the budget for the entire LSST project is nearly $400 million over a period of several years.

This project requires ultra high accuracy in temperature measurements with thermocouples with a goal of ± 0.02 °C measurement accuracy on every channel in the range -5 °C to 25 °C, and unprecedented channel to channel uniformity among all 376 channels of the system. The temperature readings from the thermocouples will be used by LSST scientists and engineers in a real time control system to control mirror temperature gradients across the entire volume of the mirror as it reacts to changes in air temperature during the night.

Precise and reliable temperature control is crucial. The mirror surface must be kept below the surrounding air temperature at all times as it changes from -10 °C to 30 °C from night to day, but no more than 0.5 °C below the air temperature. And the mirror will distort if it is not uniform in temperature. During the night, the distortion of the entire mirror surface must be limited to within 0.2 microns ( 0.000008 inch) as the mirror changes in temperature -- else it will be out of focus. Thus the need for ultra high accuracy in temperature measurements, and extreme uniformity among all thermocouple readings of better than
± 0.02 °C.

The four thermocouple instruments, along with 400 calibrated thermocouples, were delivered in 2010 and the performance exceeded the specifications of the project. See some data from these instruments on pages 8 and 9 of the following article: Achieving Ultra High Accuracy in Temperature Measurements