Viewing the Night Sky with Omega's Thermocouples

The Large Synoptic Survey Telescope (LSST) is a new type of optical telescope capable of viewing an extensive area of night sky than ever before possible. Once the LSST is operational, its tasks are to look for near-earth objects that could pose a collision threat and also capture short-lived cosmic events missed by standard telescopes. Over time, a detailed 3D map of the universe will be developed by the LSST. Astronomers will make use of this data to search for dark matter and also to help them understand dark energy.

The heart of the LSST is its large 8.4 m (27′) mirror. This unique structure is different from other optical telescopes and integrates both the primary (M1) and tertiary (M3) mirrors in a single piece of glass. Geometric precision is vital to ensure high quality images.

Problem

The “M1M3 monolith” was cast in 2008 and then went through years of grinding and polishing in order to obtain the required nanometer precision. With such a massive mirror, expansion and contraction from temperature differentials could have grave consequences on the grinding as well as polishing operations, and even in-service performance.

To identify and compensate for temperature differentials, the LSST design team conceived and then created a custom thermal control system. This needed bonding precision thermocouples to the mirror back, front, and mid-plane at 146 locations. If a temperature difference was identified between any of these locations, the control system would then apply a correction. The differential temperature measurements had to be repeatable and accurate to 0.1 ºC for satisfactory operation.

Solution

The best way to attain this level of performance is by using superior quality thermocouples made with wire from the same lot. Unlike other suppliers, OMEGA was ready and also willing to meet this and various other challenging requirements. With the large insulation extrusion operation, the company had considerable quantities of same-lot thermocouple wire readily available. After the design team reviewed the production capability and quality assurance procedures, OMEGA obtained the go-ahead to proceed with manufacture.

The thermocouples supplied were OMEGA’s standard 5TC Series models, but produced from a single lot of Special Limits of Error thermocouple wire. Furthermore, they were handled and packaged as mandated by the LSST team. All the leads were terminated with OMEGA’s strain relief connectors. Coiling in huge rolls permitted extremely high uniformity between the numerous thermocouples, significantly enhancing temperature measurement and tracking at the large number of locations involved.

Results

The LSST team reported that by using the OMEGA® 5TC thermocouples, the temperature monitoring system performed to the 0.1 ºC system requirements. After removing more than 11,000 pounds of material to develop the exact shape required, the mirror was then formally accepted on February 13th, 2015.

When the telescope is finally completed, these same thermocouples will be used for ongoing thermal monitoring of the mirror. A digital processing package will make use of this data in order to offset the distortion brought about by mirror expansion and contraction.

LSST construction started in 2011. The telescope has been scheduled to take its first look at the stars in mid-2020, and when this happens, the image quality will be partly due to the contribution made by OMEGA.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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