Creating a Bigger Picture of the Night Sky with LSST Telescope Mirror

The Large Synoptic Survey Telescope (LSST) is a new type of optical telescope capable of viewing a wider area of night sky than ever before. Once operational, its tasks are to search for near-earth objects that could be a collision threat and capture short-lived cosmic events missed by conventional telescopes. In time, the LSST will form a comprehensive 3D map of the universe. Astronomers will use this data in the search for dark matter and to help them comprehend dark energy.

The core of the LSST is its massive 8.4 m (27 ft) mirror. Compared to other optical telescopes, this unique structure integrates both the primary (M1) and tertiary (M3) mirrors in a single piece of glass. Geometric precision is vital to guarantee superior quality images.

Problem

The “M1M3 monolith” was cast in 2008 and then for years went through grinding and polishing to attain the nanometer precision required. With such a huge mirror, expansion and contraction from temperature differentials could have critical consequences on both the grinding and polishing tasks, and in-service performance.

To detect and make up for temperature differentials, the LSST design team envisaged and developed a custom thermal control system. This needed bonding precision thermocouples to the mirror back, front and mid-plane at 146 locations. If a temperature disparity was detected between any of these locations, the control system would enforce a correction. For suitable operation, the differential temperature measurements had to be repeatable and accurate to 0.1 ºC.

Solution

The ideal way to accomplish this level of performance is by employing superior quality thermocouples built with wire from the same lot. Compared to other suppliers, OMEGA was ready and willing to meet this and other challenging requirements. With OMEGA’s large insulation extrusion operation, they had considerable quantities of same-lot thermocouple wire readily available. After the design team reviewed the production capability and quality assurance procedures, they received the go-ahead to carry on with manufacture.

The thermocouples delivered were OMEGA’s standard 5TC Series models, but made from a single lot of Special Limits of Error thermocouple wire. Additionally, they were managed and packaged as directed by the LSST team. The leads were all completed with OMEGA’s strain relief connectors. Coiling in large rolls allowed for very high uniformity between the several thermocouples, significantly enhancing temperature measurement and tracking at the large number of locations involved.

Results

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

When the telescope is completed at last, these same thermocouples will be used for continuing thermal monitoring of the mirror. A digital processing package will use this data to compensate for deformation caused by mirror expansion and contraction.

LSST construction started in 2011. The telescope is programmed to take its first look at the stars in mid-2020, and when it does, the image quality will be partly because of the contribution done 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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