The Large Synoptic Survey Telescope (LSST) is an advanced kind of telescope that can boast 3.2 billion pixels due to the fact that it combines a huge 8.4 meter primary mirror with the world’s largest digital camera, which improves its field of view. This combination would enable the detection of faint objects with particularly short exposure times, twenty times faster than what is achievable at present.
Those advancements would give it the ability to survey the whole of the sky twice each week. Each night more than 30 terabytes of data would be generated, processed and stored. The LSST was scheduled to get its first look at the sky in 2015 from a mountain top on Chilean Andes, and to go fully operational in 2017.
Large Synoptic Survey Telescope (LSST)
The Large Synoptic Survey Telescope
Every night the LSST can record hundreds of images from the sky because of its exposure time of only 15 seconds. In fact, this would result in a movie from the sky, but the more important ability of the LSST is that it can detect and track near-earth objects that might present a threat of collision.
Furthermore, it can capture short-lived cosmic events that can be missed by conventional telescopes. Ultimately, the LSST would be used to create a 3D map of the universe in unprecedented detail. Scientists and astronomers hope to use this to locate dark matter and to characterize the properties dark energy, both of which, for now, remain theoretical.
Extraordinary Optical Properties
The impressive structure of the LSST that includes the 8.4 meter mirror, incorporates both the primary and tertiary mirrors of the telescope in a single piece of glass. This gives the LSST several extraordinary optical properties. Grinding of the mirror surfaces was scheduled for completion in January 2012. The process involved the removal of over 11,000 pounds of material, and hence, it would take more than two years to complete.
The 8.4 Meter Mirror in the Polishing Cell
As mentioned, the mirror is large in size. Therefore, the contraction and expansion from temperature differentials in the different sections can have a significantly detrimental effect on the final precision of the grinding and polishing operations.
Such negative effect can be disastrous because of the long time it takes to complete the process. It was in overcoming this problem that Omega Engineering was able to make a significant contribution to ensuring a successful outcome.
The LSST design team had the desire to use a custom thermal control system installed on the back surface of the mirror in order to maintain a uniform temperature throughout the structure at all times.
Precision thermocouples were to be bonded to the mirror front, back and mid-plane at 146 locations. If a difference in the temperature was observed between any of the monitored locations, that would cause the temperature control system to react to correct it. The specification required differential temperature measurements to be repeatable and accurate to 0.1 degrees Celsius.
High Quality Thermocouples
The best possible way to achieve such high performance is by using high quality thermocouples all made with wire from the same lot. Omega Engineering was different from other suppliers that the LSST team contacted in their readiness and willingness to meet all the special requirements of this application.
Omega Engineering already had significant quantities of same-lot thermocouple wire as a result of its large insulation extrusion operation. After a review of Omega’s production capability and quality assurance procedures, a green light was given and the realization of the project begun.
Back Surface of the 8.4 Meter Mirror
The thermocouples supplied were standard Omega® 5TC Series products, the only difference being that they were made from a single lot of special limit of error thermocouple wire. Additionally, they were given special handling and special packaging as mandated by the LSST team specification. The leads on all units were terminated with Omega’s strain relief connectors and they were coiled in large rolls.
By doing this, a high uniformity between the numerous thermocouples was achieved. It also improved the temperature measurements and tracking at the large number of locations involved. The achievement, as reported by the LSST team, was that the temperature monitoring system, using the Omega® thermocouples, performs to the 0.1 degree Celsius system requirements.
After the telescope is completed, the same thermocouples will be used for ongoing thermal monitoring of the mirror. During use, the relevant data can be used by the digital processing package to compensate for distortion caused by expansion and contraction of the mirror in real-world conditions.
The success of this project generated interest in Omega and it has been asked to supply further same-lot thermocouples for test and measurement applications on other phases of the project.
Omega Engineering is pleased to have a significant role in ensuring a successful grinding and polishing operation for the LSST 8.4 meter mirror. Omega’s commitment to do what it takes to supply the products and services needed by its customers is obvious through this application. Extensive custom engineering capability and vast experience in fulfilling special customer needs have made Omega the reliable choice for test and instrumentation applications.
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.