|Image: These are both images of a star, before the Next Generation Wavefront Controller (NGWFC) (right) and after NGWFC (left). After NGWFC, the peak of the image is much higher and sharper. The most commonly used metric, called the Strehl ratio, is the peak of the image relative to the peak of a “perfect” image. Before NGWFC, on a faint star like the one shown, we obtained a Strehl ratio of 23%. With NGWFC we obtain a Strehl ratio of 41%. That means that the peak (or sharpness) of the image is now almost twice as high. Original image by Marcos van Dam.
The Next Generation Wavefront Controller (NGWFC) is an upgrade to the adaptive
optics (AO) system of each of the two Keck Observatory telescopes. The improvement
in performance of the AO systems at Keck Observatory will benefit all users of
the adaptive optics systems.
By improving the quality of the correction and
the robustness of the instrument, astronomers will be able to take home more
data and data of a higher quality, allowing them to maximize the scientific productivity
of the telescopes.
|Photos: The full NGWFC project team. Photo
by Sarah Anderson.
The NGWFC project began in earnest in 2004. Paul Stomski (Project Manager), Erik Johansson (Project Engineer), and Marcos van Dam (Project Scientist) led the Keck Observatory NGWFC design team. Other team members included Roger Sumner, Jason Chin, Jim Bell, and Peter Wizinowich (Principal Investigator).
There were two problems that NGWFC was developed to address:
Problem: The old system used ten-year old computers that were aging and no longer supported by their manufacturer. The old system relied on 16 processors working in parallel, which made it very difficult to troubleshoot the system when there were problems or to make upgrades. If one of the processor boards broke, our ability to use the AO system was compromised. We were beginning to experience relatively frequent computer crashes as the system became more complex (with the addition of the laser guide star), resulting in significant lost time at night.
Solution: The new system uses robust state-of-the-art technology which will be supported by the manufacturers for many years to come.
Problem: The charge coupled device (CCD) that was used in the wavefront sensor to measure the atmospheric turbulence was old and relatively “noisy.” A CCD is an electronic imaging detector, like the ones found in every digital camera. All CCDs produce an image that is somewhat corrupted by the detector electronics. The difference between the true image and the measured image is called “noise.”
Solution: We replaced the old CCDs with a much “quieter” CCD that improves the accuracy of the wavefront measurements, leading to improved performance.
Problem: The speed of the computers was also limiting the performance of the AO system.
Solution: The NGWFC computers are much faster and the software is much more flexible, leading to improved performance. The speed at which the NGWFC can run is limited by the camera rather than by computing power. The new system can run at 2400 corrections per second, compared to 660 for the old one. The new system has about 30 times more computing power than the old one, and it is easier to use.
The result of these upgrades is that we are able to better estimate and correct for atmospheric turbulence. This means that we are able to more faithfully mimic the turbulence with the AO system’s deformable mirror, especially on windy days when the turbulence is changing quickly.
The biggest technical challenge our team faced with the NGWFC project was the short amount of time we had between when we disassembled the existing AO system (August 8) and the deadline for the new system to be fully functional (November 4). There were myriad software, hardware, and infrastructure changes to make, and a lot of development and testing that had to be completed in this brief time frame in order for the system to work. As if these technical issues weren’t enough of a challenge, we also had to deal with bad weather, which resulted in the loss of almost all of our first two observing nights, and the earthquake — which completely misaligned the optics bench and diverted significant resources away from this project.
Fortunately, the NGWFC system worked well from the start -- with only a few bugs to squash. Already, it produces better image quality than the old system, and our results are only going to get better.
We continue to work on improving Keck Observatory’s instrumentation. First light for NGWFC on Keck II is scheduled for February 26, 2007. We will be installing a laser guide star adaptive optics system on the Keck I telescope at the end of 2007 which will work with the NGWFC. Also, a revolutionary new CCD is being developed under a project led by the W.M Keck Observatory. This CCD has the advantage that it has a large number of pixels and has incredibly low noise. We plan to install this CCD on the wavefront sensor of both the Keck I and Keck II adaptive optics systems. The
new CCDs are being produced at the Massachusetts Institute of Technology’s Lincoln Laboratory.
|Photo: New adaptive optics system overcomes atmospheric turbulence. Marcos van Dam is pointing at the Polycom, which scientists at Keck Observatory headquarters in Waimea use to communicate with the telescope operators on the summit. The telescope operators are Carolyn Parker (right front), Chuck Sorenson (center), and Jason McIlroy (left rear). They are celebrating their success of overcoming both adverse weather conditions and atmospheric turbulence on the first night of using NGWFC.
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