Laser Guide Star Adaptive Optics
Astronomers have conquered the Earths atmosphere’s blurring effects on views of the Universe with Laser Guide Star Adaptive Optics.
In visible and near infrared wavelengths, Laser Guide Star Adaptive Optics has proven to be effective at neutralizing the blurring effect of the ocean of air that is above our telescopes. Previously, we were limited to atmospheric “seeing”, the turbulence that distorts starlight as it passes through our atmosphere. Resolution is a function of telescope diameter. The larger the diameter, the finer the resolution. Atmospheric “seeing” will prevent our large telescopes from attaining their theoretical maximum resolution and usually limit us to 1-3 arc-second resolution. On Mauna Kea, due to the excellent seeing in the middle of the Pacific Ocean, we can attain 0.3 arc-second resolution without correction (10-meter class telescopes can achieve 0.01 arc-second). Most other locations on this planet has 2-3 arc-second seeing due to turbulence.
Starlight in space has a flat wave front.
On entering the Earth’s atmosphere, the wave front is warped as starlight encounters varying densities of air caused by air currents. Laser Adaptive Optics consists of a laser, wave front sensor and a deformable mirror that has hundreds to thousands of tiny actuators that push and pull on the mirror’s rear surface to negate the wave front error induced by the atmosphere. The wave front sensor detects the warping and computes the correction 2000 times per second. In the time it takes the starlight to travel 2-feet to the deformable mirror, the correction is applied to the mirror and the distorted wave front is corrected. The deformable mirror is placed just in front of the camera after the main optics. The telescope is enabled to produce resolution to the design limits. In the case of the Keck 10-meter telescopes, they can resolve 5 times finer details than the Hubble Space Telescope. Hubble can resolve down to 0.05 arc-second.
The laser is used to produce a bright artificial star in the field of view.
Very often as we target a dim galaxy billions of light years away, that light is too dim to be analyzed for seeing effects. 60 miles above sea level there is a layer of vaporized meteor material that includes traces of sodium. Illuminating that layer with a laser tuned to excite the sodium results in a bright fluorescent artificial star at the top of the atmosphere. Placing the galaxy next to our artificial star enables the correction of the dim galaxies light to be resolved to the limits of the telescope.
Currently, most Laser Guide star Adaptive Optics systems use a single laser.
Only a small area of the sky around the laser star can be corrected. Future designs have multiple lasers to correct a large area for clear panoramic views for survey work. Future space telescopes need only observe the portions of the spectrum that do not penetrate the Earth’s atmosphere. Typically, a space telescope costs about $250 million per year to operate. A ground based 10-meter telescope costs tens of millions of dollars for a year of operation. Plus updating a ground based telescope with the newest technology is far less expensive. Space telescopes are already 10-years old technologically before launch due to design, certification and building time.
