Thirty Meter Telescope

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Making Your Own Stars


By Gary Sanders, TMT Project Manager

The image on the right is dramatic! The Gemini South Observatory in Chile has just made a major step in pioneering the kind of adaptive optics that TMT will use. Launching five laser beams from the top of the Gemini South 8-meter telescope, they have succeeded in creating a manmade five-star constellation about 90 kilometers above the Earth. At that altitude, a layer of sodium ions is excited by the light of a particular color.

Gemini used the desired wavelength of 589 nanometers and achieved this striking result.

While this is a beautiful image, how does it fit into the adaptive optics that Gemini and TMT will both use? We have written about adaptive optics in previous issues of Segments. To recap, to remove the blurring caused by the Earth’s atmosphere, light from a sharp “guide” star near a science object to be studied is measured by a sensor placed at the image from the telescope. This sensor measures the distortion of the wavefront from the known sharp guide star and this measurement is sent to a fast computer that sends commands to a deformable mirror that applies the opposite distortion to the image of the science object. The result is a sharp image of the science object. This sensing and correcting is carried out about 1,000 times per second. This is the basic method used in adaptive optics.

But there is a problem. What if there is no sharp guide star near the science object so this trick cannot be performed? Indeed, when this occurs adaptive optics cannot be used. The solution is to launch a laser beam from the telescope to create an artificial guide star near the science object.

This technique has been used by a number of telescopes, notably by the Lick, Palomar, Keck, Gemini, Subaru, and VLT observatories.

But there is another problem. Launching a single laser beam creates one guide star. That permits the adaptive optics system to correct the image near the guide star. This is because the optical distortions in the atmosphere correspond to turbulent thermal cells in the atmosphere and these cells have a certain size. You can correct for the atmospheric turbulence only nearby the guide star image. What if you want to correct a wider region of the image from the telescope? The solution is to use multiple deformable mirrors and several guide stars near or surrounding your science object image. This is a constellation and it is what Gemini has now done.

The Gemini image on the right clearly shows five stars. TMT will start its observing with six guide stars and will progress to nine artificial stars. By doing this and by using multiple sensor/deformable mirror combinations, the images will be sharpened over a wide visual field, sweeping away the distortions of the Earth’s atmosphere. This is Multi Conjugate Adaptive Optics, a complex name for using an artificial constellation and doubling the adaptive optics system and making it all work together.

TMT will use six small 20-watt lasers mounted down on the elevation bearings of the telescope (see the image below) and will transport the light up the side of the telescope to the back of the secondary mirror. A “launch” telescope will send the six beams to their 90 kilometer destination. We are designing the detailed optics for this system now. The design is being carried out by our collaborators from the Institute of Optics and Electronics in Chengdu, China, a part of the Chinese Academy of Sciences. Both in China and in Pasadena, we found the Gemini image to be inspiring.