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Getting the Best Images

by Alan Tokunaga, Division Chief, NASA Infrared Telescope Facility

Inside the NASA Infrared Telescope Facility dome. The large blue boxes with doors contain the electronics for the instruments on the telescope. Heat generated by the instruments is removed from the boxes to avoid having hot air near the focus of the telescope. The primary mirror above the instruments will soon be cooled to about one degree below the air temperature to eliminate air turbulence at the mirror's surface. The blue box at the very bottom of the picture contains an infrared camera and a spectrograph. Observations are taken with specially designed instruments and are recorded by computers. Photo by D. Watanabe.

Astronomers are always pushing for ever-sharper images of stars and galaxies. This is one of the reasons that Mauna Kea is one of the world's best, if not the best, site for ground-based astronomy. The atmosphere above Mauna Kea is extremely stable, which greatly reduces the atmospheric turbulence that blurs the images of stars.

Modern observatories make a considerable effort to remove the effects of the atmosphere, a job made easier on Mauna Kea, where the atmospheric pressure is only 60 percent of that at sea level. To achieve the very best images of stars that are limited by atmospheric turbulence alone, it is critical to eliminate other sources of air turbulence. Even a temperature difference of a few degrees between the outside air and the air inside the telescope dome is sufficient to cause noticeable degradation of images.

At a typical astronomical site, the atmospheric turbulence causes a star's perfect point of light to be blurred to an angular size of about one second of arc or larger. To give you an idea of its size, one second of arc is equal to about one mile at the distance of the Moon. On Mauna Kea, images of stars are typically about 0.5 second of arc and can be as good as 0.25 second of arc. For comparison, the full moon has a diameter of 30 minutes of arc. For an explanation of these astronomical measurements, see "What is a second of arc?")

IRTF dome

Open dome of the NASA Infrared Telescope Facility showing the telescope inside. Photo by Megan Novicki, digitally enhanced by Karen Teramura.

We at the NASA Infrared Telescope Facility have therefore recently made great efforts to match the air temperature within the dome to that of the outside air. Even the primary mirror of the telescope will soon be cooled so that it is slightly below the temperature of the surrounding air. This will remove the air turbulence right at the surface of the mirror. This attention to reducing air turbulence within the observatory is now a critical part of all observatory designs and daily operations. After all, light from distant stars and galaxies comes a long way, and we should not let air turbulence disturb its last few yards of travel to our cameras!

http://irtfweb.ifa.hawaii.edu/