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?")
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!