Creating artificial stars to see through the soup

Here is a picture I took during a night
of observing on the summit of Mauna Kea in Hawaii.
The laser from inside the dome at the Keck telescope
creates an artificial star in the upper atmosphere
that is used for adaptive optics. I mentioned before that there is one major obstacle that prevents us from obtaining the best resolution possible from a ground-based telescope. The key word here is of course "ground-based", as opposed to "in-orbit."

Although it is quite convenient to build a telescope in a place we can actually visit, these locations have one major drawback. For the same reason that stars twinkle, any observations from the ground suffer from degraded resolution. Where we would be able to see the beautiful spiral arms and compact core of a distant galaxy in an ideal observation, we really see a blob that resembles a snowman as much as it does a galaxy.

The cause of this twinkling, or degradation, is the Earth's atmosphere. I like to think of the atmosphere as a boiling pot of chicken noodle soup. We are in the unfortunate position of sitting at the bottom of the pot, trying to look out through all the turbulence into the infinite kitchen. It's hard to tell exactly what's happening in the neighboring pots from this perspective, not to mention the pots that are 10 billion light years away.

There are two solutions to this problem; the most obvious (and most expensive) solution is to build a telescope that will be launched into space. I’m actually working on such a project and will write a bit about it next time. The other solution is a little more complicated, a technology known as adaptive optics.

Image courtesy of CFHTWe use adaptive optics to tilt the lenses and mirrors in a telescope to correct the twinkling of the stars or galaxies that we are trying to observe. If my galaxy twinkles left, the optics register this disturbance and compensate by tilting left. Galaxy twinkles right, tilt right. This feedback loop happens in real time, with more than 100 measurements of the atmospheric disturbances every second. After the observations are finished, we have an image with a resolution that can be 20 times better than an image without adaptive optics.

Now the system needs something bright to serve as an anchor. In some cases, a bright star is near the target. It is easy to measure the position of such a star in a very short exposure, and to determine the adaptive corrections required to keep that position stable. These corrections are then applied to stabilize the observations of your target galaxy. However, most of the time no star lies close enough to the target to anchor the observations.

When there is no nearby star, we use a laser guide star instead. The most common laser guide star is created with a laser tuned to a frequency that will excite sodium atoms and cause them to glow (you'll find this glowing sodium in quite a few streetlights). The laser is pointed right next to the target of the observations into a sodium layer which resides about 60 miles high in the upper atmosphere. The sodium atoms absorb and then re-emit the light in all directions. Just like a real star, this newly created artificial star is used to model the disturbances in the atmosphere and determine the corrections required to improve the resolution of the image.

Several telescopes such as the Gemini Observatory, the Very Large Telescope, and the Keck Observatory have starting using adaptive optics, or are in the process of installing a system. The technology still has a ways to go, for example it works much better in the infra-red than in the visible part of the spectrum, it works only over a relatively small area, and produces a final resolution that can be quite difficult to interpret and model. However, adaptive optics does offer a lot of promise for greatly improving observations from all the large telescopes we build on Earth's most distant mountains. Someday I imagine that observations from these telescopes will be just as good as those from space.

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and
development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.