Wednesday, April 3, 2013

On Mountain Tops and Lasers

The Subaru Telescope (left) next to the two Keck Telescopes on Mauna Kea.
Image Credit: J. Pennington
Tonight I am sitting on top of a 14,000 foot mountain observing distant galaxies. This is the second of a three night run at the Subaru Telescope on Mauna Kea on the Big Island of Hawaii. In a previous post I talked about a run to obtain spectroscopy on one of the Keck Telescopes and some of the excitement, including ups and downs, that are part and parcel of an observing run. This run is no exception!

Initially, my collaborators from the University of Hawaii and I proposed to obtain near-infrared spectroscopy of distant luminous and ultraluminous infrared galaxies using an instrument called FMOS. Due to a problem with the telescope, that instrument turned out to be unavailable during this run so we had to scramble to change our science program to use a completely different instrument. I'm sure I'll talk more about FMOS in the future, but for now I'll focus on what we're doing this week!

The instrument we are using is called IRCS - the Infrared Camera and Spectrograph. We are only using the imaging mode of this camera. The exciting thing about this run is that we are also using the Laser guide star adaptive optics system on Subaru. Briefly, adaptive optics is a technique used by astronomers to obtain images at a higher resolution (and therefore allows us to study features at greater detail) than we normally can from the ground. Since we have to observe through the Earth's atmosphere, our images are blurrier than they would be from space. The reason for this is that the turbulence of the atmosphere causes the light from distant objects to shift in position on a very short timescale (this is why stars twinkle!). As you take an image with a camera on a telescope, this shifting adds up, causing the resulting image to be blurry. Ever notice how a picture on your digital camera is blurry if your subject moves while you are taking it?

Image of the nuclear region of a nearby galaxy (NGC 7469) taken with
and without adaptive optics at CFHT.
Image credit: Center for Adaptive Optics
We can counteract some of this by placing our telescopes on top of tall mountains (such as Mauna Kea) and therefore above some of the atmosphere. We can improve on this even more by using adaptive optics. Adaptive optics uses a bright star to correct for the effects of the atmosphere. Since we know what a star is supposed to look like (it should be point-like in images), the distortions introduced by the atmosphere can be calculated and a deformable mirror is re-shaped so that the light goes where it's supposed to. These corrections can result in images almost or just as sharp as those taken from space (check out the example to the left)! However, since the turbulence of the atmosphere is different at every point on the sky, in order to make these corrections you need to have a bright star that is very close to the object you are trying to observe. This isn't always possible and can be very difficult in our deep fields. This is where Laser Guide Star Adaptive Optics comes in!

Laser from the Subaru Telescope
Image credit: D. Birchall
Laser Guide Star Adaptive Optics is used to create an artificial star close to the object you are observing anywhere in the sky. A bright star is still needed, but it can be a little fainter and a little farther away. This technology has opened up a much larger area of the sky for this kind of imaging! As I am typing this, there is a laser from Subaru aimed at the target I am observing. Keck is also using their laser tonight. You can often see these lasers in images of the telescopes (such as the ones to the right and below).

Tonight we are targeting galaxies within COSMOS and EGS that fall outside of the CANDELS WFC3 HST coverage. In this way, we can obtain high resolution near-infrared images of select interesting targets in order to study their morphology in detail. These galaxies are all sources with extreme infrared luminosities and with this data we will be able to search for signatures of galaxy mergers and study the properties of any star-forming clumps we detect. Our first night was lost due to some problems with the dome shutter but those problems have been fixed and tonight we making our way through our list of targets! 

Using the laser always makes for a particularly busy night of observing because there are many things to consider when shining a bright laser into the sky. First of all, all of our targets must be submitted to Space Command for approval ahead of time to insure that the laser does not interfere with any passing satellites. Because of this, there are certain times of the nights where we must pause our observations or switch targets. We also have to be careful about any planes that might be passing overhead. While the laser is shining, there are always a couple of people standing outside watching, ready to turn off the laser just in case. So far, tonight is going pretty well and the forecast looks great for tomorrow as well!

Panoramic image showing lasers from Subaru and both Keck Telescopes. Image Credit: D. Birchall

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