“The Earth’s atmosphere is an imperfect window on the universe… atmospheric turbulence blurs the images of celestial objects, even when they are viewed through the most powerful ground-based telescopes.” –John Bahcall
There’s no doubt that the Hubble Space Telescope has given us some of the most spectacular, high resolution views of the Universe. From the most distant galaxies ever seen to stars here in our own galactic backyard, the Hubble Space Telescope has simply dwarfed anything we’ve been able to do from Earth’s surface.
This is the globular cluster NGC 288, separated by just over 1 degree from the famed Sculptor Galaxy, as seen through a simple 3″ telescope. Larger telescopes can, of course, do better, but from high above the Earth’s atmosphere, Hubble’s 2.4 meter primary mirror has given us this view of this remarkable object.
Absolutely amazing! For over 20 years, Hubble has been returning images like this, with a resolution of just a couple of hundred-thousandths of a degree!
The reason it can do this, of course, isn’t its size. At 2.4 meters, Hubble is pretty large, but we have plenty of 8-meter and 10-meter telescopes here on Earth, which could get much better resolution than Hubble if they were in space. No, Hubble’s advantage is its location.
While ground-based telescopes have the entire atmosphere to contend with, complete with turbulent air, a slew of different, moving layers, and intervening molecules, Hubble is literally above all that. Despite their extra size, ground based telescopes haven’t been able to compete because of the atmosphere.
But a new technology — adaptive optics — is changing all of that. Here’s how it works.
You start by shooting a powerful laser with very well-defined frequencies, like this sodium laser, creating a guide star that’s in the direction you’re taking your observational data. You’re seeing light from all of the actual stars, galaxies, etc. — you know, the real observing targets — as well as your artificial guide star. The beauty of using a sodium laser is that, around 100 km up, there’s a thin layer of sodium in Earth’s atmosphere that will absorb and re-emit the light back towards your telescope.
All the light that comes in, both from your real targets and from your guide star, gets distorted by the atmosphere. But, since you know what your guide star is supposed to look like, you can take the blurred, incoming signal from the guide star, and compute what type of weird, fun house-style mirror you’d need to un-blur the image!
Just like a fun house mirror distorts normal images, the right fun house mirror can fix distorted images, if you create just the right mirror. But if you can create the proper mirror to fix the guide star (i.e., the light from the laser), you can also fix the light from your observing targets! Creating a system that continuously adapts its mirror to the changing atmosphere, giving you an undistorted image of your observing target at the end, is the end-all goal of adaptive optics.
And when this is put into practice, adaptive optics is capable of taking what looks like turbulent, nonsense noise and turning it into a crystal-clear, real-time image of what actually lies out there in the Universe.
Want to see it in action? Take a look at this 2006 video of adaptive optics taking on a binary star system; you seriously won’t believe it.
That was then.
Just a couple of months ago, Gemini South Observatory released their first light image from GeMS/GSAOI, the world’s most advanced adaptive optics system, attached to the 8-meter Gemini Telescope. And wouldn’t you know which object they happened to take a look at for their very first image?
Wouldn’t you know: it’s globular cluster NGC 288! As the GeMS Principal Investigator, François Rigaut was absolutely amazed at this image, and said,
We couldn’t believe our eyes! The image of NGC 288 revealed thousands of pinpoint stars. Its resolution is Hubble-quality – and from the ground this is phenomenal. This is somewhat uncharted territory: no one has ever made images so large with such a high angular resolution.
Although all of that is true, I think University of Toronto Astronomer Roberto Abraham more encapsulated my reaction to this image, when he said,
This is fan-freaking-tastic!!!!!!!
And it is! If you horizontally flip and (slightly) rotate the raw image, you can actually overlay it atop the Hubble image back at the top of the page, and compare these two directly!
At this zoomed-out resolution, it doesn’t look all that impressive, especially considering the monochrome nature of the ground-based image.
But let’s take a look at a very small region — those four bright horizontal stars towards the center of the above image — with both the Hubble Space Telescope and the Gemini telescope with the new adaptive optics!
Even at first light — with its very first image — the GeMS/GSAOI adaptive optics were easily just as good as Hubble’s resolution, the first time that a ground-based telescope has ever done that!
Of course, that was like, two months ago already, so Gemini has since gone on to take even higher resolution images than Hubble can, like this one of NGC 2362.
Sorry that there’s no Hubble image of this to compare with, only a Spitzer image that really looks like a joke, particularly next to the full-resolution Gemini version. When you’re looking at the image above, remember that each quadrant is less than one ten-thousandth of a square degree! Highest. Resolution. Image. Ever.
And that’s how you defeat Hubble without ever leaving the ground!