Planet Imager will enable telescopes to image extrasolar planets directly

Sep 29, 2009 by Kathleen M. Wong, ScienceMatters@Berkeley
The first visible light image of an extrasolar planet, taken by the Hubble Space Telescope in 2006. The star Formalhaut is some 3 billion times brighter than the planet Formalhaut b (inset). The planet is only visible because its star's glare is dimmed by the coronagraph mask. Image credit: NASA, ESA, P. Kalas, J. Graham, E. Chiang, E. Kite, M. Clampin, M. Fitzgerald, K. Stapelfeldt, J. Krist

The best way to observe objects in solar systems is simply to look -- but distortions caused by Earth's atmosphere drown out much of the spectacle of space. To address this problem, Berkeley astronomer James Graham and colleagues are designing an adaptive optics system that can spot new planets.

For as long as we have viewed the and , humans have wondered about the existence of other worlds. Though in our own solar system such as Venus, Mars, Jupiter and Saturn have been known since antiquity, those circling other suns have proved far more difficult to find. In fact, the first definitive detection of an extrasolar planet orbiting a star like our Sun didn't occur until 1995.

Since then, astronomers have hunted planets primarily by observing the gravitational wobbles big planets impose on their suns. Though this method has found more than 300 extrasolar worlds to date, it tends to miss those that are small or reside far from their stars.

Now astronomers like James Graham are getting more ambitious. "We want to know the full range of objects that orbit in solar systems," says the Berkeley professor of astronomy. "And the easiest way to explore these outer regions is just to look."

Photos, Graham says, provide information about planets that can't be obtained any other way. "If you can collect light from the planet, then you can start to study what it's made of, how hot it is, even how it was made. You can start doing comparative planetology," Graham says.

Actual planet spotting, however, is tricky. Stars can be several billion times brighter than their orbiting planets. "It's much worse than seeing a firefly next to a searchlight," Graham says.

Planet Imager will enable telescopes to image extrasolar planets directly
The adaptive optics system of the Gemini Planet Imager will greatly improve scientists' ability to resolve planets in distant galaxies. This simulation of GPI data depicts a planet (green dot) about 300,000 times fainter than the star. Image credit: Bruce Macintosh and Christian Marois, Lawrence Livermore National Laboratory

A case in point is the star Formalhaut. Surrounded by a diaphanous disk of dust kicked up by collisions between rocky objects, Formalhaut has long been considered a prime spot for planet hunting.

Graham and fellow Berkeley astronomer Paul Kalas, analyzed a 2004 photo of the star taken by the Hubble Space Telescope and found a distinct gap between the dust and the Formalhaut itself. Their observations demonstrated that something very big was booting the dust and asteroids out of the way.

This orbiting housekeeper indeed turned out to be a planet. In November of 2008, Berkeley Eugene Chiang joined Graham and Kalas to announce the first-ever image of a planet outside our . A tiny dot on an image filled with light, Formalhaut b is located so far from its star that it requires 872 years to complete one orbit. A device on the Hubble Space Telescope called a coronagraph, which blocks out much of Formalhaut's glare, helps make the planet visible.

Exciting as it is, that photo also illustrates the limitations of existing telescopes. Streaks radiate from the center of the photo, the result of distortion from imperfect telescope optics. Like scratches on a windshield, these streaks make it difficult to identify additional planets within the noise.

Earth's turbulent atmosphere makes distortion even more extreme for ground-based telescopes. To illustrate, Graham switches on the lava lamp on his desk. Pink-tinged blobs of waxy paraffin expand as they are heated from below, rise, then fall as they cool. "It's what happens when sunlight heats Earth's surface and the air above it rises," Graham says.

The column of air above a telescope might contain dozens of pockets of warm air within a colder backdrop. Because they are less dense, these warm pockets convey light faster than the colder air around them. Any starlight filtering through these pockets gets distorted. Instead of focusing on one point in the telescope, the light smears into what Graham calls a "splodge." "The size of the splodge is so large that you couldn't find any planets within the star's solar neighborhood—you couldn't look that close to the star," he says.

To solve this problem, Graham and colleague Bruce Macintosh at Lawrence Livermore National Lab have been designing an adaptive optics system for the 8-meter telescopes of the international Gemini Observatory, which has facilities in Hawaii and the Chilean Andes. Called the Gemini Planet Imager, it will enable astronomers' telescopes to image directly.

The system consists of thin secondary mirrors placed behind the telescope's existing optics. Tiny actuators bend these mirrors to cancel out atmospheric errors. Sightings from a distant star are used to measure the type and degree of correction required.

Making it work is a formidable task involving an international team of engineers and scientists. "The errors projected onto the telescope are changing every few milliseconds. You need to make these measurements thousands of times a second to be able to correct for the cumulative errors, compute how to bend the mirror, then deform the mirror rapidly enough to keep up," says Graham, who began work on the project in 2003.

The $18 million system is scheduled to go online in 2011. Let's hope those distant worlds are ready for their close-ups by then.

Source: UC Berkeley, ScienceMatters@Berkeley

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WhiteJim
not rated yet Sep 29, 2009
There is a very simple solution to this. Placing a star masking object to cover the star in space outside of the atmosphere. The mask can be made to have specific sprectural properties that are known and documented. When the telescope images the vicinity of the star it also images the mask. A computer program can perform calculations on the differences between the image of the mask viewed through the atmostphere and the reference image of the mask. Each photon of light captured would be corrected by the computer program to compensate for the distotion of the atmosphere based on the simultanious measurement that occurs between the light from the star and the light from the mask.

No fancy mirror bending is required and the adjustments to the observed photons is done by software.
WhiteJim
not rated yet Sep 29, 2009
The precision possible from software to correct for distortion should be many fold more precise than bending the mirror. Each immage would record not only the light from distant objects but precise differences caused by the atmosphere to each photon based on the reference image of the mask. This allows for both the image and the correcting values needed for the atmostphere to be collected at the same time for later study.
ofrxnz
not rated yet Sep 29, 2009
The precision possible from software to correct for distortion should be many fold more precise than bending the mirror. Each immage would record not only the light from distant objects but precise differences caused by the atmosphere to each photon based on the reference image of the mask. This allows for both the image and the correcting values needed for the atmostphere to be collected at the same time for later study.


Photons and pixels are not interchangeable. There are magnitudes fewer pixels than photons (as with any camera) you would have lost tremendous amounts of data by the time you used the software if you didn't have an adaptive mirror. This is due to the fact, a pixel is an average of numerous photons either in an in instant or over a greater period of time. Correcting with the mirror you loose none of the source data before correction, initial or otherwise, is made.
WhiteJim
not rated yet Sep 29, 2009
I respectfully disagree. The corrections possible by measuring the distortion trackabe by the image on the reference mask located in space would be very mathematically precise. These corrections would be made to the collection of averaged photons. This method is not limited by the mechanical ability to bend mirrors. It results in a collection of pure raw data which can be manipulated mathematically to great precision. The bent mirror approach is always limited by the mechanical work and cannot be so precise.
ofrxnz
not rated yet Sep 29, 2009
I'm not saying software does not provide a much needed form of correction. In fact, its quite necessary. Just, there quickly comes a point where all image manipulation software generates serious artifacts that are often unpredictable: what margin of Photo Shop error does it take to create or delete a planet when that planet already makes up a relatively small percent of total pixels? While the mirror does have numerous mechanical limitations, it is essentially optics correcting optics. It does correct distortion and most artifacts it could theoretically create are usually well defined and understood distortions that probably don't loose data. Digital correction on the other hand are algorithms unpredictably interacting with algorithms creating unique and difficult to reproduce/correct/predict artifacts. Also, everything is changing so, by the time you make a reference the reference is out of date due to the turbulence atmosphere. Though, I may be misunderstanding your point.

WhiteJim
not rated yet Sep 29, 2009
yes I think your are mistaking my point. The point in the concept is to have a reference located in space that is used to mask the star. The mask is immaged together with the region of interest around the star where an exoplanet may be seen. The image of the mask will have the effects of the turbulance at exactly the same time as the star light is being recorded. By knowing the image properties of the mask every little bit of turbulance can be accounted for and the star light corrected. I am not thinking about the limitations of any particular software or algorithm (writen or yet to be written) to apply the corrections for the measured turbulance at each point in time.
WhiteJim
not rated yet Sep 29, 2009
In fact the important part is the mask in space that acts as a reference to measure the turbulance to a high degree of precision. You could use that same data to control the bending of the mirror.
ofrxnz
not rated yet Sep 29, 2009
Ah, I think I get what your saying....my background is actually some Scanning Electron Microscopy and some digital photography (among other things). At the SEM scale, artifacts are always an issue, and digital correction of artifacts, after the image is taken, is a potentially dubious practice, not only because it could signal forgery but it creates artifacts of its own. I understand the math behind the corrective optics of the mirror but, when saw only using digital methods to filter noise/distortion when the target object (exoplanet) was essentially indiscriminate from the noise to begin with, it threw up all sorts of red flags as being suspect to severe digital artifacts.

sufficive to say its probably more of a personal hesitation to trust digital manipulation in place of well tuned optics
probes
not rated yet Sep 30, 2009
What a fantastic idea. The mask would hide the light from the star at the same time be a reference for canceling the earths atmosphere artifacts. So how far away and how big should this mask be, to mask out an average sized star a few light years away , for example?
Adamas
not rated yet Sep 30, 2009
The problem cannot be solved by simply applying corrections programmatically. The adaptive optics are there to increase the resolution of the system so that valid signal information can be separated from the noise.

The atmosphere is blurring the image, essentially smoothing out a small signal barely above the noise floor. If that is smoothed enough it cannot be characterized as a signal any longer by any program, no matter how sophisticated.

It all comes back to obtaining the most accurate data initially. If you don't have these mirrors, data is being lost that cannot be recovered.
lengould100
not rated yet Sep 30, 2009
Seems (to me) is should be less costly to do the actual photon collection in space than to try to place a miniature mask the size of an observed star disk at a (very) precise location somewhere up above the geosynchronous orbit and keep it at some arbitraty (eg. non-orbital) position.
lengould100
not rated yet Sep 30, 2009
I vote for constructing a set of three 200 metre optical telescopes at the Lagrange points L2, L4 and L5 of the earth-moon system, with data from the L2 point routed to earth via the L4 and L5 positions. Makes a great long-baseline interferometer system...
WhiteJim
not rated yet Oct 04, 2009
The problem cannot be solved by simply applying corrections programmatically. The adaptive optics are there to increase the resolution of the system so that valid signal information can be separated from the noise.

The atmosphere is blurring the image, essentially smoothing out a small signal barely above the noise floor. If that is smoothed enough it cannot be characterized as a signal any longer by any program, no matter how sophisticated.

It all comes back to obtaining the most accurate data initially. If you don't have these mirrors, data is being lost that cannot be recovered.


The most accurate data of the corrections required would come from the reference mask located in space. If you still want your adaptive optics to do the correcting (rather than using math) you could plug your mirror benders directly to the input data from the reference mask for error-free instantanious adaptive optics controled by the mask's data.
yyz
not rated yet Oct 04, 2009
Why not avoid atmospheric compensation techniques entirely (as some have suggested) and build large aperture telescopes and interferometers in space or at the lunar poles. This would both negate atmospheric distortion and facilitate spectroscopic observations without contamination from telluric lines.

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