Interview with Neville Woolf
|Keck’s adaptive optics/interferometry project is a significant cornerstone of NASA’s Orgins program, which ultimately seeks to identify and characterize Earth-like planets – if they exist – around neighboring stars.|
Credit: Keck Telescope Homepage
Neville Woolf: [We'll begin our work with] the Multiple Mirror Telescope (MMT), which is now 6.5-meter and has an adaptive secondary mirror. This allows us to have very high angular resolution and very good images in the thermal infrared. Those are wavelengths longer than 2 microns. This is the first adaptive optics system that works well in the infrared.
AM: Why is that important?
NW: Because one of the ways of finding planets is by their heat emission, and the heat emission of planets tends to be rather stronger in comparison to their star than their reflected [visible] light. So it’s a way of boosting the planet signal with respect to the star.
AM: How does the MMT compare with the Keck telescope in Hawaii?
NW: [The MMT has] a smaller mirror, but it’s in a single piece, and it has an internal cooling system in it, so that it can stay at the night air temperature, and that really helps to make the images better. The Keck telescope has more light collecting power, it has in principle higher angular resolution, but when you look in detail at the images developed by the Keck telescope, even with adaptive optics, they show a great deal of the effects of the separate pieces that make up a Keck primary mirror.
|The Keck telescope.|
Credit: Lawrence Berkeley National Laboratory
AM: You also have a new telescope, the Large Binocular Telescope, currently under construction. How is that work proceeding?
NW: We will fairly soon have our first 8.4-meter mirror in the Large Binocular Telescope, and then that will tend to take over this kind of work from the MMT. Then as we put in the second mirror, then of course, we’ll get the extra light collection and angular resolution. So we have an evolutionary program.
AM: What is the advantage of the binocular telescope?
NW: It will be 23 meters edge-to-edge, so we will reconstruct images that see the detail you would get with a 23-meter telescope, and it will have the light collecting power of an 11.9-meter telescope. Now, you could say, similarly, that with the Keck telescope you get the detail of a 75-meter telescope, but it doesn’t quite work out the same way. The binocular telescope has its two mirrors on a single mount; they move together. You actually are able to pull the light together, have adaptive optics sharpening the image, and do all of that with only three warm surfaces that the light encounters. And therefore you don’t get very much heat radiated by the telescope getting into the picture. So you can have high sensitivity. This makes the LBT and the Keck Interferometer complementary – each has its different area of greatest usefulness.
AM: Can you explain what adaptive optics are?
|The LBT Enclosure erected on Mt. Graham in southeast Arizona during December 1999.|
Credit: Large Binocular Telescope Homepage
NW: Adaptive optics is a process of changing the shape of the optics to warp the light paths back to what they would be if there weren’t an atmosphere. The atmosphere causes shimmering, and it causes blurring. Both of those are caused by the rays of light not going straight to a focus, as they would in an ideal optical system.
What you do then, with the adaptive optics, is you measure exactly how far the path of the light is distorted and you tilt small areas of a mirror to [eliminate the distortion]. We take the secondary mirror of the telescope and we just bend it to the necessary shape to take out all the effects of the atmosphere.
AM: So you’ve got the effect of a bunch of little fingers on the back of the mirror pushing on it?
NW: That’s right, pushing and pulling, yes, and changing the amount of push and pull every 1/1000 of a second. There are 336 fingers on the MMT and there will be 672 on each of the secondary mirrors for the Large Binocular Telescope.
NW: All of the devices that we’ve sent into space to look at the Earth look at tiny regions of the ground, and it’s very, very hard to get a clear picture of what the Earth as a whole looks like. In particular, the cloud cover is fairly large and clouds are bright and the surface of the Earth is not very bright in comparison. So what we mostly see of Earth from space are the upper regions of clouds, and it’s very hard to predict how the light contributions to the spectrum will add up and give us an overall spectrum similar to what we expect to see with a Terrestrial Planet Finder-like device.
|Earthshine is light reflected off the Earth, and then reflected back to Earth from the Moon’s night side. (Above: Earthshine and a young crescent Moon.)|
Credit: Laurent Laveder, NASA
We’ve already learned from Earthshine that there are some features that show fairly well. Those include the Rayleigh scattering that causes our sky to be blue, and we see that in the Earthshine. And the other thing that we see is the signature of vegetation. Those are both very important as features we might look for in another [Earth-like planet]. In particular, the Rayleigh scattering is the only good external measure of the amount of nitrogen in our atmosphere, and we need to know that to interpret the spectrum.
My current work is trying to look at Earthshine in the near-infrared (1-2 microns) – and this is one of the goals that we’ve posed for our node of the [NASA Astrobiology] Institute. This is because the features of water vapor on Earth show up much better in the near-infrared, and we weren’t seeing them as well as I had hoped in the spectra we were taking from the visible.
AM: Earthshine is light reflected off the Earth, and then reflected back to Earth from the Moon’s night side. How do you go about observing this?
NW: What we do is we wait for the Moon to be nearly new. So most of it is not being lit up by the Sun, and instead it’s facing the daylight side of the Earth, and so the Moon collects the light from all over the Earth, and if you take its spectrum, you see the spectrum of the Earth.
Now, it’s slightly modified. It’s modified by the reflectivity of the Moon. But if you then compare the Earthshine spectrum with that of the crescent of the Moon that is receiving its light directly from the Sun, you can take out the effect of the Moon’s reflectivity and you can take out the effect of the last passage of the light [back down] through the Earth’s atmosphere. And you can get the answer you want: What does the Earth’s spectrum look like from space?
AM: So you have to do your observing during the daytime?
NW: No, you do it just before the Sun rises or just after the Sun sets. It’s not the easiest observation because of that. You’re trying to get as close to new Moon as you can, and of course the Moon and the Sun are horribly close together. And you try and use the Earth as a kind of sunshade to prevent you from seeing the Sun or too much lit-up sky.
It’s an interesting and tricky observation, and you have to learn how to do it and slowly work your way in. I was up on Mt. Graham [in southeastern Arizona] earlier this week, and we have actually observed the Earthshine in the infrared. Not quite as well as I would hope, or as well as we’re going to do, but we’ve proved to ourselves that it’s possible with the telescope and the equipment we have up there, and so we’re looking forward to pushing forward with that work.
Read Part 2 of this interiew.