Lecture 11

1) Until now astronomers have been limited by the atmosphere "blurring" their images so that even the largest telescopes could not make images significantly better than backyard small 10 inch amateur telescopes.
2) To solve this problem NASA launched the Hubble Space Telescope above the atmosphere to make sharp images.
3) However, Hubble is now quite small compared to the twelve 6.5-10m class telescopes operating on the ground. Each theoretically is able to make images ~3-4 times sharper than the best Hubble can achieve.
4) Hence a new technology has emerged over the last 10 years (20 years including military research) to give ground based giant telescopes the ability to observe the universe in unparalleled sharpness.
5) This new technology is called Adaptive Optics (AO) and it allows a telescope to correct (in real-time) the blurring due to the atmosphere.

The reason stars "twinkle" is the mixing of hot and cold air in the atmosphere. A good example of this is the "puddle mirage" that we see along a road in the summer time where the hot air just above the road "warps" light as it passes along the road. Here is a movie which shows how layers in the atmosphere bend light here on the surface of the earth.

1) to sense very fast the way an image is being "warped" or distorted by the atmosphere (typically 1000 times each second!).
2) based on the direction and strength of those measured distortions apply an equal but OPPOSITE distortion to a flexible "rubber" mirror.
3) continue this "closed loop" feedback to give the science camera a distortion free image.

Here is a movie showing the MMT adaptive secondary (built by the Center for Astronomical Adaptive Optics here at Steward Observatory) before closing the loop then after closing the loop
on a 0.24" double star.

Here is a movie showing the real Hokupa'a AO system working. Hokupa'a was the first (and is still) the highest order curvature AO system in the world. It was built by the Honolulu based Institute for Astronomy AO group. It is now the AO system for the 8m Gemini telescope in Hawaii shown in the previous movie. Hokupa'a means "immovable star" in Hawaiian (the name the Hawaiians gave to the pole star).

OUTLINE FOR OBSERVERS:

Since r_o varies as wavelength to the 6/5 AO images are always better in the IR than the visible.

Based on the above plot we can see that stars of V<13 will give good AO corrected
images. For guide stars fainter than this the images will become less sharp. The brighter the
guide star the better the corrected image.

The AO system can work with the derotator on. Or with the derotator off if the guide
star is the science target. (keeping the derotator off and rotating the images later in software can help -since the pupil is than fixed w.r.t. the science camera).

1) Young stars, binary stars: A very rich area of research in adaptive optics is that of binary stars and young stars.

Here is movie which shows how much better one can image a young binary (in fact a triple) star with adaptive optics at the 4th 8m VLT Telescope run by the European Southern Observatory.

Such images are in the infrared but sharper than the images Hubble can make from outside the atmosphere. Typically images made by large AO equipped telescopes will be 0.050" (20x sharper than a 1" image - which is the best one can hope for usually without AO).

2) The surface of the Sun: Another fascinating use of AO is to image the surface of the Sun. These images from the National Solar Observatory in New Mexico are a big improvement over non-AO images (as this movie proves). However, solar astronomers can combine AO and some image processing to pull up the sharpest features (see movie) to make even more impressive insights into the Sun's surface.

Also sun spot movies from the Swedish Solar telescope AO system are also amazing.

3) Planetary Astronomy: Studies of other planets in our solar system is also a ripe field of study for AO.

For example the planet Neptune has complex methane clouds that can be seen revolving as the planet turns (movie from the University of Hawaii AO group Hokupa'a images).

Also recent images of Saturn's moon Titan which has a dense methane atmosphere show how this atmosphere can act like a lens as 2 background stars pass behind it (movie from the Mt Palomar PALAO system).

However, adaptive optics are not just for the professionals on the largest telescopes. Even backyard telescopes can gain a lot from simple AO systems that cost only a few thousand dollars. Stellar Products builds a simple AO system that stabilizes and corrects the most common wavefront errors. Here is a movie of Jupiter taken in a backyard with a 9 inch telescope using such an AO system.

4) Planets Around Other Stars: The search for planets outside of our solar system is one of the great new fields in astronomy. In 1995 the first planet around another star (51 Peg) was inferred from radial velocity measurements of the primary. Today there are over 80 such stars that appear to have Jupiter like mass planets in orbit. However, there has not been a single direct detection of light from these planets.

Above we have an image of a brown dwarf around a nearby star (Mike Liu, IfA, Hokupa'a/Gemini image of 15 Sge)

It would be incredibly exciting to actually image a planet around another star. My prediction is that AO will do just this in the next year.

Already we can detect brown dwarfs around stars (see image above).
Brown Dwarfs are somewhere between 75-13 Jupiter masses. They are not massive enough to considered hydrogen burning stars, but they are likely too massive to be considered planets.

Above we show some of the first binary brown dwarfs and brown dwarf companions found with Hokupa'a/Gemini AO (see Close et al. 2002a, Close et al. 2002b). We have now found that of 40 low mass stars observed 9 have low mass companions. All of these companions could never have been found without AO. Hence it appears that brown dwarfs form into binaries as often as more massive stars but they tend to have smaller separations (typically ~4AU).

There is even hints of lower mass (planetary mass objects) in some of these images.

Almost all large D>4m telescopes have facility AO systems either running or close to operational.

Since diffraction-limited scopes gain as D⁴ power on point sources there is a clear advantage to AO on large telescopes.

AO is now quite a common technique practised by experts and general IR astronomers.

below we show a histogram of all science papers published in for purely scientific goals:

We see that AO has rapidly increased in productivity. As new generations of observers are trained in AO and as new (larger) facility AO systems come on-line the number of exciting results steadily increases.

Today (March 2008) there are well over 1000 refereed publications on adaptive optics, (roughly >200/yr new science publications). As well there are over 10,000 citations to these papers. AO is now a mainstream technique.

Here we see that some systems are far more successful than others at producing science.

A key reason for this is the site of the telescope. Most of these papers are produced at the best sites (Mauna Kea and Chile)

Another key to success is the ability to work from 1-2.5 microns and have a simple User Interface.

Another big advantage is being able to utilize faint guide stars. Currently Curvature AO Systems (CS) can reach fainter limiting magnitudes (R~16-17 at an 8m) compared to Shack Hartmann systems (SH).

Another key is to have access to the southern sky. Only Adonis could operate in the south.

Some things that don't seem to help much are the ability to work a 5 microns. Usually the thermal background (combined with the difficulty in chopping + AO) makes thermal imaging rare (accounting for only 1.4% of papers)

Also visible wavelengths are less appealing compared to 1-2.5 microns since no system (except Maui)
has enough actuators to correct in the visible on a large telescope. Small 1.5m telescopes have AO in the visible but are then competing with HST.

When this survey was done Laser Guide Stars (LGS) were not yet ready for "prime time" despite the enormous amount of work that gone into them. Only 1 published paper (from Alfa) has utilized a laser for pure science.

Now LGS systems are operational on Keck, VLT, and Gemini. The potential is great however, and so hopefully we soon see LGS science papers as commonplace. Indeed, just a few years after Keck's LGS system was offered there are now over 30 refereed publications from Keck LGS alone.

AO has been used for most types of imaging astronomy. Here is a table showing some of the popular topics and what years they have papers published using AO.

    1) Planetary science: -asteroidal surfaces, asteroidal Moons, Moons of Giant planets, clouds of giant planets etc.
    2) Stellar astronomy: -young binary stars, stellar clusters, crowded field work etc.
    3) Star Formation: -young binaries, circumstellar disks, embedded clusters, nebulae etc.
    4) Faint companions: -detection of very faint companions to nearby stars, brown dwarf companions, white dwarf companions etc.
    5) Extragalactic:-detection of host galaxies, companion galaxies, morphology, gravitational lens, interacting galaxies, the cores of nearby galaxies etc.

AO will quickly become the dominant observational technique for the following problems in solar system and galactic astronomy:

The second generation of AO systems is now under way. Here at Arizona we are developing the world's first (and only) adaptive secondary mirror. Having the "rubber" mirror placed at the location of the secondary will dramatically increase the throughput of the system and lower the amount of "heat" seen by the infrared detector.

Here is a movie of the positional error of the mirror while in the wind. The mirror worked quite well only +/-30 nm of wavefront error was recorded while the mirror was holding "flat" in the wind while using only a fraction of its range (<1%).

Here is a MOVIE of the MMT adaptive secondary closing the loop on the theta 1 Ori B.

Below are some great images of interesting binaries and single stars done with the MMT AO system last week
(the data is so new that it has not been bad pixel corrected yet...):

(note the "poor" quality of the 0.5" OFF image in the lower right, compare to the AO ON image in the
lower left -- a big difference - these are ALL log scales)

Soon astronomers will desire to have even bigger fields of view corrected by AO. To do this will require several guide stars (maybe made by lasers) and several rubber mirrors all working together. This
technique is commonly called MCAO.

Here we show what a 2 arc minute field of View looks like without AO (movie).
Here we show what this field looks like with AO and one "rubber mirror" (movie).
Here we show what it could look like if you had 3 rubber mirrors (movie).

So the future for ground based astronomy has never looked brighter (or sharper) thanks to AO.

At our own MMT there is a program to use 5 Rayleigh Beacons to do GLAO and LGS AO at our own MMT.

AO is already indispensable to large telescopes today. It will play an even bigger role in the development of the 30m class telescopes of tomorrow!

Astronomy 302

Lecture 10 & 11

Introduction to Adaptive Optics

1.0 INTRODUCTION