Why aren't ground-based observatories using adaptive optics for visible wavelengths (circa 2016)?

Question

Adaptive Optics (AO) techniques allow ground based observatories to dramatically improve resolution by actively compensating for the effects of Astronomical Seeing.

The atmospheric effects are quite variable in both time and location. A parameter called Isoplanatic Angle (IPA) is used to express the angular extent over-which a given wavefront correction optimized for one point (usually a guide star, artificial or natural) will be effective. As an example, Table 9.1 in this Giant Magellan Telescope resource shows values for IPA scaling almost linearly (actually: $\sim\lambda^{6/5}$) from 176 arcseconds at a wavelength of 20 microns to only 4.2 arcseconds at 0.9 microns.

This suggests an IPA of 2 to 3 arcseconds for visible wavelengths, which taken by itself is not a killer limitation.

However, it seems almost all currently active AO work is done exclusively in various infrared wavelengths, apparently down to 0.9 microns but no further. (AO is also implemented computationally to array data in radioastronomy.)

Is this because the observed wavelength needs to be longer than the guide star monitoring wavelength? Because it is simply much harder and there is always Hubble above the atmosphere for visible work so it's not worth the extra effort, or is there another more fundamental reason?

I'm not looking for speculation or opinion, I'd like a quantitative explanation (if that applies) - hopefully with a link for further reading - thanks!

Answer 1

There's a pretty good discussion at this page.

There are several factors at work:

1. The smaller isoplanatic angle, as you note. This limits how much of the sky you can observe with AO, since your target needs to be within the isoplanatic angle of a bright enough references star. (Even with laser guide stars, there is still a need for a reference star for "tip/tilt" correction.) The difference in angular area on the sky means that the area of the sky that can theoretically be observed with AO will be about 20 times larger in the near-IR than in the optical, just from the difference in isoplanatic angle.

2. The effects of turbulence are stronger and have shorter timescales in the optical. This has three effects:

   A. The corrective optics (e.g., deformable mirror) need to have more movable parts ("a near-perfect correction for an observation done in visible light (0.6 microns) with an 8-m telescope would require ~ 6400 actuators, whereas a similar performance at 2 microns needs only 250 actuators.") and need to operate on a faster timescale.

   B. In addition to the electromechanical complexity, you'll have to do far more in the way of calculations to drive all those actuators, and on a shorter timescale. So the computing power required goes up.

   C. In order to provide the inputs for the corrective computations, you need to observe the reference star on a much finer angular scale ("A large number of actuators requires a similarly large number of subapertures in the wavefront sensor, which means that for correction in the visible, the reference star should be ~ 25 times brighter than to correct in the infrared."). This limits how much of the sky you can do AO for even more: a star that might be bright enough in the near-IR to correct a region 20-30 arcsec wide isoplanatic patch won't be bright enough to correct the corresponding 5-arcsec-wide isoplanatic patch in the visible.

3. In order to make corrections, you need to observe the reference object in the optical. This is easy to do with a near-IR setup using an optical/IR beamsplitter: send the optical light to the AO equipment and send the near-IR light to the near-IR instrument. In the optical, you use an optical beamsplitter to send half the light to the instrument and the the other half to the AO equipment. This means that the AO equipment only gets half the light it would if it were used with a near-IR instrument, which makes it (even) harder to do the corrections.

Finally, there is an issue unrelated to the AO itself, which is that you need different science instruments depending on whether you're working in the optical or the near-IR. Optical instruments use silicon CCDs for detection; these are only sensitive out to about 0.9-1 microns. Near-IR instruments use different detectors (usually HgCdTe-based), which are good from about 1-3 microns. (Near-IR instrument also need a different design to reduce contamination from thermal emission from the telescope and optics for observations at wavelengths longer than 2 microns.) So in practice the choice has been: combine AO with a near-IR instrument and get good performance with affordable/feasible technology, or combine AO with an optical instrument and get very limited performance with more expensive (or even, until recently, unattainable) technology.

Nonetheless, there are some optical AO systems starting to appear, such as MagAO on the Magellan telescope (which has both an optical instrument and a near-IR instrument, and can correct for both simultaneously).

Answer 2

The simple answer for the wavelength part is that performance of AO systems degrades the shorter in wavelength you look. The basics of what happens is as you go to shorter the wavelengths of light, you need a finer plate scale to detect variations in seeing which requires very expensive (and in some cases non-existant) hardware. You also need a higher AO frequency (ability to measure the light and deform/refocus the telescope) to account for the higher frequency of light, this again takes very expensive hardware if it exists at all at the frequency required.

This is because some of the basic calculations (not taking Zernike polynomials into account) are based on the Strehl ratio and Here (ratio of peak intensity of an aberrated image compared to perfect image) to figure out what the intensity of the source should be and the FWHM (Full-Width Half Max - width of the profile of light at half intensity) to essentially measure where the light should be. Both of these measurements are wavelength dependent.

Basic further reading can be found at The Isac Newton Group of Telescopes. Much more in-depth reading can be found at the university of Arizona Optics department.