We briefly present the basic AO principles and how anisoplanatism affects the off-axis image quality. The basic principles of MCAO are then presented together with the parameters of the Gemini MCAO.
By compensating for the atmospheric turbulence in a 3-D fashion, MCAO provides uniform image quality (diffraction-limited in the near-IR) over a much wider field than regular AO (one to two arcmin in diameter depending on the IQ criterion).
This is particularly important in tackling a number of astrophysical problems that require a relatively wide field of view or very accurate photometry.
MCAO also removes the "cone effect" associated with the use of laser guide stars. The average sky coverage in H band is approximately 50% over the whole sky.
Anisoplanatism, Cone effect and Sky coverage: The plague of Classical AO
Although it is a rather new technique, AO has been and is continuing to provide an abundance of new scientific results. All the major large telescopes have recognized its unique and innovating value. However, wide application of AO has not yet been realized because of several well-identified problems.
Limited Anisoplanatic Angle
The atmosphere all ground-based telescopes look through is 3-dimensional. Because wavefront sensors measure the phase perturbations integrated only along the direct line of sight, in directions different from the guide star, the integrated phase perturbation is different, causing a degradation in the off-axis corrections to the wavefront. How fast this degradation manifests itself depends on a number of parameters, a few of which are: the vertical distribution of turbulence (the Cn2 profile), and the wavelength and order of the AO system (how many modes are corrected). This phenomena is known as anisoplanatism, with the isoplanatic angle defined as the angle from the guide star at which the Strehl ratio is reduced by 50%. The isoplanatic angle varies with wavelength1.2 and airmass1.6. For low to medium order AO systems currently in operation in both Cerro Pachon and Mauna Kea, typical values for the isoplanatic angle are 20” in J band, 30” in H band, and 40” in K band. The isoplanatic angle is even smaller for the new generation of higher order systems on current large telescopes.
The images below present an example of anisoplanatism. They are extracted from a 35”x35” K band image taken with the infrared camera KIR and the AO system PUEO at the Canada-France-Hawaii Telescope. They are separated by approximately 30” (center to center). The difference is immediately apparent: the Airy rings are nearly gone, the loss in Strehl is of a factor of 2 (47% Strehl left image, 24% right image) and the FWHM degrades from 0.140” to 0.185”. This is a typical result for an object at an airmass of 2, and illustrates the importance of resolution and Strehl ratio for wide-field imaging (which accounts for a significant fraction of stellar population studies). Not only is the Strehl ratio important to increase the signal-to-noise ratio, but the angular resolution also plays an important role in crowded fields.
Figure 1: Example of anisoplanatism
AO compensation using natural guide stars can only be obtained in the vicinity of relatively bright stars (R~15 mags). This puts a severe restriction on sky coverage; in fact limiting the accessibility of the sky to only 5%. This realization led to the idea of using Laser Guide Stars (LGS) to create bright guidestars that could be used for AO compensation at any position in the sky. To date, the most promising laser guide star concept uses the fluorescence of sodium atoms in the mesosphere, a layer well studied by atmospheric scientists that lies between 90 and 100 km above sea level. A number of experiments validated the LGS concept (at Calar Alto in Spain and Lick Observatory for instance) by closing an AO loop using a sodium beacon.
However, LGS does not come without limitations of its own. With the laser guide star being created at a finite distance (90-100km), the return beam that the wavefront sensor uses does not pass through the same volume as a beam coming from an astronomical object at infinity. This geometrical effect worsens as telescope diameter increases. An order of magnitude for the Strehl ratio loss is 50% at 1 micron for an 8-m telescope with a typical Cerro Pachon/Mauna Kea vertical turbulence profile. This becomes a major limitation for current large telescopes, preventing the extension of laser guide star AO to the visible part of the spectrum, thus LGS AO on the next generation of giant telescopes will be severely limited until a solution to this problem is found.
By using several natural or laser guide stars with several deformable mirrors, a uniform image compensation can be achieved on a field significantly larger than the natural isoplanatic patch. This is known as Multi-Conjugate Adaptive Optics, or MCAO. The very essence of MCAO – probing and correcting a large turbulent volume – also takes care of the cone effect when using laser guide stars, which increases the compensation performance on current 8-m telescopes and opens the gates for the application of LGS AO on giant telescopes. The wide field (over an arcminute at the diffraction limit in the current design of the Gemini MCAO) opens new frontiers for scientific discoveries – see the MCAO science case document.
Figure 2: MCAO system.
In the figure above, a sketch of a typical MCAO system, two wavefront sensors are looking at two off-axis guide stars. Perturbations at different altitudes will be seen with different shifts between the two sensors. The information from both sensors is processed by a central processing unit, which feeds it to a reconstructor that computes the command to apply to a set of deformable mirrors to minimize the WFS error signal. The 3D turbulence content is never explicitly reconstructed in this process, avoiding the extreme sensitivity to noise of the latter technique. In fact, MCAO’s sensitivity to noise is very similar to that of classical AO, which has the fortunate consequence that the guide star brightness requirements are the same (e.g. laser power).
A collection of more complete articles on the MCAO principles, limitation and performance can be found in the Gemini AO archives.
The table below presents the main characteristics of the Gemini MCAO system. More details on the instrument itself and on the expected performance are provided on the following pages in this site.
DM conjugate ranges
0, 4.5 and 9 km
16, 16 and 8 actuators across the pupil
Guide Star geometry
(0,0) and (+/-42.5, +/-42.5) arcsecs (LGS)
16 by 16 (LGS); Tip-tilt (NGS)
LGS Laser Power
Equivalent to 125 PDEs/cm2/s at WFS
Behind telescope secondary, 45cm diameter
3 times 19 (for 50% Strehl reduction in H)
33Hz (LGS); 0-90Hz (NGS)
Decoupled control of the LGS and NGS modes
Table 1: MCAO main parameters