The last year has seen good progress in developing efficient wavefront control algorithms for future MCAO and very-high-order AO systems on ELTs, and in implementing detailed simulations to evaluate MCAO performance on 8-m class telescopes. Detailed simulations of GSMT and other ELTs should be feasible in the near future, provided that existing codes are parallelized to run on supercomputers and upgraded to incorporate recent advances in control algorithms. These simulations will enable a new level of design and optimization studies for ELT AO systems, including:
- Final confirmation of first order AO system design parameters, including the order of correction, control bandwidths, deformable mirror (DM) conjugate ranges, and guide star constellation (including options for natural guide star (NGS) MCAO and Rayleigh guide star MCAO);
- Detailed point spread function (PSF) characterization and estimates of NGS magnitude limits to support the development of the science case;
- Analysis of wavefront sensor (WFS) performance with highly elongated laser guide stars (LGSs), as will be necessary to determine laser power requirements and assess a variety of suggestions that have been made to compensate for this effect;
- Evaluation of AO performance with a segmented primary mirror, and the hierarchical interaction between the AO and active optics (aO) control systems; and
- More rigorous error budgeting of many implementation error sources, such as noncommon path wavefront errors, DM-to-WFS misregistration, DM hysteresis.
Moreover, detailed performance predictions, including implementation error sources and aO errors, will soon be more accurately anchored against real-world AO performance on smaller telescopes, beginning first with NGS AO and LGS AO, and then proceeding to very-high-order AO and MCAO as 4-8-m class systems come online. Progress on advanced wavefront control algorithms is also nearing the point where work may begin on the wavefront reconstructor electronics that will eventually be needed to implement these techniques in hardware.
The wavefront sensing requirements for GSMT range from foreseeable extensions of existing technology to dramatically new concepts. The order of wavefront sensing envisioned in this report for MCAO and direct Cassegrain AO is about 64 subapertures across the telescope aperture, which could be implemented using a conventional Shack-Hartmann WFS with a high-speed, low-noise CCD array of 2562 or 5122 pixels (the larger number of pixels might be necessary for wavefront sensing with highly elongated LGSs). This type of device appears to be a feasible extension of existing 1282 wavefront sensing CCDs, although it is unlikely to be developed without technology development funding. The situation is probably different for very-high-order AO, because CCD arrays with 10242 pixels would be required to implement Shack-Hartmann sensors with up to about 2562 subapertures. Alternative wavefront sensing approaches requiring fewer pixels per subaperture should be considered, including the shearing interferometer and the Smartt, or point diffraction, interferometer. Low order versions of these sensors that can be tested in closed-loop AO systems should be developed, first in the laboratory and eventually in the field.
Additional areas where research into novel wavefront sensing concepts might eventually be justified include (1) high-order IR wavefront sensors for Mid-IR NGS AO in regions of the sky with no visible stars, and (2) LGS wavefront sensors optimized for use with highly elongated LGSs. More work on the science case and detailed AO simulations is necessary to establish an actual requirement for these concepts, however.
The situation is somewhat less advanced with respect to wavefront correctors. The DMs required for MCAO on GSMT are essentially factors of two extrapolations (in linear dimension) from the existing mirrors of order 352 with inter-actuator spacings of 7 to 9 mm. Such mirrors could probably be built today, although the cost could easily be much higher than the rate of $1,600 per actuator currently typical for smaller mirrors. However, there are at least four other areas where significant innovations in wavefront correction technology will or may be necessary:
- Adaptive secondary mirrors with 1000-2400 actuators and a large (~10 µm) dynamic range are required for all of the AO system concepts considered in this report.
- Very-high-order systems will require microelectrical mechanical systems (MEMS) DMs with good figure quality, relatively high bandwidths, moderate stroke, and somewhere between 1282 and 2562 actuators. The current state-of-the-art is 122 actuators.
- Cryogenic operation for both conventional and MEMS mirrors may be desirable for observations in K-band, but this is not yet confirmed by the science case.
- Finally, we believe that the bandwidth requirements for the 300-500 mm diameter tip-tilt mirror postulated for the present MCAO opto-mechanical design may be extremely stressing for a mirror of this size, but more work on the aO disturbance characteristics will be necessary to confirm this.
Ongoing R & D activities in the first two areas should be continued and expanded, and telescope design studies should work towards estimating the tip-tilt disturbance spectrum reasonably soon.
Sodium guide star laser technology remains an essential area for R & D funding, both for 8-m class LGS AO and MCAO systems and for ELTs. The clearest way to overcome the LGS elongation problem on ELTs would be to increase the LGS signal level by perhaps a factor of 3 to 5. Increased laser power levels would also be useful for improving AO performance at shorter wavelengths. Other possible approaches include (1) novel laser pulse formats that would allow a short pulse to be "tracked" through the sodium layer, (2) NGS MCAO systems, and (3) Rayleigh guide star MCAO systems. Laser and WFS design work for these alternate approaches can be postponed, pending analysis and simulation results confirming that they are in fact viable system concepts.
Finally, all of the AO system opto-mechanical concepts presented in this report must be subjected to further analysis and trade studies, as is planned for the follow-on stages of the GSMT design effort.