Report from the External Review Committee of the

 

Pueo-Nui Workshop

 

held on Mai 22-23, 2003 at the

Laboratoire d’Astrophysique - Observatoire de Grenoble.

 

Contributors:

 

Christophe Dumas, (Christophe.Dumas@jpl.nasa.gov)

NASA/Jet Propulsion Laboratory, Chair

Marc Ferrari, (Marc.Ferrari@observatoire.cnrs-mrs.fr)

Observatoire de Marseille

Pierre Kern, (Pierre.Kern@obs.ujf-grenoble.fr)

Observatoire de Grenoble

Norbert Hubin, (nhubin@eso.org)

European Southern Observatory

Guy Perrin, (Guy.Perrin@obspm.fr)

Observatoire de Paris-Meudon

 

 

Introduction:

 

Olivier Lai and François Ménard, the main organizers of the PUEO-NUI workshop, have requested that the External Review Committee (1) generates a review of the discussion presented during the workshop, and (2) provides its view on the scientific and technical cases for an upgrade of PUEO.

 

In order to help in this task, the committee was asked to respond to a series of specific points about: the science cases for the upgrade of PUEO, the instruments that could enhance the science results obtained with Pueo-Nui, the technological & engineering aspects of the upgrade.

 

The present document captures these responses and provides a summary of the comments and suggestions from the review board committee.
A) SCIENCE

 

1) Are there compelling science cases?

 

Several programs, which would all benefit from the upgrade of PUEO, were presented during the first day of the workshop. Some of them would simply return modest improvements in regards to the results obtained with the current PUEO system. Other programs could only be carried-out with PUEO-NUI, like the ability to image diffuse and extended structures, or to detect faint point sources (fainter that currently possible with PUEO), within the immediate proximity (0.5” to a few arcseconds) of bright objects (stars, planets). Overall, the stability and high-Strehl of the PSF produced by PUEO-NUI in the near-infrared would return the crispiest images that can be obtained at this wavelength range. A strong interest to work at shorter wavelengths (I band) was also demonstrated. At these wavelengths, PUEO-NUI would allow to obtain images with a more modest Strehl ratio while still producing diffraction limited images at a resolution of ~40mas at R band (0.65mm). The direct competitor of PUEO-NUI at visible wavelengths would be the Hubble Space Telescope.

 

 

- Solar system science: PUEO-NUI would allow to search for asteroid satellites closer-in from the primary (R-I bands), or image diffuse structures like planetary rings or atmospheric structures at high-contrast (H-K bands).

The inner systems (ring and satellites) of the giant planets Uranus and Neptune could also be studied at very high contrast in the near-infrared.

 

- Galactic: Near-infrared search for brown-dwarfs and debris disks at higher contrast and closer-in from the primary (need for coronography).

 

- Extra-galactic: QSO and deep-field observations (using nearby bright stars). Observations of the galactic center (need for near-IR wavefront sensor).

 

 

2) Are the science cases compelling and numerous enough to justify an upgrade?

 

A large part of the science programs proposed for PUEO-NUI could be equally carried-out on large 8-10m class telescopes equipped with AO. The most compelling cases for an upgrade are those based on the ability for PUEO-NUI to produce a very stable PSF in the near-infrared range. Such performances will allow very high dynamic range and would therefore make possible the detection of low contrast features/objects (atmospheric variability of Uranus and Neptune, search for asteroid satellites, study of the inner system of the giant planets, search for brown dwarfs and debris disks). In order to be competitive, the modifications of current AO system should be implemented rapidly (within the next 2-3 years), and preferably before the operation of the new generation of AO systems on 8-10m class telescopes.

 

B) INSTRUMENTS

 

1) Is the upgrade of PUEO justified with the current instrument only (many science cases require instruments not presently available: e.g. Integral Field Unit for spectroscopy, long-slit spectroscopy, coronograph)?

 

The realization of a fast upgrade of PUEO under a limited budget is in contradiction with the need for a series of new instruments (coronograph, low-to-medium spectroscopy, etc). Because it is extremely important for CFHT to remain at the forefront of the technology in the area of high-angular resolution and high-contrast imaging, a limited set of improvements of the actual system focused on the main elements (e. g. deformable mirror, wave front sensor, real time computer) should be considered during the first phase. This upgrade would allow better performances than HST in JHK bands (due to its larger mirror diameter) on bright objects. At shorter wavelengths (R band), PUEO-NUI would deliver an angular resolution equivalent to a 10-12m telescope observing in the near-infrared. Science programs like searching for circumstellar disks or low-mass companions to nearby young stars, or  exploring the close vicinity of main-belt asteroids to search for small (~km size) satellites, could be successfully investigated with such an instrument.

 

It would nevertheless be regretful to limit the science capabilities to direct imaging only and future instrumentation (team/visitor as well as general public instruments) should be investigated along with the development of PUEO-NUI.

 

 

2) Which new instrument(s) would make the upgrade very attractive?

 

In order to take advantage of the full capabilities in high contrast and high angular resolution offered by PUEO-NUI, a suite of dedicated instruments (or upgrades) is strongly recommended. The technical study of these instruments will have strong high-level requirements on the system (i.e. extreme quality of the optical elements) and should therefore be identified very early during the design phase of PUEO-NUI. The list of instruments should include, at least:

 

-       A spectrograph returning low-to-medium resolution in the 0.8-2.5mic range (cross-dispersed), but an upgrade of GRIF to obtain spectroscopic capabilities at H and K bands would be an acceptable cost-driven compromise.

 

-       A new generation coronagraph (i.e. with pupil mask in addition to occulting spot).

 

 

 

C) ENGINEERING

 

1) Are there technological show-stopper? (e.g. Deformable Mirror)

 

No new technology would need to be developed to upgrade PUEO to 105 actuators. The proposed AO system could be realized using improvements of components from available technologies. Some recent ESO results from MACAO show the possibility of improving the performances of PUEO within the applicable time and budget constraints. Nevertheless, if new technology components with sufficient performances and reliability become available in a timeframe compatible with the design phase, the opportunity to integrate them in PUEO-NUI should be taken. This is particularly important in order to keep the CFHT AO expertise at the top level with respect to the next generation of AO systems (ELT or 8m HDR).

 

2) Is considering the upgrade as a test-bench for ELT or HDR 8-m compelling? If yes, is it compelling enough to justify an upgrade?

 

The engineering aspects and integration of new technology components for PUEO are very attractive and compelling when considered as a test-bench for high dynamics on large telescopes. The engineering validation would include analysis of the optical quality, stiffness, data transfer speed, etc. Some new software developments will be required. In this context, PUEO-NUI could be considered as an intermediate instrument between the current generation of AO systems and the future Extreme-AO systems. The realization of PUEO-NUI would address important issues for Extreme-AO, while investigating new concepts for coronography and related systems like double tip-tilt stage correction or phase mask position control. PUEO-NUI would also permit exploring specific data processing products related to second order effect on image degradation. A high order adaptive optics system may increase the relative contribution of these second order aberrations to the final optical quality: diffraction, parasitic light due to optical component diffusion, remaining optical aberration corresponding to high spatial frequencies (i.e. aberrations corresponding to modulation on smaller scale than the inter-actuator distance).

 

Also, it is important to consider that PUEO-NUI would be the first (and only?) high-order AO curvature sensing system. Other high-order systems planned for large telescopes like VLT or Keck are more than likely to be based on a Shack-Hartmann concept. It is not clear how/what part of the experience acquired from the upgrade of PUEO could easily be transferred to future projects on large telescopes since the technology and the staff would be different.

 

 

3) Among the *long* wish list: laser guide star, larger wavefront sensor field-of-view, infrared wavefront sensor, higher limiting magnitude, spectroscopy at R~200-400 and R~2000-5000, polarimeter, PSF suppression techniques (coronagraphy, TRIDENT, etc), tip-tilt sensor (required for laser guide star), which one are the most needed to justify an upgrade? Are they feasible?

 

As described earlier, coronographic and spectroscopic capabilities should be considered. Additionally, fainter limiting magnitude (or laser guide star) would come next in the priority list.

 

D) SUMMARY

 

Unique AO expertise has been acquired at CFHT and the upgrade of PUEO should be considered as an opportunity to secure and enrich this knowledge for the future years. Considering the various efforts planned worldwide by other groups (ESO 8m-VLT-PF, Keck 10m-XAO, Palomar 5m-AO, etc) PUEO-NUI is most timely and should be started as soon as possible in order to make the instruments available at the telescope no later than 2006. The realization of the upgrade could also be considered as an intermediate step between the current AO systems and the future concepts of Extreme Adaptive Optics (although the transfer of knowledge from PUEO-NUI to the new generation AO systems on large telescopes is unclear).

 

Some science projects would benefit from the upgrade of PUEO, although most of the AO programs could be equally carried-out in the near-infrared on large telescopes currently equipped with an AO system. The programs that would benefit the most from the upgrade of PUEO are those requiring a very stable PSF: detection of faint and low-contrasted features such as searching for circumstellar disks around young-nearby stars, studying the variability of planetaary atmospheres, or studying the inner systems of the giant planets (rings and small satellites).

 

It would be regretful to limit the domain of investigation offered by PUEO-NUI to imaging only. For this reason a minimum suite of instruments (new or upgrades) is strongly suggested. This would include at a minimum some spectroscopic capabilities (low-to-medium resolution) and a new generation coronograph. Improving the limiting magnitude of the system to a lower limit should also be considered.