Rationale

Pueo is currently the most scientifically productive adaptive optics (AO) system. If we look to refereed astronomical journals, and consider the scientific publications based on AO observations, we find that more and more of these publications are based on Pueo results. In 1998, 36% of them were made with Pueo. In 1999, 10 out of 20 came from Pueo that is 50%. This remarkable achievement is due to the ability of Pueo to work with faint guide stars (up to at least mag 15), an ability shared only by Hokupa'a, the UH visitor instrument and only other currently operational curvature AO system.

However, Pueo has only 19 actuators which compensate the equivalent of 8 Zernike modes. For comparison, the next two most scientifically productive systems compensate the equivalent of 14 modes (Adonis) and 19 modes (Hokupa'a). Because of its moderate compensation capability, the use of Pue'o is limited to the near infrared. By comparison, when operated on the CFHT, a 36-actuator system such as Hokupa'a produces the largest image improvement in the I band ($\simeq$ 0.9 $\mu$m). Upgrading Pueo to 36-actuators would extend its capability toward shorter wavelengths well into the range of CCD detectors.

Another desirable characteristics of an AO system is its ability to use extended objects as guide sources. Currently, the field-of-view of the wave-front sensor in Pueo is limited to 3 arcseconds. This is a serious limitation. For instance, Pueo has problems guiding on Neptune and cannot guide on Uranus. It has also problems guiding on some non-stellar sources such as galaxy cores or small planetary nebulae. Artifacts are likely to appear when it is not guiding properly. A drawback of a larger field of view is the increase of the sky background contribution when guiding on a faint source. However, this drawback can easily be overcome by using an iris as a field stop. The iris could automatically close down to a preset value once the loop is closed.

Finally an AO system should produce a clean point spread function (PSF). Fig. 1 shows a deep image of a binary star recorded with Pueo. It reveals a pattern in the PSF wings with a six-fold symmetry. Such patterns are detrimental to the detection of faint companions. They are thought to be produced by the deformable mirror. Since Pueo was built, important advances have been made in the fabrication of bimorph mirrors. As demonstrated with Hokupa'a, such patterns can now be avoided. An upgrade of Pueo (hereafter called Pueo Nui) would also benefit from these advances.
 
 

Fig. 1. Deep stellar exposure made with Pue`o on the CFHT. Note the regular pattern in the PSF wings with a six-fold symmetry, an artifact produced by the deformable mirror. The angular separation of this binary star is 1.4 arcsec.