The CFHT Future

The one unique asset CFHT partners enjoy is occupying the best-seeing site in the world.

The median natural delivered image quality (DIQ) over an arbitrarily large field-of-view at the focus of a perfect telescope at the CFHT site would be 0.45" fwhm at 500 nm and 0.35" fwhm at 750 nm.

In all other respects (aperture size, field size, clear sky fraction, opto-mechanical quality and agility, MIR performance, etc.) some of their competitors are or will soon be much better equipped, on the ground or in space. A non-competitive "national" facility is of no interest.

Any future astronomical use of the CFHT site must take full advantage of t uniqueness. This is, in fact, a condition for site allocation on Mauna Kea.

The current CFHT regretfully falls short of that goal by a factor of ~1.8 in DIQ or of ~3.3 in science per unit time. The causes of that shortcoming have been known and characterized for 30 years (Racine 1984, Racine et al. 1991). SAC-recommended and Board-supported efforts to eliminate them have been modest and have had limited success. The median 500 nm MegaCam DIQ has gone from 0.69" in 2005 to 0.80" in 2008 (Salmon et al. 2009). For Subaru, that figure is 0.64" (Miyashita et al. 2004).

Wide-field, high resolution (WFHR) spectro-imaging would be a perfect scientific activity at the CFHT site. This is indeed the science driver of the current 'IMAKA project. But the 'IMAKA GLAO approach alone is insufficient (Racine 2009). Its benefit will come almost totally from the compensation of the optical turbulence generated in and by the dome and of the telescope opto mechanical imperfections. Priority should be given to preventing these diseases rather than to applying technologically exciting band-aids on the damages they cause. GLAO, or MCAO, can then be implemented to do even better.

How the natural DIQ performance of the CFHT can be made to approach the full potential of the site is an issue that needs to be examined in details. Engineering, schedule and costs are important. So is competition from Subaru, LSST et al. But a dedicated WFHR facility at the site would remain competitive for a long time. That facility need not use the general purpose telescope that the current CFHT is. Before partnership in VLOT/TMT became the LRP OIR priority, the option of a wide-field 8-m was seriously considered. It may not be too late in the life of the CFHT to make a WFHR "ng" CFHT as good as the CFHT should long have been.

References
Racine, R. (1984),"Astronomical Seeing on Mauna Kea and at the Canada-France-Hawai'i Telescope» Proc. IAU Coll. no.79, E.S.O. Garching.
Racine, R., Salmon, D., Cowley, D., and Sovka, J. (1991) "Mirror, Dome and Natural Seeing at the CFHT", PASP, 103, 1020
Racine, R. (2009) "Steps toward ‘IMAKA", Report to the ‘IMAKA Working Group, CFHT Corp. June 8, 2009.
Miyashita, A., Takato, N., Usada, T. et al. 2004, SPIE 5489, 207
Salmon, D., Cuillandre, J-C, Barrick, G., Thomas, J., Ho, K. Matsushige, G., Benedict, T. and Racine, R. (2009) PASP 121 (in press)