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Ground-layer adaptive optics wide-field imager

The Canada-France-Hawaii Telescope is exploring the possibility of constructing a wide-field ground-layer adaptive optics imager, named 'IMAKA, that will work in the optical region of the spectrum.
3 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003380

The Canada-France-Hawaii Telescope (CFHT) is located at one of the premier sites in the world for ground-based optical and infrared astronomy. While the superb seeing (the image quality as determined by atmospheric conditions, especially turbulence) on Mauna Kea has long been known, a delivered image quality commensurate with the site has yet to be fully realized, even at the most modern facilities. A facility that is able to take full advantage of the site would produce unprecedented images over large fields of view. 'IMAKA is a path to achieve this on CFHT.

Aptly named ('imaka means ‘lookout’ or ‘scenic viewpoint’ in Hawaiian), 'IMAKA will deliver angular resolutions 2–3 times better than the best current large-field optical imagers. This will enable new science and will effectively increase the telescope's aperture to compete directly with the 8–10m class telescopes. 'IMAKA will deliver angular resolutions within a factor of 2–3 of that of the Hubble Space Telescope (HST) but with a field of view about 400 times larger than the Advanced Camera for Surveys (ACS; see Figure 1).

Figure 1.The Pleiades stellar cluster showing the relative sizes of the 'IMAKA-Cassegrain design one-degree diameter field of view (the highlighted circle) compared with that of various instruments in HST. Hubble instruments are indicated by their acronyms: Fine Guidance Sensors (FGS1, 2 and 3), Space Telescope Imaging Spectrograph (STIS) CCD, Wide Field Planetary Camera (WFPC3), Near Infrared Camera and Multi-Object Spectrometer (NICMOS), and Advanced Camera for Surveys Wide Field Channel (ACS/WFC) and High-Resolution Channel (ACS/HRC). 'IMAKA's field of view is roughly 400 times larger than the ACS/WFC with an angular resolution within 2–3 times that of ACS.

CFHT is uniquely positioned to deliver and exploit the capabilities of 'IMAKA. Its community has led the field of wide-field visible-wavelength astronomical imaging with instruments such as MOCAM (the 1996 CCD mosaic camera), CFH12k (the 2000 12k × 8k pixel camera), and the 2003 CCD Mega-Cam. Each successive instrument made use of technological gains in detector sizes to push to progressively larger fields of view. We now stand poised to bring two new technological advances to this area: orthogonal-transfer CCDs and ground-layer adaptive optics systems. These advances move wide-field imaging in an orthogonal direction, that of high-resolution images.

While the adverse effects of CFHT's enclosure (the telescope's dome itself) on its delivered image quality are well established,1 the emergence of improved technologies and improved understanding of the turbulence above/at the site led to 'IMAKA's development. Recent studies at CFHT2 and on Mauna Kea3,4 show that the delivered image is degraded by two factors. One is a very confined thin-layer close to the ground/enclosure and/or sources within the CFHT facility (dome/mirror seeing and optics). The other is the relatively weak optical turbulence within the free-atmosphere (at a height superior to 1km) above the site. These are the worst offenders to optical image quality.

The fact that most of the effects are ‘local’—arising within the last few tens of meters of the optical path—means that a ground-layer adaptive optics system (GLAO) correction is significant and appropriate over a very wide field of view. In addition, the development of orthogonal-transfer CCDs (OTCCD)5 provides a means to populate a large focal plane cost-effectively and also further improves the image quality by correcting for tip/tilt in the residual GLAO correction. These techniques are advanced and all of the components are near the level of maturity required for 'IMAKA (e.g. Multiple Mirror Telescope—MMT—GLAO,6 ESO's Multi-Conjugated Adaptive Optics Demonstrator,7 and Gemini's Multi-Conjugated Adaptive Optics).8 Indeed, results from MMT and the William Herschel telescope in the near-infrared over modest fields of view (e.g. a few arcminutes) show that the image resolutions obtained match predictions very well.

In summary, 'IMAKA focuses on an instrument that enables high-resolution wide-field imaging at CFHT. It deploys an innovative, extremely wide-field ground-layer adaptive optics system with natural guide stars to correct for the local turbulence effects and a focal-plane populated with orthogonal-transfer CCDs to remove residual tip/tilt motion from the upper atmosphere. However, the path to achieving these resolutions will certainly employ a mixture of passive and active elements across the observatory as a whole. The enclosure seeing at CFHT, that is, the turbulence resulting of heat sources or uneven cooling inside the telescope's dome, is clearly a large portion of the total image degradation along the line of sight. But the planned modifications to the existing CFHT enclosure—passive measures include venting the dome to provide better air flow through the dome and flush out warm air pockets that cause turbulence—will help 'IMAKA achieve its goals. However, we recognize that many terms that adversely affect image quality are variable and/or difficult to completely control passively, so active measures, such as GLAO and OTCCDs, are a means to ensure the angular resolution is obtained.

The next step in this project is to carry out a fully detailed study of the instrument and to develop a full-blown science case. The detailed study will include an optical design and numerical simulations to help establish clearly what the image quality will be, and will determine the size of the final field of view of the instrument.

Harvey Richer
University of British Columbia
Vancouver, Canada

Harvey Richer is a professor of physics and astronomy who received his PhD from the University of Rochester. He was awarded a Fulbright fellowship in 2005 and was a distinguished scholar at the Wall Center in 2009–2010.