1  HDRT Concept

The High Dynamic Range Telescope (HDRT) expands on the 6.5 m aperture New Planetary Telescope which was devised as a replacement for the NASA Infrared Telescope Facility. The concept described here benefits from the ideas and from input from all three partner communities within the CFH consortium. As conceived, the HDRT will provide unprecedented photometric and angular resolution dynamic range. As the world's largest and highest resolution optical telescope it will also provide wide-field observations with an etendue significantly larger than even the special-purpose survey telescopes now in their planning stages. Beyond this, the HDRT allows unique opportunities for observing faint astronomical objects in the near environment of bright sources.

The primary operational capabilities of the HDRT are

• Aperture: Effective resolution of 22 m (diffraction limited over at least 1 x 1 arcmin), light collecting area equivalent to a 15.9 m unobstructed aperture

• High photometric dynamic range: This unique capability will be achieved through its unobstructed 6 x 6.5 m apertures. The wide- and narrow-field low scattered light properties of the HDRT will be unrivaled, both for direct imaging and coronagraphic applications.

• Versatility: Several operating modes involving wide (2^o) and narrow (1¢) fields-of-view (FOV). Its 6.5 m unobstructed subapertures may be coherently combined or independently used with distinct detectors.

• High angular resolution: HDRT will achieve high spatial resolution using adaptive optics while capitalizing on the unobstructed pupil.

2  Science Drivers

• Extrasolar planets

• Star formation

• Kuiper Belt objects

• Weak lensing surveys

• Wide-field galaxy and redshift surveys

• "Origins" research themes

3  Key HDRT Technical Issues

• Off-axis optical design: Current technology allows 6.5 m off-axis mirrors to be fabricated, and an unobstructed pupil offers unique capabilities for imaging faint objects in the presence of bright sources.

• Scalable curvature AO system: This telescope design will use filled-subaperture 6.5 m curvature AO "units" that we know can be built and that we understand very well. This is an effective route for achieving an affordable, working, and maintainable AO system for a 22+ m telescope.

• Thin mirror technology: Trading steel for active optical alignment mechanisms and software has proven to be effective in modern telescopes. In addition to advantageous thermal and mass issues, a thin mirror allows small conic changes that can yield very wide-field performance from a parabolic primary. Building the telescope from 6.5 m unobscured off-axis mirror segments minimizes technical risks and allows for a scalable design.

• Multimode optical configuration: The off-axis unit subaperture design has fundamental advantages for flexible instrument implementation schemes (since the optical path is accessible). Ultralow emissivity IR performance, adaptive secondaries, and optimized instrument-secondary mirror configurations are a natural advantage of this configuration. The proposed off-axis Gregorian configuration has important advantages for scattered light reduction and adaptive optics because of the accessible pupil image.

• Compact beam alignment optics: Considerable work has been done by the partners (following Roddier) in implementing membrane mirrors and rotational shearing interferometers for achieving piston, alignment, and focus adjustment of the subapertures.

• Telescope mount and enclosure: This will require imaginative and cost effective approaches to meet the design goals of the facility within the constraints of the CFHT envelope and Mauna Kea master plan.

¹Institute for Astronomy, UH