Hokupa'a Introduction

Hokupa'a was used at Gemini North from 2000B to 2002A and was replaced by Altair in 2003

Hokupa'a is a natural guide star (NGS), curvature-sensing adaptive optics system (AOS) built by the University of Hawaii. It gives near diffraction-limited resolutions in the K and H bands. We have arranged for its use as a loaned instrument on Gemini. A 1-2.5Ám near infrared camera (QUIRC) is located at the exit focal-plane.

Hokupa'a is based around a 36-element curvature wavefront sensor (WFS) and bimorph mirror. In brief, Hokupa'a makes wavefront measurements using pairs of extra-focal images whose differences are proportional to the curvature of the wavefront. A lenslet array and a set of avalanche photo-diodes (APDs) define the sub-elements in the measurement. A deformable mirror (DM) with an arrangement of actuators matched to the wavefront sensor is shaped into a conjugate of the measured aberrations to correct the wavefront. The corrected light with wavelength shortward of 1Ám is sent to the WFS while light longward of 1Ám is sent to the science instrument. See the Performance and Use section for more details.

The requirement of a wavefront reference source for the adaptive optics system leads to several important considerations in the selection of targets for the AOS. Foremost, there must be a suitably bright point-like source within 30 arcseconds of the science target. This requirement is set by the acquisition field of the WFS acquisition steering mirror. In addition, this source flux must be well within the WFS's 4 arcsecond field of view and must have some contrast (i.e. be more or less point-like). The actual magnitude limit depends on the degree of correction required but note that for very bright guide stars the system performance is limited by the number of actuators rather than photon noise. In addition, a guide star is typically required for use with the peripheral wavefront sensor to provide active optics (aO) correction of the primary mirror and fast-guiding correction to the secondary.

Hokupa'a is intended to deliver images with near diffraction-limited resolutions at 2.2 microns in 1" or better seeing. Under favorable seeing conditions, FWHM of about twice the diffraction limit can be in the J-band. The actual delivered performance of the system depends strongly on several factors: the guide-star characteristics, the intrinsic spatial and temporal properties of the atmosphere, telescope wind shake, and so forth. For further information, see the Performance and Use section.

The corrected output beam of Hokupa'a is fed at f/26 to a near-infrared camera (QUIRC). With QUIRC, Hokupa'a provides a fixed plate-scale of 20 milliarcseconds per pixel (diffraction limit is critically sampled at H and K).

From here the first place to go is the Performance and Use page. If you're new to adaptive optics, Hokupa'a or QUIRC you may want to check out the AO Primer, Hokupaa Basics, and QUIRC Basics pages. Proposal writers should check out the Proposal Checklist for things to consider including in proposals for time with Hokupa'a/QUIRC.


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Last update 23 February 2002; Phil Puxley