Hokupa'a Guide Star Requirements
Hokupaa requires two wavefront reference stars: one for the active optics correction of the primary mirror and one for the adaptive optics measurement. These have very different requirements. For information on the active optics and peripheral wavefront sensor requirements see the peripheral wavefront sensor page.
Adaptive Optics wavefront reference: The use of an adaptive optics system requires a
wavefront reference source to be near the object of interest. This requirement leads to a
number of important considerations when choosing science targets for the system.
The wavefront reference can be either the science object of interest or a nearby bright star. The criteria for using the science object as a wavefront reference is that it has a surface brightness profile which is point-like (although objects of angular extent up to 1.5 arcsec have been successful used at a cost of lowered performance) and the flux contained within the central 2 arcsec is sufficiently bright.
The most critical guide star consideration in the performance of Hokupaa is the brightness of the wavefront reference star. Full correction (up to the limitation of the number of actuators) is given for R<~14 mag. Some correction is expected to R~18 mag, but these corrections will be minimal under all but the very best seeing conditions. Performance predictions are available in the Hokupa'a Simulations page.
The wavelength response of the wavefront sensor channel is basically limited by the response of the APD's silicon. The following plot shows the wavelength sensitvity of the APDs as given in the EG&G SPCM-AQR Series Datasheet. This is not a scan of the throughput of to the WFS andis only given to give a sense of the wavelength sensitivity. The total throughput to the WFS is approximately 20-25%.
The brightest guide star that can be used is set by the maximum count rate in the APDs and the strength of the neutral density filters in front of the Hokupaa wavefront sensor. The maximum APD count rate is 50 x 106 counts per APD per second. The following table lists the available neutral density filters in Hokupaa. Hokupaa has been used in engineering tests on stars as bright as R~3 mag.
Differences in the wavefront aberrations along different paths through the atmosphere limit the field of view over which the correction of the adaptive optics system is valid. This translates into a variation of the PSF (Strehl and FWHM) across the field of view. How large an effect this is depends strongly on the wavelength and seeing conditions (mainly determined by the Fried parameter, r0 where the seeing FWHM ~ wavelength/r0). Performance predictions are available in the Hokupa'a Simulations page.
Hokupa'a can acquire AOWFS stars as far as 30" off-axis.
NOTE: The center of the
acquisition range is offset ~1" N and ~1" W from the center of the QUIRC detector array.
Under engineering tests we have operated Hokupaa with guide stars as far off-axis as 40"
but vignetting of the wavefront sensor introduces fixed aberrations in the focal-plane.
We do not recommend observing guide stars past the 30" limit. For guide stars within 30"
any arbitrary field orientation can be obtained.
Last update August 22, 2000 Mark Chun