CFHT, Instruments, Imaging, AOB, Performance
The CFHT Adaptive Optics Bonnette :
The median seeing at the F/8 Cassegrain focus of the CFHT telescope is 0.65
arcsec. Under this conditions and on a bright guide star, the Strehl ratios
of J, H and K images is 0.27, 0.41 and 0.56 respectively, excluding the
optical aberrations of the imaging device (but including those of the
telescope and PUEO's internal optical aberrations).
The magnitude for which the Strehl ratio is attenuated by 50% relative to
its value using a bright guide star is 14.3 at J band, 15.0 at H band and
15.7 at K band.
A typical value of the isoplanatic patch at H band corresponds to an
attenuation of 50% of the Strehl ratio at 30 arcsec from the guide star.
The isoplanatic patch varies as lambda to the power 1.2. Therefore this
value of 30 arcsec translates into 20 arcsec in the J band and 40 arcsec in
the K band.
These effects (Strehl ratio on bright guide star, loss on Strehl due to
noise - faint guide star - and loss of Strehl due to the use of an off-axis
guide star) are cumulative : They multiply. Let's summarize. You want to
observe a 15 magnitude star at H band under median seeing conditions. The
Strehl ratio of your image will be 40% x 50% (15 magnitude star, H band) =
20% (still almost diffraction limited in FWHM). If now you are imaging,
say, a QSO 30 arcsec away from this 15 magnitude star, then the Strehl
ratio on the QSO will also be affected by the isoplanatic
deccorelation. Final Strehl on the QSO will be 20% x 50% (30 arcsec, H
band) = 10%. The corresponding FWHM is roughly 0.2 arcsec (see below the
article on performance for a more accurate correspondence between Strehl
ratio and FWHM).
Beware of extended objects. Empirically, one arcsec seems to be the maximum
extend tolerable by the wavefront sensor. At this extent, the price to pay
is quite high, and only fairly bright objects are feasible.
Extended and faint objects are typically difficult targets for PUEO.
For more information :
For most up-to-date and detailed informations, you can download the article
entitled "Performance of the Canada-France-Hawaii
Telescope Adaptive Optics Bonnette", published in PASP, 110,
The old performance estimator is still available for
reference. Please note that there is some differences between what was
predicted and what is actually achieved. The major difference is in the
limiting magnitude. The old estimator was optimistic by approximately one
magnitude with respect to what is achieved at the telescope.
Created 4 March 1995.
Page maintained by Jean-Luc Beuzit.
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