Summary :
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 be additionnaly affected by the isoplanatic deccorelation. Final Strehl
on the QSO will be 20% x 50% (30 arcsec, H band) = 10%. The corresponding FWHM
is roughtly 0.2 arcsec (see below the article on performance for a more
accurate correspondance 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 the most up-to-date informations, you can download
this draft of an article entitled
"Performance of the Canada-France-Hawaii
Telescope Adaptive Optics Bonnette", to be submitted.
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 optimitic by approximately one magnitude with respect
to what is achieved at the telescope.
Created 4 March 1995. Page maintained by Jean-Luc Beuzit.
Please send comments to beuzit@cfht.hawaii.edu Back to PUEO home page
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