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CFHT Observatory Manual - Site (Sec.2) http://www.cfht.hawaii.edu/Instruments/ObservatoryManual/CFH...
Weather
CFHT Observatory Manual
Section 2 - SITE CHARACTERISTICS
TABLE OF CONTENTS
Mean minimum temperatures at the summit are around 0 C (summer) and -4 C (winter). Extreme
temperatures hardly ever go lower than -10 C. Daytime temperatures are normally about 10 C in
summer and 3 C in winter. Weather conditions in the Hawaiian Islands are determined largely by
the strong persistent Northeast Pacific Ocean anticyclone, which usually gives rise to easterly
(trade) winds in Hawaii, especially during the summer season. Trade winds give an inversion layer
with an average height of 2000m; air above this inversion tends to be both dry and stable, hence
giving the good astronomical quality usually experienced at the Observatory. At the mesoscale
level, the summit of Mauna Kea is generally intercepting a free flow of air, thus preserving this good
quality. However, high altitude cirrus can be a problem; in some years it has been present about
30% of the time. The mean annual precipitation at the summit of Mauna Kea is ~15 cm, most of
which falls as snow during the winter.
Site Quality
General characteristics include: 80% usable nights (55% photometric, 25% spectroscopic), median precipitable
water
vapor 0.9 mm.
The median seeing (free atmosphere) is ~0.40 arc sec, with a likely systematic variation between winter (0.45)
and
summer (0.35). The 10 percentile is probably of the order of 0.25 arc sec. The summit of Mauna Kea appears to
be in
that respect, the best known site on earth. Observers must be cautioned, however, that seeing characteristics
are often
highly variable, even during the course of a single night.
Image Quality
A large sample of CCD images, either at prime or F/8 Cassegrain focus, have allowed good statistical
study of the
image quality with CFHT. Images are at the subarcsec level at least 75% of the time and long-exposure
images with
FWHM better than 0.4 arcsec have been obtained. The figure below shows the evolution of image quality
as
documented by science images taken since the beginning of CFHT operations. Note that the HRCam and
SIS images
have been taken with the instruments’ fast tip/tilt systems, and that the MOS images are badly
under-sampled.
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