CFHT Observatory Manual

Section 2 - SITE CHARACTERISTICS

TABLE OF CONTENTS
Weather
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.

Median image quality with FOCAM is slightly better than 0.8 arc sec. Optical quality of the telescope, dome and mirror
seeing, image motion and guiding errors play a substantial role, and the free atmosphere seeing is usually better as
noted above.

Sky Brightness

Average sky brightness at zenith during dark time is given in the table below.

Color	Equivalent ()	Brightness [mag/(")²]	Flux [phot./cm²/s/microns/(")²]
U B V R I J H K	0.36 0.44 0.55 0.64 0.79 1.23 1.66 2.22	21.6 22.3 21.1 20.3 19.2 14.8 13.4 12.6	1.74x10e-2 1.76x10e-2 3.62x10e-2 5.50x10e-2 1.02x10e-1 2.49 4.20 3.98

Night sky brightness in U increases by a factor of 5 at quarter moon and 65 at full moon. Corresponding values in V
are 1.3 at quarter and 5 at full moon. These rough estimates are of are, of course, for clear (cirrus-free) nights.

The diagram below shows a typical spectrum of visible night sky emission at Mauna Kea (reproduced courtesy of Paul Hickson and Alan Stockton).

From 1.5 microns to 2.2 microns the spectrum of night sky emission is dominated by OH emission lines; between 2.2m
and 2.55m H2O lines and thermal continuum are the dominant contributors.

(1) A spectrum from Kitt Peak, by Broadfoot and Kendall, in the near infrared region is included for reference.

(2) Typical spectra, taken from ESO (Chile) and from UKIRT (Mauna Kea) are included for reference.

Average background fluxes are quite variable in the infrared. Over a few minutes they typically vary by 1% in J, 2% in H
and 0.3% in K. These figures by T. Gerball, obtained at UKIRT, are highly variable, however, especially for J and H.
Longward of 2.5 m the background emission is set by thermal radiation from the telescope and from the atmosphere.
Mean sky emissivity is 0.35 at 20 m and 0.67 at 27 m.

City lighting is relatively small, and quite often completely damped from cloud cover at the 2000-3000 m level. A county
ordinance has been adopted, which restrict most lights of the Big Island to low-pressure sodium lamps.

For an interesting look at our night light environment at CFHT, see The Light Environment of Mauna Kea.

Precipitable Water

The summit of Mauna Kea is especially dry and, on clear nights, typical total water content is ~1 mm. It is thus a good
site for observations in the near to mid-infrared (1 micron to 25 micron).

Wind

Throughout the year, the wind rose is clearly bi-modal; a large percentage of the time winds are either easterly or westerly.
50% of the time, wind speed is less than 7 m/s and 84% of the time it is less than 12 m/s. About 5% of the time, they are
more than 30 m/s and the telescope must be closed. When observing during strong winds it helps to point only at objects
which are situated roughly leeward. Note that these are average values, and that the percentage of very high winds is
extremely variable from one period to another.

Extinction and Refraction

The mean extinction coefficient and refraction versus wavelength for Mauna Kea are shown below.

Extinction Curve for Mauna Kea

Atmospheric Refraction for Mauna Kea

Airmass Values

An airmass nomograph for Mauna Kea is given here. Note that a unit airmass at Mauna Kea (with a mean barometric pressure of 605 millibars) is equivalent to 0.60 airmass at sea level.

Nomogram to estimate airmass

Astronomical Calendar

The time of sunset and sunrise at Mauna Kea throughout the year, and the corresponding sidereal time are provided in the accompanying figures.

Sunrise and Sunset times for Mauna Kea

Siderial time through the year

Site characteristics references

Bely, P.Y.: Weather and Seeing on Mauna Kea 1987, Publ. Astron. Soc. Pac., 99, 560.
Boulade et al 1987 Atmospheric Extinction in the region 310 to 390nm (Boulade, 1987 CFHT Bulletin 17, p.13)
Broadfoot, A.L., Kendall, K.R.: The Airglow Spectrum, 3100-10000 Å Journal of Geoph. Res. Space Phys., 73, 426.
Erasmus, D.A.: Meteorological Conditions Affecting Observing Quality on Mauna Kea 1986, Publ. Astron. Soc. Pac., 98, 254.
Krisciunas, K.: Atmospheric Extinction and Night-sky Brightness at Mauna Kea 1987, Publ. Astron. Soc. Pac., 99, 887.
McCord, T.B., Clark, R.N.: Atmospheric Extinction 0.65-2.50 microns Above Mauna Kea 1979, Publ. Astron. Soc. Pac., 91, 571.
Morrison, D., et al.: Evaluation of Mauna Kea, Hawaii, as an Observatory Site 1973, Publ. Astron. Soc. Pac., 85, 255.
Racine, R., D. Salmon, D. Cowley, and J. Sovka: Mirror, Dome, and Natural Seeing at CFHT 1991, Publ. Astron. Soc. Pacific, 103, 1020.
Warner, J.W.: Comparative Water Vapor Measurements for Infrared Sites 1977, Publ. Astron. Soc. Pac., 89, 724.

Version 1.0 January, 2003