Mauna Kea - General Infos on Observing Conditions

Keck Observatory (Blue Book, Wallace W. Sargent)

The W. M. Keck Observatory consists of two ten-meter telescopes constructed and operated by Caltech and the University of California on top of Mauna Kea, an extinct volcano 13,800 feet high on the island of Hawaii. Ultimately, the two telescopes can be combined for interferometry on a baseline of 85 meters. After allocations to the University of Hawaii, which owns the site, and to NASA, which is buying into one-sixth of the project, UC and Caltech will each have 73 percent of the time on a single ten meter telescope when the observatory is completed. Until then they each have 45 percent of the time on Keck I. Routine operation of Keck I is now being phased in-- at the end of 1994 80 percent of the nights are being scheduled for scientific observations. Keck II is expected to be completed in the Fall of 1996. None of the other 8 to 10-meter telescopes in the world are expected to come into operation before 1998 at the earliest so that the Keck observing community will enjoy several years in which to skim the cream at the latest frontier of optical and infra-red astronomy.

Mauna Kea is a superb site because of the very stable airflow over the long reach of the Pacific ocean which leads to very good images. The median ``seeing'' on Keck I has been measured to be 0.55" (FWHM) and the best 0.25". The telescope optics have reached the design goal of putting 80 percent of the light into a circle 0.4" in diameter (corresponding to a FWHM of 0.2"). The site also has extremely good transmission in the infrared, due to the high altitude.

Astronomers observing on all of the radio and optical telescopes on Mauna Kea eat their main meals and sleep at Hale Pohaku, a large complex maintained by the University of Hawaii at the 9000 foot level on the south side of the mountain. The road from Hale Pohaku to the summit is 8 miles long and only the upper half is paved. The air pressure at the summit is 60 percent of that at sea level and water boils at 86 deg C. Observers are required to spend at least 24 hours at Hale Pohaku in order to become at least partially acclimated to the high altitude before going to observe at the summit. Despite this, about 75 percent of observers experience mild headaches on the summit and a few experience more severe symptoms.

The Hawaii operations of CARA, the organization formed jointly by UC and Caltech to build and operate the observatory, are conducted from a large complex of buildings in Waimea, a small community about 20 miles north of the summit of Mauna Kea. Waimea has an altitude of 2500 feet and is situated in a saddle which joins the ``wet'' and ``dry'' sides of the Big Island. The complex includes living quarters for visiting scientists and a large operations room which will be ultimately used to operate the telescopes remotely. The nearest airport with connections to Honolulu and the US mainland is at Kailua-Kona, about 35 miles from Waimea. The Keck Observatory now has 66 employees in Hawaii.

Keck Observatory (Image Quality, Mike DiVittorio, Apr. 24, 1996)

On Keck I we are doing the following with regards to image quality measurements:
Our baseline is a 20 sec image in the red (not R). We use 20 seconds because the image is not degraded by open loop tracking but it is long enough to smooth seeing. We also use IRAF's IMEXAM tool. The camera which we use for these images has ~.1" pixels.

Over the past 2 years the median FWHM on this instrument has been .56". The best image has been .31".

Due to the segmented nature of primary we are able to make another measurement, segment image quality. By introducing appropriate tilts to the segments, the mirror is spread into 36 images. Not surprisingly, the best segment image quality is better than the "stacked" image, .45" median and .24" best. This "best" segment image quality is the upper limit on seeing, at least at 1.8m.

Gemini Web site

Mauna Kea Data logged nightly by the UKIRT Telescope Operators over a ten-year period (1985 September 13 to 1996 August 4) have been analysed. Their reporting includes cloud cover (in eighths), cloud type and the number of usable and available hours. The data may be summarised thus: Usable time: excluding nights for which no information was available (e.g. due to telescope shutdown) approximately three-quarters of the available time (31419 out of 43427 hr) was identified as "usable" i.e. the telescope was open and data were collected. Cloudless: of the usable time 62% (19371 hr) was noted as being "cloudless". It is recognised that this is a subjective assessment e.g. it is difficult visually to detect thin cirrus in a dark sky. There are, however, two caveats: (a) we have included in this value only nights which were classified as cloudless throughout and (b) there may be a partial compensation from nights which were recorded as having some cloud cover (1/8 or 2/8, say), and which are treated as having these conditions all night, but which may have experienced substantial clear periods. Nevertheless, for the purposes of the Gemini observing condition constraint we have assumed that only 50% of the usable time is actually photometric. Thin cloud: the UKIRT cloud cover and cloud type often were logged as a single value for an entire night. To estimate the time during which thin cloud was present, we have taken the nights explicitly reported as "thin cirrus" and added the fraction (1.0 - cloud cover) of nights reported as having "cirrus" (with a cloud cover of 4/8 or less). The UKIRT data shows these conditions occuring 23% (7096 hr) of the usable time. For the Gemini observing condition constraint we have assumed that thin cloud is present 20% of the usable time. Thick cloud: for the Gemini observing condition constraint we have assumed that thick cloud is present 20% of the usable time. (excerpt from Cloudiness and more).

CTIO Web site

Photometric Skies for Paranal, Tololo, Mauna kea and Mt. Graham

(excerpt from A Satellite Survey of Water Vapor and Cirrus Cloud Cover in Northern Chile by Dr. D. A. Erasmus

Keck Report 90: at Mauna Kea, the wind is under 14m/s 95% of the time. The nightime temperature is within 2 +/- 4 C, 90% of the time.