The Observatorio del Roque de los Muchachos (ORM), belonging to the
Instituto de Astrofısica de Canarias (IAC), is located on the island
of La Palma, one of the seven islands which constitute the Canarian
archipelago. The Canaries are located around 28
N latitude and 17W
longitude, 1300 km south to the European continent. The climatology of the
Canary Islands has been studied in great detail through long data series
(Font-Tullot 1956; Huetz de Lemps 1969). The main characteristic which
defines their excellence as observing sites is the stability of the height
of the temperature inversion layer, typically below 1500 m. Being situated
at 2400 m above sea level on an isolated peak, the ORM benefits most of the
time from dry, stable and transparent atmosphere (Ardeberg 1984;
Murdin
1985; Muñoz-Tuñón & Fuentes 1991).
Since the last century, there have been many studies of the quality for astronomical observations both at the ORM and the Observatorio del Teide (Tenerife). A very comprehensive review from the beginning up to 1985 can be found in Murdin (1985). Much other useful information can also be found in this issue of Vistas, which is entirely dedicated to the observatories of the Canary Islands.
This paper is devoted to night-time observations. Day-time site-testing campaigns have also been carried out at the Canaries. Among these we can mention the JOSO (Joint Organization for Solar Observations, Brand & Righini 1985) and, more recently, the LEST (Large Earth-based Solar Telescope) campaigns.
A key parameter in determining how good is an astronomical site is is surely the
image quality (see Coulman 1985, and references therein). In the last few decades,
a huge effort has been devoted to the development of new-generation large-diameter telescopes and to the implementing of very sophisticated
post-focus
instrumentation which require a very precise site characterization. In this
sense, since the pioneering work by ESO in Chile, "new philosophy'' intensive
site-testing campaigns (Vernin & Muñoz-Tuñón 1992) have been performed at candidate sites and in already
existing observatories to verify whether they matched the
necessary criteria for these new-generation astronomical facilities.
Since 1985, site assessment campaigns have been carried out by some of the
authors
at Mount Locke
(Texas), La Silla and Paranal (Chile), Mauna Kea (Hawaii) and the ORM (La Palma). The main aim of these campaigns was to
assess,
by means of a variety of simultaneous experiments (see e.g.
Vernin & Muñoz-Tuñón 1992),
the vertical
profile of 
and therefore the relative contribution and identification of
individual turbulent layers (Vernin & Muñoz-Tuñón 1994;
Coulman et al. 1995). The measurement
of simultaneous other atmospheric parameters, such as temperature, humidity and
wind, are of major importance in the interpretation of the occurrence of optical turbulence.
Many different attempts have been made to measure seeing since the pioneering
work
by Rösch (1963). Although the parameters relevant to image degradation are
related to geophysics (turbulent fluctuations of air density), all seeing
monitors are optical instruments; balloon data
(Vernin & Muñoz-Tuñón 1992, 1994) can be used
but cannot monitor it versus time. Different concepts have
been used: the Danjon method, Polaris trails, shearing interferometers
(Roddier
1976), the
Scidar technique (Azouit & Vernin 1980), differential image motion monitors
(DIMMs)
(Stock & Keller 1960; Sarazin & Roddier 1990), the grating scale monitor
(Martin et al. 1994). These use different receivers such as the eye,
photographic plates,
photomultipliers or intensified CCDs.
To monitor the seeing, the Département d'Astrophysique (DA) of the University of Nice and the
Instituto de Astrofısica de Canarias (IAC) developed a DIMM based on a principle first proposed by
Stock & Keller (1960). The DA/IAC DIMM was cross-calibrated with other
existing ESO DIMMs during a joint campaign at Mount Paranal, Chile. The results
from
this comparison were excellent (Vernin & Muñoz-Tuñón 1994). A very comprehensive description of the
fundamentals and capabilities of the instrument can be found in Vernin & Muñoz-Tuñón (1995).
Although many new instruments and many intensive site-testing campaigns have been developed in the recent years, very little long-term seeing climatology is available yet. Even very good observatories, such as Mount Graham or Hawaii, lack of this kind of database, which nevertheless seems imperative for a proper site comparison to be done. Only La Silla and Paranal, and now the ORM (present data) have systematic seeing measurements taken with cross-calibrated instruments.
In this paper, we analyse seeing data taken at the ORM with two DA/IAC DIMMs. In Sect. 2 there is a brief summary of the DIMM principle along with the data aquisition and processing. In Sect. 3 the results of the statistical analysis is described. In Sect. 4 two different areas at the ORM are compared. In Sect. 5 the temporal dependence of seeing values is studied, and the summary and final remarks are given in Sect. 6.