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The four sections below summarise information about observing conditions at the observatory:
The IAC has produced a colour brochure (2001) including some of this information.
Measurements of the temperature, wind speed, humidity etc from 1999 are available from the the ING meteorology and seeing archive. On average, 25% of observing time at the William Herschel Telescope is lost to bad weather (when cloud, wind speed, dust, humidity, dew or ice exceed the operational limits (semester A runs from February to July, and semester B from August to January):
RoboDIMM seeing monitor, a few 10s of m north of the WHT building, the median seeing at zenith is 0.80 arcsec April - November (0.72 arcsec in August - September), and 1.07 arcsec December - March.
The RoboDIMM measures the seeing every few minutes on most nights. The measurements are made by observing a convenient star with zenith distance < 30 deg, and correcting to be representative of zenith.
The RoboDIMM was not intended as a site-characterisation tool, and it's known to under-detect very good seeing (< 0.5 arcsec), so the numbers quoted here are likely to be slightly pessimistic. Nevertheless, on a given night there's generally reasonable agreement with other DIMMs on the mountain-top, and the RoboDIMM measurements give a good idea of the shape of the seeing distribution, and of the seasonal variation.
The variation of RoboDIMM median seeing with month is similar from year to year, and the monthly medians for 2012-15 are:
Month: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Seeing (arcsec): 1.04 1.13 1.00 0.87 0.83 0.83 0.84 0.72 0.72 0.79 0.87 1.13
The distributions of 2012-15 RoboDIMM seeing during April - November, and December - March, are shown below:
There is no significant correlation between seeing and time during the night.
Seeing is expected to vary roughly as airmass0.6.
Dome seeing at the WHT is usually negligible. There may be some mirror seeing when the primary mirror is more than 3 degC warmer than the dome internal temperature (which is the case on ~ 10 - 20% of nights).
The finite PSF delivered by the WHT itself (optical aberrations, < 0.3 arcsec) will significantly degrade only the very best site seeing.
As is evident from the histograms above, seeing of just a few tenths of an arcsec is occasionally recorded by the DIMM monitor outside the WHT. On the night of 11 January 2018, it recorded seeing ~ 0.3 arcsec for several minutes (screenshot by Fiona Riddick):
The seeing at the INT tends to be worse than at the WHT, probably due to the thermal inertia of the building below the telescope.
varies with wavelength. See La Palma technical note 31 (King 1985) for further information.
During July, August, September, a pall of neutral-extinction ('grey') Saharan dust (properties described by Murdin 1986 in La Palma technical note 41) hangs over the islands, and the total extinction may be as high as 1 mag.
The sky is brighter at low ecliptic latitude (by 0.4 mag); at solar maximum (by 0.4 mag, and at high airmass (0.25 mag brighter at airmass 1.5). Next solar minimum is due ~ 2020.
The mean brightness of the sky varies by < 0.1 mag with time after astronomical twilight.
Light pollution (from street lamps etc) is visible to the naked eye at certain azimuths on the horizon, but its contribution to the continuum brightness at the zenith is < 0.03 mag in all bands. The NaD emission from street lamps brightens the sky in both V and R broad bands by about 0.07 mag. Total contamination (line + continuum) at the zenith is < 0.03 mag in U, approx 0.02 mag in B, approx 0.10 mag in V, and approx 0.10 mag in R.
The brightness of the sky shows no dependence on atmospheric extinction AV, for AV < 0.25 mag (as is the case on 80% of nights). In very dusty conditions (extinction more than a few tenths of a mag), the sky brightness may be several tenths of a mag brighter, probably because of enhanced back-scattering of streetlighting by the dust layer.
For a more recent investigation of light pollution, see Marco Pedani's article "An Updated View of the Light Pollution at the Roque de los Muchachos Observatory" in the ING newsletter, 9, 28.
A typical La Palma night-sky spectrum is shown below:
The continuum is dominated by airglow (actually pseudo-continuum) and zodiacal light. The bright OH lines in the near-IR and at 5577, 5890/6 (some of the emission), 6302 and 6364 Å are airglow.
The remainder of the 5890/6 NaD line is light from La Palma's low-pressure sodium street-lights, back-scattered from the atmosphere above the telescopes. The faint 4358 and 5460 Å lines are from mercury street-lights.
Typical broad-band filter bandpasses are shown.
When the moon is 60 deg from zenith, with extinction 0.15 mag, the zenith sky will brighten by ~ dmag magnitudes as tabulated below:
New Crescent Quarter Gibbous Full Phase angle (deg) 180 135 90 45 0 Approx day: 1 4 8 12 15 D, G or B: D G G B B Illum. frac. % 0 25 50 75 100 dmag (U, B, V) 0 0.5 2.0 3.1 4.3 dmag (R) 0 0.3 1.3 2.4 3.5 dmag (I) 0 0.2 1.1 2.2 3.3Sky brightness for other values of lunar phase, lunar zenith angle, sky position and extinction, can be estimated with SIGNAL's sky-brightness calculator.
Note that the quarter moon (i.e. half disc illuminated) is a factor of 10 (not 2) fainter than full, due to the opposition effect (also responsible for gegenschein on the ecliptic and dry heiligenschein on earth).
The contribution of moonlight in V has been calculated according to the scattering formula of Krisciunas & Schaefer (1991, PASP, 103, 1033), normalised (multiplied by a factor of 2.4) to agree with measurements of sky brightness made at the JKT on a dust-free night in 7/98. The moonlight contribution in the other bands is calculated according to the U-B, B-V, V-R, R-I colours of moonlight measured on the same night in 7/98. These numbers provide a rough guide to the brightness of the moonlit sky. The actual brightness on a given night will depend strongly on how much dust and cloud is present.
Definitions of dark, grey and bright time can be found in La Palma technical note 127 (Skillen 2002).
Measurements of twilight sky brightness vs time after sunset (or before sunrise) can be found on the twilight sky brightness page.
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