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Site Quality

The four sections below summarise information about observing conditions at the observatory:
  • Weather - ~ 75% of nights are clear
  • Seeing - median seeing at the WHT is 0.7 arcsec
  • Extinction - typically 0.12 mag in V, higher in summer
  • Sky brightness - 22.7, 21.9 and 21.0 mag/arcsec2 in B, V and R in the darkest conditions

The IAC has produced a colour brochure (2001) including some of this information.

Weather

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, humidity, dust, dew or ice exceed the operational limits).

Fraction of observing time lost to bad weather at the WHT (blue curve) and INT (yellow curve). [ PNG | PDF ].

Monthly relative-to-night-length WHT weather downtime from 1989 to present. The average downtime is ~20% in semester A (February to July) and ~30% in semester B (August to January) [ PNG | PDF ].

Seasonal variation of weather downtime and length of night from 1989 to present. The thick black line indicates the duration of astronomical night, in hours. The border of the pink area indicates the (rather small) seasonal variation in the number of hours of good weather available, from a minimum in May, to a maximum in August. [ PNG | PDF ].

Seeing

The median seeing at the William Herschel Telescope is 0.7 arcsec. More information can be found on the ING seeing pages.

Seeing statistics as measured by ING's DIMM monitor, which was installed mainly in order to monitor the seeing on adaptive-optics nights.

The median seeing during the summer is probably ~ 0.15 arcsec better than the median during the winter.

Further information about site seeing can be found on the IAC's Sky Quality Group web pages. (which include a comparison between IAC and ING DIMM seeing measurements).

Extinction

Atmospheric extinction above the observatory on La Palma is typically 0.12 mag in V band, and 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 Mercator Telescope provides half-hourly measurements of site extinction at night. The Carlsberg Meridian Telescope records the extinction nightly.

Sky brightness

The median moonless night sky brightness above La Palma at high elevation, high galactic latitude and high ecliptic latitude, at sunspot minimum, is B = 22.7, V = 21.9, R = 21.0, rms 0.1 mag/arcsec2. The sky brightness in U and I is less well-determined, U approx 22.0 mag/arcsec2 (few data), I approx 20.0 mag/arcsec2 (variable). As at other dark sites, the main contributions to sky brightness are airglow and zodiacal light, in the ratio 2.5:1 at high ecliptic latitude.

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).

The mean brightness of the sky varies by < 0.1 mag with time after astronomical twilight.

Light pollution 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 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.

Further details can be found in ING technical note 115 (Benn C.R., Ellison S, 1998, also available as a postscript file).

For a 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.

Moonlight

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    3.3
Sky 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).

Moonlight diagrams can be found here.

Information about artificial satellites (including flare predictions) can be found here.



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Last modified: 11 May 2013