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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 0.80 arcsec April - November
  • 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.

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):

Relative-to-night-length weather downtime per year (semesters A+B) excluding from the calculation S/D/commissioning nights as well as nights when visiting instruments were used [ PNG ].

Relative-to-night-length weather downtime per semester excluding from the calculation S/D/commissioning nights as well as nights when visiting instruments were used. Median values for both telescopes (around 24% weather downtime) are shown. [ PNG ].

Average fraction of observing time lost to bad weather at the WHT, 1989 to present, and polynomial fit. The average downtime is ~18% in semester A (February to July) and ~27% in semester B (August to January). A shoulder can be seen in February-March which is a characteristic of the climatic condition at the geographical location of the Roque de Los Muchachos Observatory (for comparison, see this article on the climatology of the Izaña Observatory) [ PNG ].

Squares: average fraction of observing time lost to bad weather at the WHT, 1991 to present, excluding the maximum and the minimum values; diamonds: maximum and minimum values; shadow area: one-sigma interval of the average defined above. Winter months show higher dispersions and extreme cases than summer ones. July is the most stable month. [ PNG ].

Monthly variation of usable observing time and length of night from 1989 to present. The thick black line represents the duration of astronomical night in hours. The border of the pink area shows the (rather small) monthly variation in the number of hours of good weather available, from a minimum in April, to a maximum in August. Average of usable observing time in semester A is 6.6h and 7.1h in semester B [ PNG ].

Monthly variation of percentage of nights with some weather downtime from 2002 to present. The dashed line shows the percentage of time lost to bad weather on the WHT for reference. More than 40% of the autumn and winter nights have some time lost to bad weather, in summer time less than 20%. [ PNG ].

As measured by the RoboDIMM seeing monitor, a few 10s of m north of the WHT building, the median seeing 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 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:

The distributions are roughly log-normal. The RoboDIMM seeing is worse than 1.8 arcsec during about 5% of April - November, and 16% of December - March.

There is no significant correlation between seeing and time during the night.

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.

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.

Further information can be found on the ING seeing pages, and on the IAC's sky-quality web pages.

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 Carlsberg Meridian Telescope (CMT) provided a nightly record of extinction between 1984 and 2013. García-Gil et al. (2010, PASP, 122, 1109) carried out a statistical analysis of the CMT data.

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 (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.

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

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    3.3
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.


Measurements of twilight sky brightness vs time after sunset (or before sunrise) can be found on the twilight sky brightness page.

Artificial satellites

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

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Contact:  (WHT Manager)
Last modified: 17 May 2016