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Seeing FAQ

Unless otherwise stated, all these questions are mainly related to the Canary Islands Astronomical Observatories:

- Roque de los Muchachos Observatory (island of La Palma): “ORM”

- Teide Observatory (island of Tenerife):OT”

How can I get the seeing and extinction data of a particular night?

Seeing has been measured in differents periods and locations at ORM since 1995. At present, we are operating a DIMM instrument regulary every other week. Please follow the steps indicated in the section “Statistics and Data/Seeing Database” to get the available data:


-You can obtain the seeing value from the“full width at half maximum (FWHM)” graphs-


There are also other seeing measurements at ORM with daily data available. For example, the ROBODIMM instrument operated by the Isaac Newton Group of Telescopes:


Here you can find a comparison of different seeing measurements:


Extintion data at OT are available from:


Extintion data at ORM are available from the Carlsberg Meridian Telescope:






(IAC DIMM) Statistics and Data/Seeing Database

Carlsberg Meridian Telescope





(IAC DIMM) -coming soon-

(IAC extiction)

What are the parameters, instruments and tools for site testing?

Please have a look at the section “Parameters for Astronomical Observations and ELT Design” (http://www.otri.iac.es/sitesting/index.php?flash=1&pag=7) where the main parameters are listed with detailed information.


You can also read the proceeding “Instruments and tools for site testing“ (Muñoz-Tuñón, García-Lorenzo and Varela) prepared for GW3-ESO- site Evaluation (Feb,06):


What is seeing?

Seeing is the effect of the atmosphere turbulence in the light of astronomical objects.


Atmosphere turbulence alters the light coming from stars (or other astronomical objects) due to variations induced in the refraction index. This distortion changes at a high rate, typically more frequently than 100 times a second. While an astronomical image is exposed (seconds or even minutes) the different images of a star average out as a filled disk that is what is usually called "seeing disk". We can give the seeing value as the statistical ”full width at half maximum (FWHM)”. Or we can estimate it as the seeing disk angular diameter.


As we said, atmospheric turbulence drastically affect the seeing value, varying from aprox. 0.3 arcsec(''), in an astronomical observatory, to even 10 or more arcsec ('') in non adequate locations.


Seeing also depends upon the wavelenght of the light observed (see a next question for explanation)

How is seeing related to the astronomical image?

Seeing and image quality are inversely proportional: when seeing increase, image quality gets worse.


The diffraction limit  of a telescope is given by:

q = 1.22· (l/D)                       ; [rad]

where  l is the wavelenght and D the diameter of the telescope, both in [cm]. This value is taken as the minimum angular separation of two sources that can be distinguished by the telescope.


But, in practice, under typical conditions, all telescopes larger than ~10cm are going to be limited by the present seeing conditions (seeing limitted): when observing astronomical objects we are really observing disturbed images of that object, because of atmospheric turbulence.


The next table shows the described effect in the middle visible range under standar conditions. We can observe how angular resolution is the same for of all the telescopes larger than 10cm, independet of its diameter.



D [m]

telescope (e.g)

diffraction limit q [''] (middle visible: 0.5mm)

angular resolution [''] (middle visible: 0.5mm)


















Adaptive optics techniques come to overcome this limitations.

How does seeing go with the wavelength? How is it in the infrared?

Seeing depends on the turbulence intensity and the wavelenght. The dependence on the wavelenght is proportional to l-1/5. So seeing drastically reduces while wavelenght increase. For example, in the same atmospheric conditions, near IR seeing is ~ 25% less than the visible.

How does dust (calima) affect seeing data?

Seeing is mainly governed by the atmospheric turbulence through the variation of the refraction index. In this sense, dust stabilize the atmosphere reducing the turbulence and hence, seeing is improved. On the other hand, the extinction severely get worse.

What's the basic operation principle of a seeing monitor?

Typical seeing monitor is based on the Differential Image Motion Technique. In the section “The Astroclimatic Station” you are going to find references, detailed explanations and comparisons between different instruments based on this principle. There is also a link to the SCIDAR technique (SCIntillation Detection And Ranging), that allows to measure vertical profiles of turbulence:


We are interested in acquiring a seeing monitor. Brief recommendations.

How is corrected the atmospheric effect in the astronomical images?

The atmospheric effect in astronomical images can be compensated with several techniques. The most developed are the Adaptive Optics techniques (do not confuse with “active optics” that acts over the telescope mirror to compensate the mirror or lens distorsion in longer time scales). These techniques are based in three main modules:

- Wavefront sensor, that allows measure the fast fluctuations in the light wave phase.

- Compensation System. This system has to include a deformable mirror (or a set of them), the most common with piezoellectric actuators. These mirrors allow to compensate the measured phase distorsion in real time.

- A Control system.

Because a science target is often too faint to be used as a reference star for measuring the phase, a nearby brighter guide star can be used instead. But in practice, the field of view that can be corrected with an Adaptive Optics systems is limited, typically to less than 1 arcmin surrounding the reference star, by the effects of anisoplanatism. So, there is an important limitation in finding adequate stars for any field of view. To overcome this problem, alternative light reference has been proposed based on laser beams. The reference may be the Rayleigh scattered ligh (10-15km) or a sodium spot generated by a convenient exitation of the atoms present in the mesosphere (~90km).

In any case, natural or artificial references, it is crucial to characterize the typical turbulence and mesospheric sodium behaviour to achieve an adequate Adaptive Optics system.

Basic recommendations for an idoneous place selection for an astronomical observation.

How is the sky quality legally protected? Whta's the Sky Law (Ley del Cielo)?

Sky quality is protected in Spain through a law best known as “Sky Law”. On the 31st October 1988 the Spanish Government passed the Law for the Protection of the Astronomical Quality of the IAC Observatories (Law 31/1998), which was proposed by the parliament of the Canary Islands.

The law deals with four main areas:

-          Light Pollution

-          Radioelectrical Pollution

-          Atmospheric Pollution

-          Aviation Routes


More details in http://www.iac.es/eno.php?op1=4&op2=10&lang=en

What are the typical meteorological values?

(A) ORM:
  • Average day Temperature is 9.9±6.1ºC.
  • Average night Temperature is 7.6±5.5ºC.
  • Minimum and maximum recorded Temp. (2000-2003) are: -9.60ºC and 28.10ºC
  • Average wind speed: 4.8±3.0m/s (day) and 5.2±1.6m/s (night).
  • The dominant wind direction (GTC location) is North-East.
(B) OT:
  • Average Temperature is 9.8ºC.
  • Average Minimum Temperature is 5.9ºC (extreme -9.8ºC)
  • Average Maximum Temperature is 13.6ºC (extreme 30.4ºC)
  • Median wind speed: 8.3m/s.
  • The dominant wind direction is North-West.

What are the typical seeing values?

In the following table appear typical seeing values at ORM measured with DIMM technique. Showed data was recorded at differents locations from 1995 to 2003.

Year 1995/96 1997 1998 1999 2000 2001 2002 2003 2005 2006
Ndata 87978 55545 50661 53178 66348 48115* 111561** 75311*** 28794 38322
NHours 806.5 509.2 464.4 487.5 294.9 213.8 513.6 334.7 319.9 425.8
Min (") 0.18 0.20 0.20 0.17 0.18 0.11 0.14 0.15 0.19 0.15
Mean(") 0.75 0.66 0.71 0.78 0.84 0.62 0.88 0.69 0.77 0.71
std (") 0.40 0.30 0.34 0.40 0.52 0.41 0.44 0.43 0.47 0.41
Median(") 0.65 0.58 0.63 0.68 0.68 0.53 0.76 0.56 0.64 0.60


You can find more detail in the section “Statistics and Data/Monthly Statistical Seeing Results”:


Or even at “Statistics and Data/Annual Seeing Statistical Results”:


Are sky quality measurements intercorrelated between both observatories?

There is a generally very good correlation between both observatories atmospheric conditions. However, it may appear small local cirrus levels or a few hours delay in turning up a particular atmospheric phenomena. Wind roses are also strongly dependent on the local topography. So it is advisable to use the data with care.

Are satellites adequate for site testing characterization?

Spatial resolution provided by instruments on board satellites (e.g. OMI-AURA, MODIS-TERRA & AQUA, SEVIRI-MSG1, GOME-ERS2, SCIAMACHY-ENVISAT, etc, typically larger than a few km2 ) usually limits the use for site testing purposes

Astronomical observatories locations has usually very particular conditions which make the difference with maybe less than 1000m away.

This is the case in the Canary Islands observatories, both located in the edge of volcanic calderas. An horizontal displacement of only 100m in ORM (e.g) may fall several hundreds meters in the topographic height.

So, it is common that the pixel evaluating the observatory contains significant information of “non-observatory” phenomena falsifying the result.

Nevertheless, there are good works related with any site testing parameters, for example Erasmus and van Rooyen, 2006 (Final Report to ESO), reported a 83.7% of phtometric time for ORM. See “Parameters for Astronomical Observations and ELT Design/% photometric nights” for a summary (http://www.otri.iac.es/sitesting/index.php?flash=1&pag=7-124)

You can read the proceeding ”Use of satellite Data for Astronomical Site Testing Characterization” (Varela, Bertolini, Muñoz-Tuñón, Fuensalida and Ortolani) or the paper "Astronomical site selection: on the use of satellites data for aerosol content monitoring (Varela, Bertolin, Muñoz-Tuñón, Fuensalida and Ortolani, MNRAS 2008) for a complete view of the topic.