BEATING THE SEEING
di Brusasca Marta
Università statale di Milano Bicocca
“L’unico rimedio è un’aria assai serena e tranquilla, quale
si trova forse sulle vette delle montagne più alte, sopra le
nubi più dense.”
Isaac Newton
“Ottica” 1704
THE ATMOSPHERE
The Earth's atmosphere has usually different effect that are divided in:
• Refraction (atmosferic dispersion) by dust and molecules causes the sky
background to have a certain intrinsic brightness, and it is this which impose a limit
upon the faintest detectable object through the telescope.
• Extinction (atmosferic absorption): is due mostly to the molecular absorption bands
of the gases forming the atmosphere; the two well know window in the spectrum
extended from about 360 nm to 100 m and from 10 mm to 10 m, but even in these
regions there is some absorption so visible light is decrease in its intensity by 10 or
20% for vertical incidence.
• Seeing is the distortion of the radiation wavefront of a cosmic source, produced by
variation in the index of refraction of air.
It’s usually divided in:
ATMOSPHERIC SEEING
DOME SEEING
ATMOSPHERIC SEEING
Poor seeing is caused by atmospheric turbulence. It can occur in three main regions
within the telescope viewing path, each of which is associated with different mechanisms
of turbulence generation.
From the lowest to the highest altitudes, these regions are near ground seeing (0 – 100
m) central troposphere (100m – 2km), and high troposphere (6-12km).
These scales are defined as:
the macro-scale
the meso-scale
the micro-scale
THE MACRO-SCALE
Jet stream (fast moving “rivers” of air )
One of the major causes of turbulence is the strong wind (speeds sometimes exceeding 300 mph)
and steep temperature gradients found in the vicinity of the jet stream.
Areas of the Northern hemisphere most affected by the Polar jet stream are the Central US,
Canada, North Africa, and Northern Japan. The Jet stream’s position varies with the seasons,
tending to move further South during the winter and spring months.
Cold front
A weather front exists at the interface of two air masses of different temperatures and/or moisture
content. The cold air undercuts warm air, forcing it aloft. Because of this vertical lifting, and
because cold air is susceptible to convection, the turbulence in the vicinity of a cold front is
usually great.
Thermal instability
Instability is a term used to describe the tendency for the air to undergo vertical motion. An
unstable atmosphere encourages air to rise initially, then fall after undergoing adiabatic cooling.
This tends to result in the generation of convective currents of air. Strong surface heating and cold
temperatures aloft will produce the greatest levels of instability. Conversely, a stable atmosphere
tends to discourage vertical air motion. Atmospheric processes that tend to cause cooling at the
surface and warming aloft (thermal inversion) produce highly stable air that is largely free of this
type of turbulence.
THE MESO-SCALE
Surface Pressure Gradient
Winds at the ground normally blow in response to broad-scale pressure patterns Once it is
set into motion, air becomes susceptible to certain kinds of turbulence. While in theory it is
true that extremely stable moving air can be turbulent-free (laminar flow), such conditions
rarely occur. The assumption here is that higher wind velocities tend to result in higher levels
of turbulence.
Terrain Roughness
At the lowest level of the atmosphere the horizontal movement of air is impeded by the
ground. This frictional drag introduces a form of mechanical turbulence into the boundary
layer of the atmosphere. A relatively smooth, uncluttered ground will produce low levels of
turbulence, while a rough ground with high relief will disturb the air to a greater degree.
The turbulence at these
altitudes is determined largely
by the topography upwind of
the observing site.
THE MICRO-SCALE
Site Terrain
The type of terrain that surrounds an observing site
may cause movements of air that degrade the quality
of seeing. If there are uneven ground features nearby
(such as the steep slopes of a mountain or hill,
gorges or gullies), then nocturnal drainage may
generate moving currents of relatively dense air that
can adversely impact image quality.
Ground Type
The type of ground cover immediately surrounding an
observing site will often impact the quality of seeing.
If the ground is allowed to heat up during the daylight
hours by absorbing the sun’s short-wave radiation,
that stored heat will be reradiated back into space
during the night, causing micro-scale turbulence in
the process. Surfaces that either dissipate or reflect
sunlight minimize this effect. Natural ground cover
such as grass or leafy shrubs tends to distribute the
sun’s energy in ways that keep the temperature of
these surfaces relatively cool.
THE DOME SEEING
Also the telescope itself can perturb the image, if it hasn’t reached ambient temperature, this will
result in a “boiling effect” when viewing. One should leave their scope for at least 1 hr prior to
observing and probably longer. Certain types of telescope and observatory are more prone to
turbulence: Newtonian reflectors can be troublesome if not properly ventilated, as can Schmidt
Cassegrain’s if not left to cool for long enough.
If the objective is not at air temperature, it will
surround itself with a wavy, irregular
envelope of air slightly warmer or cooler than
the ambient night.
THE DOME SEEING (2)
Installing a fan behind a reflector's mirror has become a popular way to speed cooling and blow
out mixed-temperature air
Left: Cooling fans are traditionally mounted behind a reflector's primary mirror, but inventor Alan
Adler has shown that you can break up heat waves better by placing the fan in the tube's side
so it blows across the mirror's face.
Right: Opposite the fan, Adler put exhaust holes that allow warm air to exit the tube. Note that
they're slightly offset to the rear of the tube to help ensure that the flowing air "scrubs" the
mirror before leaving.
THE DOME SEEING (3)
An example:
A constant gentle breeze across a
reflector's primary mirror is so
important that astronomers built a
dome with removable sides for the
8-meter Gemini North reflector
atop Mauna Kea in Hawaii.
ZENITH
The seeing is always worse at low altitude in the sky.
The altitude influence the seeing because when we
observe a point down the horizon, the light must
cross a lot of air boundaries than a point placed at the
zenith. The light of an object placed at 25° on the
horizon has to cross 4 time the air that pass an object
at the zenith. For this reason is better to observe an
object when is at the max height.
Atmospheric dispersion elongates a star into a
colorful little spectrum; close to the horizon this effect
overtake even poor seeing as a cause of blurry
images.
A SCALE OF SEEING
Many scales have been devised to rate how steady the atmosphere is on a given night. This
scale of seeing is the Pickering Scale, devised by Harvard Observatory's William H. Pickering
(1858-1938). Pickering used a 5-inch refractor to devise the scale.
1. Star image is usually about twice the diameter of the third diffraction ring if the ring could be
seen; star image 13" in diameter.
2. Image occasionally twice the diameter of the third ring (13").
3. Image about the same diameter as the third ring (6.7"), and brighter at the centre.
4. The central Airy diffraction disk often visible; arcs of diffraction rings sometimes seen on
brighter stars.
5. Airy disk always visible; arcs frequently seen on brighter stars.
6. Airy disk always visible; short arcs constantly seen.
7. Disk sometimes sharply defined; diffraction rings seen as long arcs or complete circles.
8. Disk always sharply defined rings seen as long arcs or complete circles, but always in motion.
9. The inner diffraction ring is stationary. Outer rings momentarily stationary.
10. The complete diffraction pattern is stationary.
Note: On this scale 1-2 is very poor, 3-4 is poor, 5 is fair, 6-7 is good, 7-8 very good, and 8-10
excellent.
These photos show
the double star Zeta
Aquarii which has a
separation of 2
arcseconds being
messed up by
atmospheric seeing.
A SCALE OF SEEING (2)
In Italy, to measure the seeing we use the Antoniardi’s scale that gives a value from
I to V taking in account the effect of distortion.
I = excellent condition: the image is practically perfect
II = good condition: the image is stable for long periods
III = medium condition: the image is troubled and blurry but there are brief period of
stable images
IV = bad condition: the image is always fuzzy
V = it’s impossible to do observations
The turbulence is something instable and it’s non simple to assign the right value of
seeing. Usually it’s possible to use middle value as I-II or III-IV.
A drawing of Jupiter
simulated to show 3
different views: (far
left) under excellent
seeing, (centre) under
fair seeing, and (far
right) very poor seeing.
THE SITE
Good intrinsic site seeing quality is important, if not crucial to most astronomical observations.
The world’s finest locations for a stable atmosphere are mountain top observatories, located
above frequently occurring temperature inversion layers, where the prevailing winds have
crossed many miles of ocean. Sites such as these are: La Palma, Tenerife, Hawaii, Paranal
etc, that frequently enjoy superb seeing much of the year. Also a major factor is generally
unvarying weather patterns, dominated by large anti-cyclones (High pressure systems).
ENO – European North Observatory
Observatory
Location
TEIDE
Tenerife
Surface area
50 hectares
Altitude
2.390 metres
Longitude
16º 30' 35" West
Latitude
28º 18' 00" North
Observatory
ROQUE DE
LOS MUCHACHOS
Location
Surface area
Altitude
Longitude
Latitude
La Palma
189 hectares
2.396 metres
17º 52' 34" West
28º 45' 34" North
ENO – TEIDE
VTT (Vacuum Tower
Telescope)
Solar labour
Infrared
telescope
"Carlos
Sánchez"
VTT
Solar telescope
(France-Italy)
THEMIS
ENO - ROQUE DE LOS MUCHACHOS
WHT
New solar telescope
Sueco (NSST), 1 m
Solar telescope
Abierto Holandés (45
cm)
Telescope "William Herschel",
(WHT) 4,2 metre
TNG - Telescopio Nazionale Galileo
The Telescopio Nazionale Galileo (TNG), with a primary mirror of 3.58m, is the
national facility of the Italian astronomical community.
ENO – Historical Perspective
The quality of the Canarian skies for observational astronomy
has long been recognised. As far back as 1856, the
Astronomer Royal for Scotland conducted astronomical
experiments on the mountain summits of the island of
Tenerife. He concluded that the skies above the Canary
Islands were ideal for astronomical Observations.
These early conclusions were later confirmed by, among others,
Jean Mascart in 1910 during an expedition to study Halley's
Comet: Mascart reported that the cold ocean currents
surrounding the Canary Islands in combination with the trade
winds, provide a unique stable climate with little atmospheric
turbulence.
In 1968 a collaboration was established among a number of European institutes and an
extensive site-testing campaign began, identifying La Palma and Tenerife as the best observing
sites.This international collaboration was formalized in 1979 with the signing of the International
Agreements and a specific law approved in 1988. This makes the IAC's Observatories a legally
protected site (in effect an astronomical "reserve") where continued dark skies, low radio
frequency fields, and control over other sky-polluting effects are guaranteed.
ENO - Sky Background
The brightness of the moonless night sky above La Palma has been measured from hundreds
of CCD images taken with the 2.5-m Isaac Newton and 1.0-m Jacobus Kapteyn Telescopes
between 1987 and 1996. The 2 median sky brightness, in units of magnitude per arcsec , at
high elevation, high galactic latitude and high ecliptic latitude, at sunspot minimum, is B=22.7,
V=21.9, R=21.0, U~22.0, I~20.0. The contribution of light pollution to the continuum brightness
at the zenith is <0.03 mag in all bands. The Sodium D emission brightens the sky in both V and
R broad bands by about 0.07 mag. Total contamination (line plus continuum) at zenith is <0.03
mag in U, ~0.02 in B, ~0.10 in V, and ~0.10 in R.
Typical spectrum of the La Palma sky
on a moonless night. Most of the
distinctive features of the night-sky
spectrum are due to airglow. The NaD
emission at 5890/6 A is partly due to
street lighting, while the mercury
emission at 4358, 5461 A is wholly so.
With the exception of the 8645-A O_2
line, the features dominating the
spectrum redward of 6500 A are
rotation-vibration bands of OH.
ENO - Atmospheric Transmission
Long-baseline extinction values for ORM have been measured by the Carlsberg Meridian
Telescope in the V band (and more recently in the Sloan r' band). During photometric, dust-free
nights median extinction is: 0.19 mag (at 480 nm), 0.09 mag (at 625 nm) and 0.05 mag (at 767
nm).
In summer over 75% of the nights are free
of dust (there are short periods of higher
extinction due to Saharan dust in the
atmosphere), while at other times of the
year over 90% of the nights are dust free.
Extinction in V is less than 0.2 mag on
observing nights over approximately 88% of
the nights, and extinction in excess of 0.5
mag only occurs less than 1% of the nights.
Frequency of extinction over the ORM
during winter (top) and summer period
(centre). In both cases the modal value is
found
at
0.11
mag/airmass.
Their
Corresponding cumulative frequencies are
also shown (bottom). The vertical line
indicates the extinction coefficient limit for
dusty nights, k > 0.153 v mag/airmass.
ENO - Image Quality and Atmospheric Turbulence
Good intrinsic site seeing quality is important, if not crucial to
most astronomical observations. A number of site testing
campaigns now provide objective evidence for the quality of the
site. Using a Differential Image Motion Monitor (DIMM) the free
atmosphere seeing was measured over long time intervals and
at different places (at times simultaneously) at the ORM.
These results show that the mean seeing over the years is
0.67 arcsec. Under typical seeing conditions, image quality
does not depend on the particular location and shows a
high degree of homogeneity over the whole Observatory. A
seasonal variation is noticeable, better seeing conditions
appearing during the summer, coinciding with a welldefined inversion layer due to the high prevalence of trade
winds. During the summer, 50% of the time seeing is
better than 0.54", value which reaches down to 0.4-0.46"
during June-July.
ENO - Image Quality and Atmospheric Turbulence (2)
Only La Silla, Paranal and ORM have
systematic seeing measurements taken
with calibrated instruments.
To evaluate the contribution of the free
atmosphere, the boundary layer and the
surface layer to image degradation, several
intensive site-testing campaigns have been
carried out using: equipped balloon
soundings (CN2 profiles, water vapour,
wind velocity and direction) + DIMMs and
meteorological towers equipped with
microthermal sensors.
TNG
The FREE ATMOSPHERE has
an exceptionally low contribution
(0.4"), comparable to the values
measured at La Silla (0.34") and
Mauna Kea (0.46"). The
SURFACE LAYER (from 6 up to
12m) is almost negligible (0.08").
ESO – European Southern Observatory
ESO was created in 1962 and is supported by eleven countries: Belgium, Denmark, Finland,
France, Germany, Italy, the Netherlands, Portugal, Sweden, Switzerland and United Kingdom.
ESO operates three sites in the Atacama desert region in Chile.
The Very Large Telescope (VLT) on Paranal is located on a 2.600 m high mountain some 130
km south of Antofagasta. The VLT consists of four 8.2-meter and several 1.8-meter telescopes.
These telescopes can also be used in combination as the VLT interferometer (VLTI). All four
telescopes and five large state-of-the-art multi-mode astronomical instruments are now in
operation. The VLTI had "First Light" in March 2001.
The La Silla Observatory is located 600 km north of Santiago de Chile, at 2.400 m altitude, and
consists of a series of optical telescopes with diameters up to 3.6 m .
The third site is the 5,000 meter high Llano de Chajnantor, near San Pedro de Atacama. Here a
new submillimeter telescope (APEX) is being completed, and a large submillimeter
interferometer array of 64 antennas (ALMA) is under development.
ESO -The Very Large Telescope (VLT)
Definitions and Terminology
(1)
Astronomical seeing is the distortion of the radiation wavefront of a cosmic source,
produced by variations in the index of refraction of air. In conditions that are
conducive to astronomical observations, pressure and water vapor fluctuations are
negligible, thus variations in the index of refraction result primarily from thermal
fluctuations associated with turbulent air flow.
The temperature structure function of an atmospheric layer at altitude h above the
ground is defined in the form:
DT (r, h) =< T(r, h)2 >
where T(r, h) is the temperature difference between two points separated by a distance
r, at constant altitude.
DT is generally described by a power law of the form:
(2)
DT(r) ∝ rb
where b = 2/3 (Tatarski 1961). The proportionality in equation 2 is mediated by the
scale independent temperature structure parameter (Roddier 1981):
(3)
CT2 
DT ( r )
r 2/3
which relates to the refractive index structure parameter via:
(4)

C n 2 (h)  CT2 (h) 80 106 P(h) / T 2 (h)

2
where the units of C2n, pressure P(h) and temperature T(h) are respectively m-2/3, mb
and K.
Definitions and Terminology (2)
The spatial coherence scale of atmospheric turbulence is expressed by the Fried
3 / 5
parameter r0 (in m):

(5)
r0  0.423k 2 sec( ZA) Cn2 (h)dh


h0
where k is the wavenumber in m-1, ZA the zenith angle of the line of sight and h0 the
elevation of the telescope above the ground in m.
Finally, the full–width at half–maximum of a stellar image at a wavelength is
(6)
 fwhm  0.98

r0
In other words, for a telescope of diameter D turbulence degrades the image
resolution from Qdiff=l/D to the diffraction limit which would be observed with a
telescope of aperture r0, Qfwhm/ =l/r0. Since r0 ∝ k-6/5, as indicated by Equation 5, and
l= 2p/k, then
(7)
 fwhm  0.2
All layers of the atmosphere contribute to seeing, as illustrated by Equation (5)
HOW MUCH DOES THE SEEING COST?
NASA spent $2.1 billion to escape from poor atmospheric seeing;
that's what it cost to put the Hubble Space Telescope in orbit.
BIBLIOGRAFIA
• “Astronomical seeing” Ron Thorkildson, web site of Rose City Astronomers
• “Paul Conteau on atmosferic seeing” web site of Rose City Astronomers
• “The atmosphere and observing” Damian Peach
• “Beating the seeing” Alan M. MacRobert,
• “Astrophysical techniques” C. R. Kitchin
• “Reflecting Telescope OpticsII” R.N.Wilson
• “Il Seeing” di G. De Santis e L. Zucchi
• “The Optical/Infrared Astronomical Quality of High Atacama Sites. I.Preliminary
Results of Optical Seeing” Giovanelli et al.
•“Ottica Adattiva”J.W.Hardly (articolo Scienze agosto 1994)
• www.iac.es
• www.tng.iac.es
• www.eso.com
• www.skyandtelescope.com
• www.rca-omsi.org