- What does "good seeing" mean?
- The atmosphere is a complex and ever changing mass of air which can
drastically affect how well you can see with your telescope. To the
naked eye, on what would appear to be a clear night, stars and planets
might look just fine. But through a telescope, focusing may actually be
next to impossible.
- Observing planets, planetary nebulae or any celestial object with
details at high power requires excellent seeing conditions. The seeing
is the term used in astronomy to quantify the steadiness or the
turbulence of the atmosphere. Seeing should not be confused with sky
transparency, which is the terminology used to qualify the darkness of
the sky. When we look at planets, we need high power to see all the fine
details but most of the time we are limited by turbulence occurring in
the telescope (local seeing) and/or in the atmosphere. During a night of
bad seeing we are usually limited to see only two bands on the Jupiter
disc and we can hardly use power over 100-150x. On excellent seeing
conditions we can use high power and see many bands, white spots,
festoons and details in the great red spot. Excellent seeing with high
quality telescopes can also show details on the largest moon of Jupiter,
Ganymede. What we are seeking is the best nights where we can boost our
telescopes to their limits… which reach as high as 50X per inch
diameter for quality telescopes… which means 500x for a quality
10-inch ( 25cm ) instrument.
- A night of exceptionally good seeing, a night where the detail seen on
Jupiter causes observers to swoon and swear, is thought to be rare. It
would be boon to a know in advance when good and bad seeing might occur.
- see Mars for an
example of effects of seeing
- twinkling; irregular changes in the brightness of a star caused by
atmospheric turbulence. A star will appear to wander around its
average position in a telescope since the image is being disturbed
by the atmosphere.
- contrary to popular belief, crisp, clear Winter nights, with the
stars twinkling like Christmas lights, are the worst possible for
- factors affecting seeing:
- haze can be due to a number of different factors, but most
commonly is pollution. For example, if no wind has
blown, smoke from cars and factories accumulates close to
ground level. Haze can also be due to an environmental effect called
inversion. That's when cooler air (along with air born
pollutants) is trapped below a layer of warmer air. This obviously
affects seeing by reducing detail and brightness of objects. Expect
only the planets and brightest of stars to show through during these
times. Light Pollution also increases due to haze or smog in
- Light pollution
- The Jet Stream/Upper Level Winds:
- if the jet stream is overhead or just the fact that upper level
winds are acting up (winds several kilometres up that cannot be felt
at ground level), it'll look as if everything was under flowing
water when you look through your telescope.
- keep in mind that your telescope doesn't just magnify the objects
you want to see, it also magnifies the atmosphere as well. That's
why the Hubble
telescope is in space and can get those remarkable images. Focusing
at higher magnifications will be next to impossible to accomplish
during these times. And unless you have sophisticated technology,
like that found on the Keck
telescopes on Mauna Kea in Hawaii, there's nothing you can do about
it, but wait for a better day.
- local air disturbances:
- "surface layer seeing":
- lower atmosphere temperature differentials and wind shear
effects especially in the lowest 200m above ground level
- in general, the higher up in altitude, the better the seeing
because there is less atmosphere to see through. This is one
reason professional observatories are usually located high up on
mountain tops. However, some sea level locations (southern
Florida for example) can be nearly as good at certain times and
some locations such as the Midwestern U.S. are nearly always
- telescopes with top of opening at least 10m above ground level
can be expected to perform much better than those at ground
- rising heat:
- try to use a position
which is not looking at the planets over the top of the house
especially if the house heater is on. The heat fluctuations
coming off the top of the house get between you and the planets.
- bad seeing caused by local effects, like a hot driveway, is
properly called ground seeing.
- "dome seeing"
- poor seeing due to air refraction with telescope domes
- air disturbances "tube currents" within the telescope
causing "mirror seeing":
- Reflectors are notorious for their tube currents. Any
open-ended tube should be ventilated as well as possible.
Suspending a fan behind a reflector's mirror has become a
popular way to speed cooling and blow out mixed-temperature
- It's easy to check whether tube currents trouble your
- Turn a bright star far out of focus until its a big,
uniform disk of light.
- Tube currents will show as thin lines of light and shadow
slowly looping and curling across the disk.
- if the out-of-focus star disk swarms with wrinkles that
scoot across the view, entering one edge and leaving the
other, then there is local seeing near the telescope.
- assuming the more common warm telescope inside house, going to
cooler outside environment scenario:
- The heat stored within your telescope, which includes not only
the optics, but the tube and other parts as well, creates
turbulences in the air and physical deformations in your optics
when you take it outside. When looking through an eyepiece,
these turbulences and optical deformations translate into a
fuzzy and poor quality image. As your telescope comes closer to
achieving equilibrium with the surrounding temperature, the
turbulence in the air inside your telescope calms down, and your
optics also begin to settle in to their new shapes. Remember,
when things cool off, they contract.
- Now depending on the temperature difference, you may be able
to see very well with your telescope immediately after taking it
outside, like on a warm summer night, or you may need to let it
sit out for about 3 hours, like on a cold wintry day. A 10 inch
Newtonian takes approx. 3 hours to fully stabilize when the
temperature difference is 30 degrees Celsius (+22C inside, -8C
outside). As refractors are sealed tubes, those with many
lenses will take a long term to reach thermal equilibrium.
- So if you're taking pictures of the sky, let your telescope
- When and Where is "Good Seeing" Possible?
- The "where" part of this question isn't so difficult. A
nice dark place far away from light pollution, will most likely also
take you away from air pollution as well. Incredibly, light pollution
from a city can actually affect the seeing qualities as much as 50
kilometres outside of the city limits! So the further you are away from
the "big city" the better.
- The higher altitudes also help "seeing", as you're going to
be looking through less atmosphere.
- Ideal locations are remote mountain peaks with prevailing winds coming
from the ocean. Hawaii, the Chilean ranges and the Canary Islands are
prime examples. Avoiding weather fronts and locating instruments on
grass may help.
- As for the "when" part, that can get tricky! The summer
season yields somewhat better seeing conditions than the others, as the
air is less dense and frankly the temperature is more comfortable for
- The best time to observe is just before dawn, when the air is stillest
after the Earth has given off it's heat over night. Looking through the
least amount of atmosphere by observing when the object is overhead or
at it's highest.
- As you use a telescope on
different nights, you will find every night is different depending on
the weather, pollution, heat, humidity and dust.etc. One night you won't
be able to use more than 200x magnification, then on the next night you
can. There are different ways to tell roughly. How much the stars
twinkle is one way or finding out the UV (ultra-violet) rating for the
day on the weather is another. After a while you can tell just by
looking at an object you know.
- "Airy Disk":
- the disc-like image of a planet or star (or any point source) which is
seen through an optical system with a circular aperture.
- the majority of the light from the object is within this disc, and
this is what limits the resolving power of a telescope.
- it is a series of concentric
rings around a bright star and the ability to see it indicates excellent
optics and seeing conditions.
- the central disk is known as the Airy disk and it's size in inversely
proportional to the size of the telescope objective.
- That is why a large telescope can see more detail under perfect
conditions than a small one.
- Because of physical limits the Airy disk is the smallest detail that
can be seen at maximum magnification and the smaller it is, the less it
intrudes on the detail. Makes little difference when looking at a star
which can never be resolved because of distance but when looking at the
surface of Mars or the Moon, every feature is just a lot of Airy disks
all jumbled together and the larger they are, the fuzzier the image.
- Measuring "seeing":
- Professional astronomers and more advanced astro-amateurs evaluate the
seeing with a scale 1-10. Through a telescope, they measure the star
diameter which usually ranges from bad seeing at 5-8 arcsec to excellent
seeing at 0.5-0.2 arcsec. Astro-amateurs, can also use a qualitative way
to measure the seeing. They look through their telescope at the zenith
for a 2-3 magnitude star at about 30-40X per inch diameter ( 300-400x
for a 10 inch telescope ) and from the look of the diffraction pattern
they estimate the seeing on a scale I-V.
- the seeing can be rated through astro-amateur telescopes with the
following guidance incl. arc-seconds diameters:
- V ….. Perfect motionless diffraction pattern....<0.4"
- IV….. Light undulations across diffraction rings.....0.4-0.9"
- III….. Central disc deformations. Broken diffraction rings.....1.0-2.0"
- II…… Important eddy streams in the central disc. Missing or
partly missing diffraction rings.....3.0-4.0"
- I……. Boiling image without any sign of diffraction pattern......>4"
- see photo examples here: http://www.weatheroffice.ec.gc.ca/astro/seeing_e.html
- the diffraction pattern diameter is related to the aperture of the
telescope. The diffraction pattern of a 4 inch telescope is twice as
large as for an 8 inch instrument. So the seeing rating with this method
will depend of the diameter of the telescope. An astro-amateur rating
the seeing at 4/5 with a 6 inch telescope will certainly appear as a 3/5
with a 12-14 inch optical instrument.
- Pickering scale:
- a system developed by William H. Pickering of Harvard at the turn
of the century
- The popular Pickering 1 to 10 scale is in common use by
professionals and amateurs alike. The Pickering scale is based on
what a highly magnified star looks like when carefully focused, in a
- A star at high magnification, under perfect seeing (P-10) looks
like a bull's eye. A small central disk surrounded by one or more
concentric rings. At P-1, it is just an amorphous blob.
- One fact little understood by purchasers of new telescopes is that
the effects of poor seeing increase dramatically as the size of
the telescope is increased. This is simply because a small
telescope has to look through a much smaller column of air than a
large one. A fairly good night with a small scope might be not worth
taking out a large one. Pickering established his system using a
5" diameter telescope and his scale would have to be fudged
when used with a scope of larger or smaller aperture.
- At P-7, a 16" reflector is about the same as a 5"
refractor at P-4 in the ability to resolve detail and not just the
ability to see dim objects. The large scope always prevails in the
latter but when viewing the surface of Mars or the Moon, for
example, no more detail can be seen on a poor night with a larger
- it is very difficult to photograph the diffraction pattern but
there are more pragmatic ways of demonstrating the effects of
seeing. Because seeing not only varies from location to location and
from night to night but also changes drastically from moment to
moment, particularly on poor nights. A P-3 night can have instants
of P-6 and a patient observer can often snatch good views if persistent
enough and does not blink at the right moment. Because of the fast
and continuous frame capture of video, it is very easy to
demonstrate these moments just by attaching a video camera to a
telescope and pointing it at the moon.
- details of the scale:
- P-1 Star image is usually about twice the diameter of the
third diffraction ring (if the ring could be seen.
- P-2 Image occasionally twice the diameter of the third ring.
- P-3 Image about the same diameter as the third ring and
brighter at the center.
- P-4 The central disk often visible; arcs of diffraction rings
- P-5 Disk always visible; arcs frequently seen.
- P-6 Disk always visible; short arcs constantly seen.
- P-7 Disk sometimes sharply defined; rings seen as
long arcs or complete circles.
- P-8 Disk always sharply defined; rings as long arcs
or complete but in motion.
- P-9 Inner ring stationary. Outer rings momentarily stationary.
- P-10 Complete diffraction pattern is stationary.
- Measuring Seeing, the DIMM
method - sophisticated (ppt)
Physics of Seeing
The physical relationship between atmospheric turbulence and seeing quality
has been reviewed in detail by Roddier (1981) and Coulman (1985). When a plane
wave of light with uniform amplitude propagates through a refractively
nonuniform medium such as the atmosphere, it exhibits amplitude and phase
fluctuations. When such a wave front is focused, the resulting image varies in
intensity, sharpness, and position. These variations are commonly referred to as
scintillation, image blurring, and image motion, respectively.
In turbulent flows, there is a range of eddy sizes that are large enough to
avoid dissipation by friction and yet are too small to be imparting kinetic
energy to the flow, called the inertial subrange. At separations (r) of the
order of inertial subrange scales, the temperature structure coefficient (CT2)
in a locally isotropic field has the form:
CT2 = [T (x) - T (x +r)]2 /r2/3
where 0.1m <~ r <~ 1.0m is the separation vector and T is temperature.
Seeing quality is therefore related to high-frequency temperature fluctuations
associated with atmospheric turbulence.
These high frequency temperature fluctuations produce variations in the
refractive index of light in the atmosphere. The refractive index structure
parameter (Cn2), which is a measure of the average
variability of the refractive index of light in the atmosphere, is related to CT2
Cn2 = CT2 [7.9x10-5P/T2]2
where P is the pressure in mb and T is the temperature in K.
The total effect of atmospheric turbulence is derived from the integral of Cn2
(z) for all atmospheric layers. The Fried parameter (ro) is a
commonly used measure of the total image degradation due to atmospheric
turbulence. It is related to Cn2 as follows:
ro = [ 0.06 w2 / Cn2 (z) dz ]3/5
where w is the optical wavelength (usually taken as 550 nm).
Meteorology of Seeing
There are three main types of turbulent motion that affect image quality.
i) Turbulence in the free atmosphere: In the free atmosphere, microthermal
activity is associated with strong wind speed and temperature gradients that
generally occur in the vicinity of the upper tropospheric jet stream at an
altitude of about 12 km.
ii) Turbulence in the atmospheric boundary layer: At the boundary between the
atmosphere and the Earth's surface, frictional effects cause the atmospheric
boundary layer flow to be turbulent. This region is also characterized by strong
iii) Turbulence in and around the telescope dome: The telescope dome
interacts with the boundary layer flow in a manner that enhances turbulence in
and around the dome. The effects of the telescope dome on seeing quality are
dependent largely on the design and thermal characteristics of the structure
itself and not on the site at which the facility is located.
The effects of ground turbulence are strongly dependent on local variations
in surface roughness, thermal forcing, and topography. These factors affect the
local wind speed and temperature gradients which are directly related to
turbulence generation via the Richardson number (Ri).
Ri = (g/T)[(dT/dz)DALR - dT/dz)] / (dV/dz)2
where (dT/dz)DALR is the dry adiabatic temperature lapse rate with
height, dT/dz is the observed temperature lapse rate with height, g is
gravitational acceleration, T is the mean temperature in the layer and dV/dz is
the wind speed versus height gradient. Wyngaard et al. (1971) have
developed a semiempirical theory relating CT2 to Ri
in the surface boundary layer. The values of CT2 computed
using their technique and those measured directly show remarkably good
agreement. Thus the theoretical bases for relating CT2 to
ambient parameters has been confirmed by observations.
Mahrt (1985) studied the structure of turbulence in a very stable boundary
layer. He found that enhanced turbulence may occur at the top of the surface
inversion layer where the nocturnal drainage flow interacts with the synoptic
flow regime. This phenomenon usually occurs in the upper boundary layer at
heights of 200-500 m above the surface. A stable drainage flow accompanied by a
surface inversion layer does develop on the slopes of mountains at night so the
interactions described by Mahrt (1985) may be occurring at observatory sites. In
addition, mass continuity demands that the air removed from the summit at night
by the drainage flow be replaced by enhanced subsidence above the mountain. This
may produce adjacent layers of different potential temperatures (an inversion).
If mixing occurs under these circumstances, microthermal activity may result.
These two mechanisms may be responsible for turbulence generation in the layer
between 30 m and 1000 m.
Free atmosphere effects are determined by synoptic scale meteorological
systems. Since these systems are migratory or undergo temporal oscillations in
intensity and have scales of between 500 and 5000 km, they induce changes in
atmospheric conditions at a particular locality with a time scale of between 1
and 5 days. Available data on the latitudinal position and strength of the
subtropical westerly jet stream at the longitude of Hawaii (Sadler, 1975), for
example, indicates that the level of microthermal activity in the free
atmosphere above Mauna Kea is highly variable since the jet stream occurs in a
region of strong temperature and wind speed gradients, these variations are
likely to be associated with changes in seeing quality.
Van Zandt et al (1978, 1981) have developed a model that simulates profiles
of Cn2 (z) in the free atmosphere. Limited comparison of
model simulations with observations (Green et al, 1984) have been made.
Deviations between simulated and observed profiles of Cn2
(z) occurred in the lower troposphere due to high humidity and low static
stability in the area where the observations were made. These conditions are not
typical of the free atmosphere above observatory sites where the air is extemely
dry and stable. Thus it is likely that the Van Zandt model can be used to
quantify the contributions of free atmosphere turbulence to image quality
The discussion above shows that the theoretical basis for using
meteorological parameters to quantify the effects of atmospheric turbulence on
seeing quality exists. There is good reason to believe that seeing quality can
be related to ambient meteorological conditions. Therefore the potential exists
to use these data to quantify and possibly forecast seeing quality at telescope