User Tools

Site Tools


photo:ast_seeing

astronomic "seeing"

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
  • Scintillation: 
    • 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 serious observing.
  • factors affecting seeing:
  • Haze/Smog:
    • 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 the air.
  • 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 bad.
      • telescopes with top of opening at least 10m above ground level can be expected to perform much better than those at ground level.
    • 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 air. 
    • It's easy to check whether tube currents trouble your images:
      • 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 cool completely.

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 us.
  • 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“

  • 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 small telescope.
  • 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 scope.
  • 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 sometimes seen.
    • 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.

The 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 ® 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 (1)

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 as follows:

Cn2 = CT2 [7.9×10-5P/T2]2 (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 (m)

where w is the optical wavelength (usually taken as 550 nm).

The 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 temperature gradients.

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 (4)

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 degradation.

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 sites.

 

photo/ast_seeing.txt · Last modified: 2013/02/01 12:57 by gary1