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Part 3: Observing Techniques
1. Seeing Evaluation
In order to understand seeing, understand its relationship to and possible origin in the site location, season, weather, jet stream, instrument and cooldown, the astronomer must be able to assess the atmospheric turbulence. This means the astronomer must have a reliable and consistent method of seeing evaluation.
Turbulence varies both in frequency and amplitude. These two factors are generally correlated as two different signs of the amount of energy driving the turbulence. However seeing is usually the cumulative optical effect of several different layers of atmosphere, so it is important that the astronomer has the means to determine where the turbulence arises, as this can determine whether the turbulence will change across the diurnal cycle. Both the aperture diameter and the relative mixture of dancing, speckling and flashing can help resolve this question.
Zenith Distance Scintillation
Although disparaged by many astronomers as crude and inaccurate, the 19th century method of seeing evaluation based on the naked eye scintillation or "twinkling" of brighter stars can be remarkably informative, especially if it is supplemented with observation through a tripod supported or motion suppresion binocular. It does not require the set up and cool down of the telescope, and can suggest whether conditions are worth that effort.
Naked eye twinkling presents four characteristics for inspection:
• The frequency of scintillation, as both the speed of the flickering, and whether the flickering is continuous or intermittent
• The smallest zenith distance of stars that show scintillation
• The lowest visual magnitude of stars that show scintillation.
Brightness affects visual sensitivity to a flicker stimulus: a constant flicker is more difficult to perceive in dim rather than bright lights, but flicker that is slower and more extreme can be perceived in fainter lights.
Third magnitude stars twinkling at the zenith when looked at directly indicate the worst seeing. First magnitude stars that do not twinkle even when 70° from the zenith (or 20° above the horizon) indicate very good seeing, especially if that condition holds around large areas of the horizon. (Typically, seeing varies with compass direction at low visual angles.)
When there are multiple sources of turbulence, the larger amplitude (surface layer and geographic) turbulence will mask the quicker and less visible high altitude turbulence. High altitude, high frequency turbulence may not be visible at all to the naked eye: stars that appear absolutely steady to the naked eye can appear to be inflated and boiling with speckles in a telescope. Discrimination can be improved with binocular observation, which will suppress the low frequency variations in brightness and show the high frequency variation more clearly.
The upshot is that twinkling is a poor method for assessing high frequency, high altitude turbulence and mixed turbulence, but is generally a reliable indicator of low frequency surface and geographic turbulence. By itself it signals momentary conditions that can easily change, but combined with an understanding of local weather conditions and diurnal cooling patterns, it can be sufficient to indicate whether it is worthwhile to set up for the evening.
A Compact Seeing Scale
Once a telescope has been set up or an observatory opened for work, it is more practical to assess seeing by means of a telescopic image. For that purpose a scale of seeing evaluation is valuable.
The key attributes of a reliable and useful rating scale are that the categories are easy to apply, the judgments will be consistent and replicable across observers and conditions, the categories adequately capture the underlying variation, and the ratings are useful for purposes of analysis or prediction.
In the previous page, I pointed out the problems with current seeing scales. The Antoniadi and APLO scales are too subjective, while the Pickering Douglass Standard Scale uses 10 intervals, too many for reliable use given the basic variability of atmospheric turbulence. Time averaged photographic FWHM is highly reliable but does not assess the visual quality of seeing and cannot distinguish the frequency and amplitude of the turbulence.
Based on my observing experience, I have adapted the Standard Scale to make it compact, simple and more robust to short term variations in turbulence. My version anchors each step on the stable appearance of a single specific feature, or the lack of any recognizable structure. The scale is based on the common academic rating A to F (omitting E), and the "grades" can be tweaked if desirable by applying plus (+) or minus (–) according to the "severity of turbulence" criterion.
Note that an aperture of about 6 inches (150mm) is approximately the smallest aperture at which complex variations in seeing are noticeable. In smaller apertures seeing is perceptible as the "bodily motion" or "dancing" of the stellar image.
• The "Airy disk" is the evenly bright, round disk, bordered by a dark band, at the center of an undistorted star image at high magnification (diagram, above).
In general, low frequency (instrument and surface) turbulence evolves from the undistorted diffraction pattern through small dancing movements to large dancing movements and increasing amounts of flashing or "spiking" around the star image. In contrast, high frequency (high geographic and high altitude) turbulence evolves from the undistorted diffraction pattern through coarse speckling of the diffraction rings, to medium speckling that includes the Airy disk, through high frequency speckling that is too rapid for the eye to resolve and flashing that lights up a large area of aerosol diffusion.
Motion in the Lunar Image
An effective method for further evaluation, when feasible, is observing the terminator or limb of the moon or a large planet with medium magnification. On the moon in particular, turbulence shadows throw in relief a variety of lunar surface detail of all angular sizes. Low frequency (boundary layer) turbulence will cause large features to undulate as if underneath a large flowing stream of water, and make very fine detail visible in long glimpses; high frequency (tropopause) turbulence will cause shadow edges to vibrate or jitter, will give the image an overall soft or fuzzy appearance, and will erase small details completely.
Starting with the object in best focus, the focus should be moved in the extrafocal direction (backing the eyepiece away from the objective, or turning an SCT mirror focusing knob clockwise). This longer focal length brings into focus atmospheric layers closer to the observer. Although it is possible to determine the approximate altitude of the disturbance from the amount of backfocus, it is most convenient simply to examine the separate effects of the high altitude and low altitude layers.
Typically the surface turbulence changes significantly (and more unpredictably) across several hours, while high altitude turbulence is more stable and more predictable across an entire evening.
Turbulence Shadows
The out of focus telescope image of a star at moderately high magnification (~200x) can reveal the actual turbulence currents in both the atmosphere and in the instrument itself.
First, center a bright (magnitude 0 to 3) star in the eyepiece field, then defocus the star until the disk of light is quite large and noticeably fainter — the central obstruction will appear as a shadow rather than an area of concetric diffraction rings, but the rim is still resolved into a series of concentric rings (if these disappear, the image has been defocused too much).
Within this disk will appear faint flowing shadows and overlapping bright streaks: these are shadows of the turbulence. The three main variations are shown below: (A) high frequency and high altitude turbulence, which almost always moves horizontally in the same direction as the jet stream flow; (B) low frequency surface turbulence, which can move in a horizontal direction in the presence of wind, but more often moves in a vertical direction in the absence of wind; and (C) mirror seeing or mirror thermal discharge, which appears as a slowly flickering elongated area extending upward from the image center and across the top half of the image. In addition, tube currents can appear as leisurely, large undulations around the edges of the image.
The principal method for controlling the effects of seeing is to choose an observing site with optimal atmospheric conditions. This strategy is one reason that modern large terrestrial telescopes have been located in clusters at a handful of locations — the sites found after extensive surveys to have the best average seeing and weather conditions year round.
Amateurs typically have limited capability to select sites because work is done at or near their residence. However they can significantly improve their routine seeing through careful choice of station within their property or at a remote site offering convenient access for portable instruments.
A number of guidelines have been developed to guide selection of observing sites:
• Nearby vegetation such as grassy fields, groves of shrubs or trees will usually improve seeing by insulating the ground from solar heating.
• Obstructive or funneling structures such as narrow valleys, tall buildings, nearby ridges or hill crests can increase turbulence when these are upwind from the observing site.
• Usually increased elevation improves seeing, especially if it is above diurnal winds or breezes, but the crest and slopes of hills exposed to wind, the slope or foot of hills exposed to downward moving cold air currents, and locations at the bottom of small valleys generally experience poor seeing. The best seeing conditions are usually on an elevated plateau, away from any descending slope and nearby larger hills or mountains.
• A nearby ocean, lake or large river can produce steady laminar air flows, although sometimes with increased humidity that can cause dew, mist or fog. A cold ocean or lake is usually preferable to a warm one: with the exception of Florida, seeing is usually better along the cold Pacific than the warm Atlantic coast — a major reason that many large reflecting telescope observatories of the early 20th century were sited in California.
Turbulence is generally at a minimum, and seeing therefore the best, near the zenith, although instrumental thermal effects can occur at almost any viewing angle, especially when rising heat from the observer's body crosses the optical path in the winter.
Heat from the observer's body becomes more pronounced during cold nights where the temperature difference with the air is greatest, but primarily affects Newtonian reflectors (because the observer is positioned near the opening of the optical tube).
A significant amount of turbulence can arise from the observatory structure or telescope storage shelter. In general, massive masonry structures will hold and release more heat than light wooden or metal structures. Aluminum or zinc siding and roofing will reflect heat and shield the interior from solar heating, then will quickly radiate away heat after sunset.
Domes, especially when heated during the winter, will produce thermal "chimneys" through the observing window; roll off roof structures release heat the fastest and will not trap heat from the observer's body. Dome openings and roof edges can produce turbulence in a slight breeze or evening air currents.
After site selection, the major factor affecting atmospheric turbulence is the seasonal and diurnal cycle of temperature and wind, which affects turbulence up to several kilometers from the ground.
Seasonal variation
Seasonal weather patterns have long been recognized as a key factor. The chart (below) from a paper by Teare & Thompson (2002) shows the number of days of subarcsecond seeing per month at the Mt. Wilson Observatory from 1940 to 1945. As is typical of many locations along the Pacific Coast, seeing is worst in winter and spring, improving to peak periods of good seeing in the summer and early fall. This specific pattern is not universal, but all areas will experience a characteristic seasonal cycle of some kind.
The Mt. Wilson study also illustrates the variation in weather patterns from one year to the next. Examining long term trends from 1920 through 2000, the researchers identified weather cycles of 11 years (good seeing might correspond to periods of peak solar sunspot activity, when solar radiance is reduced) and of 20 years, possibly related to climate cycles such as the Pacific Coast El Niño and La Niña.
Weather
Within each season, weather determines the nightly conditions. A falling barometer (approach of a low pressure system) is usually less favorable than a rising barometer. The formation of clouds (especially cumulus clouds) can release large amounts of heat, which creates turbulence as it rises through the atmosphere. High cirrus clouds often indicate rapidly moving high altitude air. The approach and passing of storms, winds or air currents from mountains, and a rapidly changing barometer are generally bad. A light haze, steady air, ground snow cover and high humidity are often good, despite the effort necessary to combat dew.
The prevailing wind will often signal better or worse seeing, depending on the specific geographic location and the upwind sources of heat or turbulence. Usually winds descending from mountains, across hills or ridges, or from inland are less favorable than winds arriving from across plains or oceans. However winds are almost always less favorable than steady air, and afternoon breezes less favorable than morning breezes.
Temperature inversion layers, caused by the heating of stratospheric or surface ozone by the sun, create a ceiling on convection currents and can limit the rise of heat from the troposphere into the stratosphere.
The Jet Stream
Across the entire continental United States, Canada and Europe, major transitions in the weather patterns are produced by the energetic and highly variable currents of the jet stream. Most modern observatories with large telescopes are located at low latitudes (below the "furious forties" and out of reach of the northern and southern jet streams) to escape its influence.
Astronomers can find real time information about jet stream conditions at several different weather or aeronautical web sites. On the west coast, the California Regional Weather Service maps report current jet steam conditions (within the past 12 hours) and the forecasted jet stream path for the next 5 days.
img src="http://squall.sfsu.edu/gif/jetstream_pac_init_00.gif" width=783 height=630>
current jet stream map for the western United States and Canada
The current map for my region is piped above; maps are also available for Canada and Europe.
Diurnal Variation
The fourth factor affecting atmospheric turbulence — after season, weather and jet steam — is the daily cycle of solar warming and evening cooling. Again, this cycle will depend somewhat on location and the effects of prevailing air currents, but the common pattern is:
• Turbulence declines in the late afternoon and reaches a minimum at the end of twilight; surface turbulence is suppressed as the Earth's surface cools and a temperature boundary can form that prevents mixing with higher altitude air.
• Turbulence can worsen in the late evening as local variations in surface temperature and topography produce surface air currents and increased mixing of warm and cool air; this can persist past midnight into the early morning.
• Turbulence declines to a daily minimum from late morning until just after sunrise.
The general guidance is that solar observations are best made in the mid morning and dark sky observations in the early evening or early morning, taking into account the specific exceptions produced by surface currents across the local geography.
In his 1824 description of the Great Dorpat Reflector, Joseph von Fraunhofer commented that "when observing with large telescopes, the largest obstacle is the imperfect air, and here mainly its apparent undulations." He noted that "the air, everywhere in space, is perfect in this sense only on a few nights per year." Similarly, Eugène Michel Antoniadi observed that good seeing is rare both from one night to the next and even within a single night. He commented that nights of good seeing were "about one in fifty" and remembered the specific date of a night in which excellent seeing lasted for an entire two hours! Diurnal variations make that kind of stability truly exceptional.
To combat instrument turbulence, telescopes are thermally stabilized or "cooled down" before use, telescope tubes are constructed to control or dissipate thermal instabilities, observers using Newtonian telescopes stand if possible down wind from the optical path, and observatory buildings are temperature controlled to minimize temperature differences with the exterior.
Modern, high end reflecting telescopes are often equipped with small fans to move air quickly across the surface of the mirror and through the telescope tube. Owners of truss or open tubes telescope use a household fan to achieve the same effect — another illustration that it is not moving air, but moving bodies of air of different temperatures, that produce optical turbulence.
The English astronomer William Herschel was perhaps the first to notice the significance of thermal turbulence in the instrument, commenting in 1782 on his observations of the double star 20 Draconis [H I 19], that "It is in vain to look for [the pair of stars] if every circumstance is not favorable. The observer as well as the instrument must have been long enough out in the open air to acquire the same temperature. In very cold weather, an hour at least will be required; but in a moderate temperature, half an hour will be sufficient."
Modern experience suggests that this advice is too optimistic. The graph (below) shows the measured temperature difference when the air temperature starts at 20°C; operating equilibrium is not reached until almost two hours later.
Sidgwick discussed tube currents in the instrument and across the objective (1971, pp.200-202), recommending the following steps to reduce instrument turbulence:
• Keep the instrument cool during the day
Nevertheless, general awareness of this problem is recent, and credit is often assigned to recent articles in Sky & Telescope magazine by Alan Alder and Bryan Greer (listed below under "Further Reading").
The recommendation made to me by an SCT manufacturer and retail sales representative was to tilt the OTA so that the corrector plate faced toward the ground, and remove the eyepiece adapter from the focuser to allow heat to escape out the visual back (diagram, left). But this merely traps the rising heat against the visual back, because the central baffle slide is firmly fixed to it.
The procedure I now use is to point the optical axis vertically: this causes the warm air within the OTA to rise to the transparent corrector plate where it quicky radiates away. Because the mirror is perfectly level, this also radiates heat equally from all sides, eliminating a "hot edge". The same procedure is very effective with a Newtonian or Cassegrain telescope.
The time required for cooldown depends on the mass of the mirror and the relative difference in its temperature relative to the surrounding air. Cooldown is also prolonged when temperatures continue to fall after sunset. In general refractors require less cooldown.
5. Apodizing or Aperture Reducing Masks
Because Fried's r0 specifies the boundary between diffraction limited and atmospherically limited optics, the observer can use aperture stops to evaluate the turbulence angular size when seeing is relatively poor.
An aperture stop is easily made by cutting a circular disk, equal in diameter to the interior width of the telescope opening, from stiff cardboard or plastic. In this disk is cut a circular opening of much smaller diameter; in reflectors, the openings must be cut to one side so that they are clear of the secondary mirror support. These masks allow the observer to assess seeing by "filtering out" increasingly smaller spatial frequencies.
A 10" telescope can for example be stopped down successively to 4", 2" and 1". As this is done, the star image will transform from bright, steady image troubled by speckles to a faint but relatively consistent diffraction image that appears to wander rapidly around a central location. The point where this transition occurs provides an approximate value of Fried's r0.
Astronomers learn early that different visual tasks are best performed at different seeing levels. The common strategies when task shifting under poor seeing conditions is to change to a smaller aperture, shorter focal length, faster f/ratio instrument and/or a longer focal length eyepiece and/or shorter exposure or lucky imaging astrophotography.
Using a lower power eyepiece — "no higher magnification than the seeing permits" — is common advice to beginning astronomers who only have one telescope to work with. This reduces the visual scale of the image, pushing the fuzzy and wobbly effects of poor seeing below the threshhold resolution of the eye: made small enough, all the effects of poor seeing disappear.
Using a smaller aperture corresponds to the reduced value of Fried's r0 under worse seeing: this eliminates most of the effect of low frequency turbulence, including instrument turbulence, on the star diffraction image. The worst effects of poor seeing never make it into the image.
Using a shorter focal length telescope reduces the magnification of the objective, therefore the magnification delivered by the same eyepiece. Alternately, using a smaller ƒ ratio (e.g., ƒ/4 instead of ƒ/8) for the same aperture reduces magnification and increases the light delivered to the image area.
For recreational visual astronomers, astronomical binoculars (aperture at least 60mm, field of view around 5°) are an important accessory. They provide many pleasurable views than most telescopes — more vivid star colors, clearer images of the Milky Way and extended nebulae and star clusters. They invite the pleasure of simply wandering the night sky looking for interesting and novel features.
Visual astronomers working with telescopes will shift from detail work such as lunar drawings, planetary observations or binary star measurements to star clusters and deep sky objects that have little fine structure and appear best in very low power eyepieces.
Astrophotographers will use lens/corrector systems that deliver a shorter effective focal length or shorter f ratio, increasing the angular scale of images delivered to the CCD and shortening the exposure time necessary to capture images. At the extreme, the astronomer may prefer to switch from astrophotography to visual astronomy, and use the time to evaluate targets for astrophotography when conditions improve.
The tactic of last resort is simply to wait for conditions to improve or to search out areas of best seeing in the sky. This requires general familiarity with weather patterns and the diurnal cycles in weather conditions, and experience with the local weather at the observing site.
As Antoniadi knew, seeing does not remain constant across the same night. The evening might be excellent for observing planetary detail at 8pm, deep sky objects at 10pm, and double stars at midnight. Awareness of these changes in turn often requires the observer to shift observing tasks, equipment, eyepieces, even area of sky studied as the night progresses.
In his "Atmosphere, Telescope and Observer" (1897), A.E. Douglass offered some useful insight:
Local and geographic turbulence, and sometimes tropopause turbulence, is rarely homogenous across the entire sky. There will almost always be areas of better or worse seeing. These areas are often associated with prevailing winds or the location of significant geographic features. Under conditions of worsening seeing, it is always helpful to assess the four quadrants of the sky to see whether seeing remains good in a specific direction.
Antoniadi described the typical experience as "more or less boiling"; detailed images were "preceded by a period of slight rippling of the disk, very detrimental to the detection of fine detail. Then the undulations would cease suddenly, when the perfectly calm image of Mars revealed a host of bewildering details." Percival Lowell called those moments "revelation peeps".
Further Reading
"Dewing-Up and Tube Currents". Section 11 of Amateur Astronomer's Handbook (3rd ed.) by J.B. Sidgwick. (New York NY: Dover Publications, 1971/1980).
"Understanding Thermal Behavior in Newtonian Reflectors" by Bryan Greer. Sky & Telescope (September 2000, pp.125-132).
Telescope Optics Topics - Bryan Greer's page of videos showing the optical effects of surface convection from telescope mirrors.
"Improving the Thermal Properties of Newtonian Reflectors — Part 1" by Bryan Greer. Sky & Telescope (May 2004, pp.125-132).
"Improving the Thermal Properties of Newtonian Reflectors — Part 2" by Bryan Greer. Sky & Telescope (June 2004, pp.132-132).
"Thermal Management in Newtonian Reflectors" by Alan Adler. Sky & Telescope (January 2002, pp.132-136).
"Beating the Seeing" by Alan MacRobert. Sky & Telescope (April 1995, pp.40-43).
"Long Term Periodic Behavior in the Subarcsecond Seeing at Mount Wilson Observatory" by Scott Teare & Laird Thompson. Publications of the Astronomical Society of the Pacific (January 2002, pp.125-127).
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Cooldown is the standard procedure for minimizing the heat that has accumulated in the instrument, especially the objective, during the day.