Seeing is turbulence in the air between you and the star
Seeing is the blurring and dancing of a star's image caused by turbulence in Earth's atmosphere. Air at different temperatures bends light by slightly different amounts. As pockets of warm and cool air drift across the telescope, they bend the light one way and then another, many times a second.
To the naked eye the result is twinkling. In a telescope at high power a star becomes a shivering blob, and the edge of the Moon or Jupiter ripples. Turbulence comes from three layers:
- High altitude: the jet stream, 30,000 to 40,000 ft up, brings the worst seeing when it passes overhead. Forecasts show where it is.
- Near the ground: heat rising from pavement, roofs, decks and nearby houses after a warm day. Observing over grass or water helps; looking over a rooftop does not.
- Inside the telescope: a warm mirror or lens heats the air in the tube and makes its own turbulence, called tube currents. A large telescope needs 30 minutes to two hours outside to cool; Schmidt-Cassegrains and Maksutovs, with closed tubes, are slowest.
Seeing, not aperture, limits detail beyond about 5 inches on most nights
A telescope's theoretical resolving power is about 116 divided by its aperture in millimeters, in arcseconds (the Dawes limit). Turbulence sets its own limit, measured as the width of a star's blurred image. Whichever is larger wins. Numbers below are the finest detail each telescope shows, in arcseconds; "(air)" marks where the air, not the telescope, is the limit.
| Telescope | Dawes limit | Excellent (0.5) | Good (1) | Average (2) | Poor (3) |
|---|---|---|---|---|---|
| 80 mm refractor (80 mm) | 1.45 | 1.4 | 1.4 | 2.0 (air) | 3.0 (air) |
| 4-inch (102 mm) | 1.14 | 1.1 | 1.1 | 2.0 (air) | 3.0 (air) |
| 6-inch (150 mm) | 0.77 | 0.8 | 1.0 (air) | 2.0 (air) | 3.0 (air) |
| 8-inch (203 mm) | 0.57 | 0.6 | 1.0 (air) | 2.0 (air) | 3.0 (air) |
| 10-inch (254 mm) | 0.46 | 0.5 (air) | 1.0 (air) | 2.0 (air) | 3.0 (air) |
| 12-inch (305 mm) | 0.38 | 0.5 (air) | 1.0 (air) | 2.0 (air) | 3.0 (air) |
| 16-inch (406 mm) | 0.29 | 0.5 (air) | 1.0 (air) | 2.0 (air) | 3.0 (air) |
On an average night of 2-arcsecond seeing, every telescope from an 80 mm refractor up shows about the same detail. A big telescope still gathers more light, so the planets look brighter and show more color, and on the rare excellent night it shows far more.
Astronomers describe turbulence with the Fried parameter, r0: the width of a column of air that stays steady enough to act as one piece. A telescope smaller than r0 sees a sharp image that wanders; one larger than r0 sees a blur. Image blur is about 0.98 times the wavelength divided by r0:
| r0 (green light) | Star blur (arcsec) | What it is like |
|---|---|---|
| 3 cm | 3.7 | Poor: stars boil, planets show little |
| 5 cm | 2.2 | Below average |
| 10 cm | 1.1 | Good: most fine planetary detail shows |
| 15 cm | 0.7 | Very good |
| 20 cm | 0.6 | Excellent: a few nights a year at most sites |
The Pickering scale rates seeing from 1 to 10 by what a star looks like
William Pickering devised his scale at Harvard in the early 1900s using a small refractor. Defocus nothing; just look at a bright star at high power and compare:
| Pickering | The star at high power | Good for |
|---|---|---|
| 1 to 2 | A boiling blob about twice the size of the outer diffraction ring; no rings visible | Wide-field and deep-sky observing only |
| 3 to 4 | A blob with occasional glimpses of rings; the disk rarely seen | The Moon at low power, bright planets in outline |
| 5 | The central disk shows; rings broken and moving | Jupiter's main belts, Saturn's rings |
| 6 to 7 | Disk sharp most of the time, inner ring complete | Fine planetary detail, close double stars |
| 8 to 9 | Disk and rings steady, with brief motion | The best planetary views; splitting pairs at the Dawes limit |
| 10 | A perfect, motionless diffraction pattern | Rare anywhere |
Transparency is how clear the air is, and it decides how faint you can see
Transparency is how much light the air lets through. Water vapor, haze, dust and wildfire smoke scatter and absorb starlight, dimming faint objects and brightening the sky background. Transparency matters most for galaxies, nebulae and the Milky Way, whose light is faint and spread out. It matters little for the planets, which are bright.
You can judge it with the naked eye. Find the faintest star you can see near the zenith (the point overhead) and compare with a chart: that is your limiting magnitude, which combines transparency and light pollution. A clear-blue daytime sky to the horizon usually means a transparent night; a milky-white sky means haze.
Steady hazy nights suit the planets; crisp windy nights suit galaxies
Seeing and transparency often work against each other. After a cold front passes, the air is dry and very clear, but fast winds aloft make it turbulent. A warm, still, slightly hazy night under high pressure can bring the steadiest air of the season.
| The night | Seeing | Transparency | Observe |
|---|---|---|---|
| Just after a cold front: crisp, windy, deep-blue sky | Poor | Excellent | Galaxies, nebulae, the Milky Way |
| Still, humid, slightly hazy, stars barely twinkling | Good to excellent | Poor to fair | Jupiter, Saturn, Mars, the Moon, double stars |
| Calm and dry under high pressure | Good | Good | Everything |
| Wildfire smoke or dust aloft | Often good | Poor | The Moon and planets |
A quick test: if the bright stars near the horizon are twinkling hard, seeing is poor. If stars overhead shine with a steady light, try high power on a planet.
Astronomy forecasts predict seeing and transparency hour by hour
General weather apps forecast cloud but not seeing or transparency. Several free services built for astronomers do:
- Clear Outside (from First Light Optics, UK): hourly cloud at three heights, transparency and moon phase, as a website and app.
- Astrospheric (Canada): seeing and transparency maps from the Canadian Meteorological Centre models, best for North America.
- Meteoblue astronomy seeing (Switzerland): seeing in arcseconds, jet stream speed and bad layers in the atmosphere.
- Xasteria: an iPhone app combining several forecast models.
Use them to pick the target for the night: planets when the seeing forecast is good, galaxies when transparency is.