Seeing vs. Transparency: Why a Clear Sky Can Still Give Soft Planetary Images

Seeing is how steady the air is. Transparency is how much light gets through it. A clear-sky forecast promises neither, which is why Jupiter can look soft and restless on a night without a single cloud. This Mac Observatory guide explains the difference between seeing and transparency, why humidity isn't a seeing forecast, and how to check the three things that decide a planetary session: steady air, clear air and dry optics.

You've probably had the night. The forecast says clear, Jupiter is bright, and the telescope is ready. Then the planet on the screen softens, stretches, snaps into focus for an instant and smears again. It's tempting to blame the camera, the focuser or the humidity. Before you change any equipment, ask three separate questions: how stable is the air, how much light is reaching you, and are the optics staying dry?

Dew is the odd one out, because it happens at the telescope rather than in the sky, and it gets its own section below. Seeing and transparency are the pair people mix up, mostly because a forecast shows them side by side.

What is seeing in astronomy?

Astronomical seeing is the blurring and wobbling of a telescope image caused by turbulent air along the line of sight. The Clear Sky Chart forecast key traces it to turbulence combined with temperature differences in the atmosphere. In bad seeing, it says, planets ripple as though you were looking at them through moving water.

Seeing belongs to the air, and ESO draws that line carefully. It treats seeing as a property of the turbulence itself, quoted at the zenith, and image quality as the star size an instrument actually records, which also depends on wavelength and on how much air you're looking through. Your telescope can't improve the seeing. It can add problems of its own, which is why the clues below deserve a second look.

At the telescope, look for detail that changes quickly: a crater edge wavering, Jupiter's belts dissolving and re-forming, a star that won't settle into a point. Those are good clues, but focus, vibration and a telescope that's still cooling down can spoil an image in similar ways.

If you don't have a forecast handy, the Astronomical League seeing scale needs nothing but your eyes. It grades the night by twinkling: excellent when even bright stars hold still, very good when stars twinkle slightly but bright planets don't, and poor when everything twinkles too much to observe.

How do you read a seeing forecast?

Seeing forecasts come in two common forms, and they run in opposite directions. An arcsecond figure describes the size of the blur, so smaller numbers are steadier. A rating from 1/5 to 5/5 describes categories, so bigger numbers are steadier. Environment and Climate Change Canada, whose Canadian Meteorological Centre forecasts feed the Clear Sky Chart, publishes the key that connects the two.

Two details on that page matter as much as the table. First, the forecast is built from cloud, wind shear, mixing near the ground and the trend in surface temperature, and it describes seeing at the zenith. Second, it's calibrated against 28 and 35 cm telescopes, the size of an 11-inch or 14-inch SCT, and bigger apertures feel the seeing more. ECCC's own example is a night a 15 cm owner rates 4/5, which looks more like 3/5 through a 25 to 35 cm scope. If you image with a small refractor, these forecasts can read a little pessimistic.

What is transparency in astronomy?

Transparency is how much starlight survives the trip through the atmosphere. Haze, smoke, dust and thin cloud all dim stars, even on a night when plenty of them are still visible. The Clear Sky Chart key's own example of poor transparency is thick haze in a cloudless sky.

The Astronomical League treats transparency as a separate observation from seeing, and its check is just as simple. In the northern hemisphere, count how many of the Little Dipper's seven stars you can see. Polaris alone means roughly magnitude 2 skies. All seven means about magnitude 6. Make the check from the same spot with similar dark adaptation each time, and note the Moon and any local lighting beside the result, because both change the count.

The two conditions don't move together. The Clear Sky Chart key warns that bad seeing can arrive in perfectly clear weather and that good seeing often comes with poor transparency. The difference shows up in a star's image.

That's why the two matter differently depending on the target. A bright planet has light to spare, so steady air usually decides the night. For a faint galaxy, lost light and a brighter background do more damage, and the Clear Sky Chart key notes that bad seeing probably leaves a visual observer's view of galaxies undiminished.

Does humidity affect seeing?

Not directly, and this is where most of the confusion lives. Relative humidity depends on temperature, so it climbs as the night cools even when the amount of water in the air hasn't changed. The National Weather Service recommends dew point as the better measure: it's the temperature the air has to cool to before relative humidity reaches 100%.

Neither number describes turbulence. Environment Canada doesn't list humidity among the inputs to its seeing forecast, and the Clear Sky Chart says its seeing has little to do with the air's water vapor. Its transparency forecast, on the other hand, is calculated from it. Humidity is a transparency and dew question first.

Observers' experience is all over the map, which is what you'd expect from two things that only sometimes travel together. In a long Cloudy Nights thread on humidity and seeing, observers in southwest Florida and the Midwest said their humid summers brought their worst seeing. Michael Rapp of Dickinson, Texas, pointed out that two leading planetary imagers of the day, Ed Grafton in Houston and Don Parker in southeast Florida, both lived where humidity runs very high. Tom Polakis countered that humidity isn't required for good seeing at all, since the best sites in northern Chile pair single-digit humidity with sub-arcsecond seeing.

Tony Flanders added a useful correction in that same thread: water vapor itself is transparent to visible light, and the haze comes from droplets and other aerosols that form more readily in humid air. So a humid evening deserves attention to haze and to your optics, but it can't tell you how sharply Jupiter will resolve.

Is it dew or the sky?

Dew spread is the gap between air temperature and dew point. Air at 24°C (75°F) with a dew point of 23°C (73°F) has a 1°C spread, about 2°F. That's a thin margin, and your optics can be colder than the air. A telescope pointed at a clear sky radiates heat away and can cool below ambient, which Ronald Holzlöhner and colleagues describe as subcooling of telescope structures. At the Atacama observatory sites they studied, the effective temperature of the night sky overhead sat 20 to 50 kelvin below the air near the ground.

If the image loses contrast slowly over an hour while the stars still look steady, check the front of the telescope with a flashlight before you blame the sky. A dew shield and a heater strip solve that problem. It's a separate job from the seeing, and no seeing forecast will warn you about it.

Partner

Agena AstroProducts

Telescopes, cameras, mounts, and accessories from the brands that matter.

ZWO · Celestron · Sky-Watcher · Explore Scientific · William Optics

Shop Agena Astro

Mac Observatory earns a commission

How should you plan a planetary session around the conditions?

Transit, the Mac Observatory menu bar app I build for macOS 15 and later, covers the weather half of this. Its Weather score, on a 0 to 100 scale, combines cloud, humidity, dew spread and wind from Apple's WeatherKit, with the Moon shown on its own line instead of folded in. It also draws an hourly forecast for the night with a best window, and for each planet it lists when it rises, when it clears an altitude floor you choose, how high it climbs and when, and when it sets.

What Transit doesn't do is forecast seeing or transparency, and that's deliberate. I removed both in version 1.5, because I couldn't license a seeing forecast for a paid app on workable terms, and surface weather data can't honestly support one. A 0 to 100 Weather score and a 1/5 to 5/5 seeing rating measure different things, so check what any app's number means before you compare it with another.

So use Transit to find a usable weather window, then check a dedicated astronomy forecast for seeing and transparency. In North America, the Clear Sky Chart, created by the late Attilla Danko from forecasts Allan Rahill developed at the Canadian Meteorological Centre, shows cloud, transparency and seeing by the hour for thousands of sites. Check the forecast time and location, then let the telescope have the last word.

Favor a higher target when your schedule allows, because light from high in the sky crosses less air. Astronomer Andrew T. Young's airmass page, hosted by San Diego State University, defines relative airmass as the amount of air along your line of sight compared with straight up. In a flat-atmosphere approximation it's the secant of the zenith distance, so a planet 30° above the horizon looks through twice as much air as one overhead, and at 60° the factor is only about 1.15.

That's path length rather than a sharpness forecast. More air gives turbulence and haze more room to work, but the factors don't convert into seeing, and the simple formula breaks down near the horizon, where Young puts the real airmass at about 38 at sea level. In Transit, setting the altitude floor to the lowest height you're willing to image at turns this into a time on each planet's row.

How can you tell which problem you have at the telescope?

Before a long capture, take a short test and write down the target's altitude, any cloud or haze, the dew spread, your focus position, and whether the image is steady or wavering. If conditions change, test again before you change several settings at once. Then read the symptom.

If you capture with Laminar, Mac Observatory's native planetary capture app for Apple Silicon Macs, its live frame-quality readout and focus tracking make that test easier to read. Leave the focuser alone and watch how much the quality reading moves. A number that swings while nothing at the telescope changes is the air. Bad seeing isn't always a lost night, either, because sharp moments still arrive, and frame selection is how you keep them.

These are the questions that come up most often when a clear night disappoints.

What should you do on your next clear night?

A clear forecast earns the trip outside. The seeing decides what you do once you're there. Check the weather window, check the seeing and transparency rows, pick the highest target that suits the night, and give the telescope time to cool. Then let a two-minute test decide whether it's a night for fine planetary detail, an easier target, or an early bedtime.