Seeing vs. Transparency: Two Different Kinds of "Good Sky"

Forecasts for astronomers usually report two separate numbers that both get casually called "sky quality," but they measure different things. A night can be great on one and rough on the other.

Seeing: how steady the air is

Seeing describes atmospheric turbulence — pockets of air at slightly different temperatures bending starlight as it passes through, blurring fine detail the same way heat shimmer blurs a distant view on a hot road. Bad seeing doesn't dim anything; it just smears sharp detail into a kind of soft blob. It matters most for planetary and short exposure imaging, where the whole point is resolving fine detail in a small, bright target. A night of poor seeing can cap your resolution no matter how good your optics are. Deep-sky imaging, which stacks many longer exposures of faint, extended targets, is more forgiving of soft seeing.

StarYard scores seeing on the forecast's 1–8 turbulence index and flags it as low, moderate, or high risk. Low would be steady air worth pointing a planetary setup at while high is better spent on wide-field deep-sky targets instead.

Transparency: how clear the air is

Transparency describes haze, thin cloud, humidity, and general atmospheric clarity. Basically, how much of a target's light actually reaches your sensor rather than getting scattered or absorbed on the way down. Poor transparency dims everything and washes out contrast, which matters most for faint, extended deep-sky targets like nebulae and galaxies that are already don't have contrast to spare. A bright planet or double star, by contrast, barely notices moderate haze.

StarYard scores transparency the same way, low/moderate/high, from the forecast's own 1–8 haze index.

Putting the two together

A night can be steady but hazy (fine for planetary work, rough for faint nebulae), or crystal clear but turbulent (fine for deep-sky, disappointing for planetary detail). On the Results page, both factors are shown alongside each object's recommendation. Read them together with the target's type, not as a single combined "sky quality" score. Also worth checking: your location's Bortle class, which is a separate, mostly-fixed factor (light pollution) rather than a nightly weather condition.