The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.
Related terms
- Transparency
How clear the air is: free of haze, moisture, dust and thin cloud. Good transparency shows faint stars and contrasty nebulae. It does not always go with good seeing: hazy, still nights are often excellent for the planets.
- Maximum useful magnification
The magnification beyond which nothing new is revealed: about twice the aperture in millimetres, so 400× for a 200 mm telescope. That limit assumes perfect optics: with real-world mirrors and lenses and imperfect collimation, the image often softens from about 1× per millimetre (200× for a 200 mm). And the steadiness of the air usually caps it at 150× to 250×, whatever the instrument. The “600×” printed on boxes of small telescopes is therefore misleading.
- Cooldown
The time a telescope brought out of a warm house needs to reach the air temperature. While the mirror is warmer, currents of warm air rise in the tube and blur the image. Allow 30 minutes to an hour for a 200 mm mirror, more for a Maksutov; a fan speeds things up.
- Lucky imaging
The technique behind planetary and lunar photos: record thousands of very short frames, then software keeps only the sharpest, taken in the moments when the air is calm. Often 5 to 20% of the frames are stacked. It goes well beyond what the seeing lets you see at the eyepiece.
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