How does seeing affect our astrophotos? What effect does the blurring effect of Earth's atmosphere have on our deep-sky, planetary and lunar images?
If we lived on a planet without an atmosphere, we would certainly be able to capture clearer images of the night sky. Starlight would appear as sharp as our astrophotography instruments are able to deliver.
But since Earth does have an atmosphere, starlight wobbles as it passes through fast-moving regions of differing density and temperature – an effect known as ‘seeing’.
Over the course of a long exposure, the wobbles add up.
Atmospheric seeing is quantitatively measured by a value known as ‘full width at half maximum’ (FWHM).
This value describes the highest resolution that you can expect to obtain on a particular night, if you take into consideration the interference introduced by atmospheric turbulence.
The best observatories in the world can experience seeing with a FWHM value of 0.4 arcseconds.
However, for the rest of us this figure is considerably larger and typically in the range of 1.5–4 arcseconds.
High-resolution imaging, whether it’s of the Moon, planets or deep-sky objects, depends on good seeing if you want to capture the sharpest detail.
Lunar and planetary imagers use high frame-rate techniques to reduce seeing effects.
In the case of deep-sky astrophotography, where long exposures are the norm, each element of detail will effectively be blurred by poor seeing.
Seeing only really becomes a critically important issue if your goal is high-resolution deep-sky imaging – in other words, if you’re taking close-up shots of distant deep-sky objects.
For the most part, if you’re taking wide-field images of large deep-sky objects, the effects of poor seeing aren’t all that noticeable.
