Seeing and FWHM
Seeing is a measure of air turbulence and the resulting image blur due to swirling of air layers and the reason for the wobbling of planets resp. the twinkling/flickering of stars. Here in the video at the capturing of the moon and a star to see:
The swirls seen in the video are caused both in the atmosphere and in the nearer surroundings of the telescope, e.g. by houses, or dew caps that are set too warm. The worse the seeing is, the more the stars "wobble" over a pixel area and form more or less large star slices.
William Henry Pickering (1858 - 1938) has divided the seeing quality in a Pickering scale named after him. On this page (http://www.amateurastronomie.com/anfang/seeing/index.htm) the 10 levels (1 - very poor to 10 - perfect) were shown.
The quality of the seeing also depends on the wavelength of the light and is defined by the FWHM value and given in arcseconds so that the seeing can also be included mathematically. FWHM means Full Width at Half Maximum and describes the half width of the brightness distribution of the diffraction disk of a star.
 | At https://www.meteoblue.com/de/wetter/outdoorsports/seeing/ an hourly exact seeing for the own location can be found
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The larger the FWHM-value is, the more "smeared" the stars are imaged. Normally in Germany there is a FWHM-value of 2-4 arc seconds [''], if all influencing factors are included. This means that in the best case a star can have an angular extent of 2" on the chip. This is important in determining which telescope-camera-system should be chosen. The system must be set up in a way that this 2" of arc slice can be distributed to the pixels so that neither over- nor undersampling occurs. For this, the image scale of the telescope and the Nyquist-Shannon-Sampling-Theorem are used.
The seeing-values are defined at Wikipedia https://de.wikipedia.org/wiki/Seeing so that 1" represents good seeing, 2" is in the middle range and 5" and more is poor seeing. Under very good conditions, the value can also drop below 1". From this the following ranges can be defined:
| Seeing-definition | Value range FWHM |
| very good | 0,5“ – 1“ |
| good | 1“ – 2“ |
| OK | 2“ – 4“ |
| poor | 4“ – 5“ |
| very poor | 5“ – 6“ |
The seeing values are also strongly dependent on the height of the object above the horizon. The closer an object is to the horizon, the longer the path of light through the atmosphere and the greater the effect of air turbulence on the image.
The diagram below shows the relationship between air mass (a measure of the path length of light through the atmosphere) and the angle above the horizon. (The formula for the calculation can be found at https://en.wikipedia.org/wiki/Air_mass_(astronomy).)
From an air mass of 1.5 (approx. 42° above the horizon) the seeing has an increasing effect on the image quality due to the longer distance through the atmosphere.