Figure 1: Example of seeing for each category
Amateur, experienced or professional astronomers evaluate the seeing with a scale of 1 to 10. Using their telescope, they measure or estimate the diameter of stars, which varies between 5 and 8 arc seconds under poor scintillation conditions and between 0.2 and 0.5 arc seconds under excellent conditions. They can also use a qualitative method to measure seeing. To do this, astronomers aim at a star near the zenith of magnitude 2 to 3 and then visualize the diffraction pattern of the star. Finally, they estimate the seeing using a scale ranging from 1 to 5”.
| Categories | Seeing in arc-second | Description |
|---|---|---|
| I | > 4" | Boiling of the image tending towards the planetary aspect |
| II | ~ 2.0-4.0." | Significant central disc eddies; evanescent or absent rings |
| III | ~ 1.0-2.0" | Deformations of the central disc; interrupted rings |
| IV | ~ 0.4-0.9" | Slight undulations running through the diffraction rings |
| V | < 0.4" | Perfect and motionless image |
Figure 1: Example of seeing for each category
It is important to note that scintillation sensitivity is greater for a large-aperture telescope. For example, if the owner of a 15 cm telescope estimates a scintillation of 4/5, the scintillation will be 3/5 for a 25-35 cm aperture telescope. It is therefore imperative to take this limit into account. The seeing forecast is calibrated thanks to a study made from telescopes of 28 and 35 cm diameter, representative diameters of modern instruments of amateur astronomers. Amateur astronomers with smaller diameter telescopes may find these predictions slightly pessimistic, but the color index can be adjusted with their own observations. Amateur astronomers who own a telescope of 20-50 cm in diameter should find these predictions useful.
The presence of cloud, weighted wind shear, momentum flux in the boundary layer and surface temperature trend are the elements used to produce the seeing forecasts. These forecasts may not be fully developed in mountains, although topography is an integral part of the calculation elements. The scintillation index is represented by 5 levels or shades of blue. Dark blue indicates the best scintillation conditions and grey indicates the worst conditions. White areas are where the weather model predicts clouds.
| Categories I to IV aka 1/5 to 5/5 | Color | Seeing conditions | ||
|---|---|---|---|---|
| 0/5 | White | Zero seeing (cloudy sky) | ||
| I | 1/5 | Grey | Very bad seeing | |
| II | 2/5 | Light blue | Poor seeing | |
| III | 3/5 | Medium blue | Average seeing | |
| IV | 4/5 | Bright blue | Good seeing | |
| V | 5/5 | Dark blue | Excellent seeing | |
The latest seeing forecasts are summarized on each clear sky chart page.