Air Mass And Wavelength Dependence

In a previous paragraph we saw that there is a dependence of air mass with the altitude (zenith angle) is by the equation 1.2. So as the air mass changes so seeing will do. From theoretical consideration one can derive that the seeing $S$ is proportional to the 0.6th power of the airmass $a$ :

$$S=S_0\cdot a^{0.6}$$ (3.18)

Where $S_0$ is the value of seeing at zenith. Equation 3.1 is used to correct DIMM data from different altitudes and is essential so the data to be self-consistent.

The seeing also varies with the wavelength according to the fowllowing equation :

$$S=S_0\cdot \lambda^{-0.2}$$ (3.19)

Where $\lambda$ is the wavelength. Seeing is defined as the image size (FWHM) in arcsec on a long exposure limited by the atmosphere (ie. telescope without optical aberrations nor dome/mirror seeing) as observed at zenith and at wavelength 0.5 micron. So when DIMM data are collected a correction of these two effects must be done. An extapolation to the zenith and to $0.5$ micron wavelenqth.