Methods to Calculate Astronomical Seeing - The DIMM

``DifferentialImagingMakesMagic ...''

Probably the most common way of describing the seeing is by specifying the full width half maximum (FWHM) of the image, which may be estimated either by direct inspection or by fitting a function (usually a Gaussian), note the correspondence of FWHM to $\sigma$ of a gaussian: $FWHM = 2.355\sigma$. This is fact is shown in figure 3.1

Figure 3.1: The Full Width Half Maximum (FWHM)

We can measure FWHM in a astronomical image very easy with many programs that have that feature. The problem is that such a measurment will not be accurate for various reasons :

• Guiding problems
• Focus problems

All these reasons cause the FWHM to be overestimated. Thus we need a method to overcome all these problems. Such a method is the DIMM method that will be analyzed in the next paragraph.