Seeing calculator

FWHM (arcseconds) = FWHM (pixels) × (Pixel Size in microns / Focal Length in mm) × 206.265 

0.5-1.0″ = Exceptional Seeing
1.0-2.0″= Good Seeing
2.0-4.0″ = OK Seeing
4.0-5,0″ = Poor Seeing
5.0-6.0″= Very Poor Seeing