Focusing Info Window
There are two calculators provided with your choice of English or Metric units. The Critical Focus Zone calculator gives the depth of the CFZ as a function of the wavelength of light and the Focal Ratio (F/#) of your system. This calculator can be useful to determine how often you need to refocus with temperature changes or even how repeatable your focusing is. It now uses the more aggressive calculation based on the Goldman/Megdal article, In Perfect Focus, in Sky & Telescope, August, 2010.
The other calculator is useful for determining where a dust particle is by measuring its resultant dust donut diameter in pixels. Simply measure the diameter of the dust donut in pixels and define the binning used. Based on the unbinned plate scale entered on the Settings page, the distance from the imaging plane to the dust particle is calculated.