Focusing Aid allows achieving perfect focus using the HFD (Half Flux Diameter) and/or FWHM (Full Width Half Maximum) metrics to measure the focus quality. The lowest value is the best one. It is advisable to relay on HFD instead of FWHM, because the first is more stable against the atmosphere turbulence. HFD works with the whole star area while FWHM works only with the line of the star profile.
Note: In CCD Mode you have to switch to 1:1 Preview Mode in order to use the raw FITS data for calculation of HFD/FWHM. Using the raw data gives much more accurate focus quality measurements that the values calculated on screen data. In DSLR mode also switch to 1:1 Preview Mode.
The Focusing Aid use the common aid behavior in APT- dragging the window and placing the Source frame over the image part that you want to use.
Source
It is a "transparent" window that shows the image under the Focusing Aid and defines what is the source to be used in the Aid. It automatically detects the brightest star in the frame and analyze it.
In left are the FWHM and HFD values for the current image. In right are the best measured values since last reset. The best values are reset if Reset Best button is used.
Used in EOS mode when LiveView is activated. Defines how many images from the LiveView stream to stack before the star quality analysis. Bigger value reduces the influence of the atmosphere turbulence and the image noise, but requires giving time after focus change before you can see the actual effect - stacking leads to less "live" image.
Shows graph of the FWHM or HFD value changes. It is valuable to see if you are tuning the focus in the right direction or if the focus point is passed. Lowering graph means that you are moving in the right direction.
Double click on the graph to switch between FWHM or HFD metrics.
The Focusing Aid is accessible from 
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