We will not be adjusting the focus of the camera during normal KELT-S operations.  Instead, we fix the lens in the position of optimal focus. 

In order to determine optimal focus, we took eight images at different focus settings.  Through rough tests we previously found that tightest focus is around the 1-meter setting on the lens.  (Since we use our own custom adapter to attach the lens to the camera, we select a non-standard flange focal distance, so that the distance settings on the lens do not correspond to true focus settings for our telescope.)  We used the width of the 1-meter tic on the lens as a position measure, and took 5 full-size images of a random area of sky, with 10s exposures.  The first image was right on the 1-m tic, and each subsequent image was one additional tic-width from the 1-m tic, in the direction of the 0.9m tic.  We then returned to the 1m tic and took three more images, in the same distribution, but in the direction of the 1.2m tic.  In this manner we sample focus parameter space as well as possible across perfect focus.

We analyzed the resulting images using the program fwhm.c