Making Fuzzy Clear: toward sub-millimagnitude precision at the Thacher Observatory.

Attaining sustained sub-millimagnitude photometric precision on small, altitude-azimuth (Alt-Az) telescopes presents a variety of challenges. The changing parallactic angle of sources results in a changing orientation of the telescope optics with respect to the camera. This, in turn, can result in a flat field that is a function of camera orientation due to, for example, vignetting. To address this issue we have furthered our work employing both a near and far field flat to calibrate non-uniform illumination of the CCD, and we present on-sky demonstrations of the measured improvement in photometric accuracy and precision. In addition, we have added the option of using two different diffusers to the optical path of our system which allows us to spread the light from targets over 1-2 orders of magnitude more pixels than a focused image thereby reducing the Poisson noise accordingly. We demonstrate the performance of the diffusers with observations of bright stars with transiting exoplanets.