This document summarizes the design of a variable aperture slit (VAS) system for an optical detector. The VAS system uses a semi-hexagonal holder that can rotate to select between different slit sizes, including 50μm, 100μm, and 200μm slits. Testing showed the system can achieve repeatability of ±25μm in the sensitive direction and ±150μm in the less sensitive direction. An Arduino controller and MATLAB GUI were developed to automate rotation between slits. The total estimated cost is $1,342.33 for the VAS system and controller.