Abstract: A modular optical system for a Fourier transform infrared spectrometer which has a baseplate assembly with a baseplate having a top surface and two mirrors mounted to focus at a point above the baseplate. At least two pins extend upwardly from the baseplate top surface and are fixed with respect to the foci of the mirrors. An accessory module has a flat bottomed positioning plate with portions defining pin holes to coincide with the pins of the face plate. A sample holding accessory platform extends from the positioning plate and is fixed with respect to the pin holes so that when the pin holes of the positioning plate are positioned over the pins of the baseplate and the positioning plate bottom surface is engaged with the locator pads on the baseplate, the sample is located at the foci of the mirrors.