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A method for fabricating an achromatic, athermalized quarter-wave retarder is presented that involves monitoring retardance during polishing. A design specified by thicknesses alone is unlikely to meet specification due to uncertainties in birefringence. This method facilitates successful fabrication to a retardance specification despite these uncertainties. A retarder made from sapphire, MgF_2, and quartz was designed, fabricated, and its performance validated for the 0.470 to 0.865μm wavelength region. Its specifications are as follows: at wavebands centered at 0.470, 0.660, and 0.865μm, the band-averaged retardance should be 90°±10° for all fields and retardance should change less than 0.1° for a 1° change in temperature. Retarder fabrication accommodated birefringence and thickness uncertainties via the following steps. The first plate was polished to a target thickness. The retardance spectrum of the first …