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 0.865 (mu)m wavelength region. Its specifications are as follows: at wavebands centered at 0.470, 0.660, and 0.865 (mu)m, the band-averaged retardance should be 90 deg +- 10 deg for all fields and retardance should change less than 0.1 deg for a 1 deg 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 plate was then measured and used to determine a retardance target for the second plate. The retardance spectrum of the combined first and second plates was then used to specify a retardance target for the third plate. The retardance spectrum of the three plates in combination was then used to determine when the final thickness of the third plate was reached.
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