... Reference Data for Calibration and Tools . ... There are two independent directories which contain the spectra of ultraviolet and optical standard stars. The CALSPEC directory contains the composite ultraviolet and optical absolute calibrated reference spectra of the HST standards. ... Details of the CALSPEC data, along with another supplemental CALSPEC directory, are contained in the above web page. ... Another directory of supplemental calspec data is available and detailed on the calspec page. ...