Calibration of UV and FUV Transmission of the EUVE Lexan/Boron Filters

The Extreme Ultraviolet Explorer (EUVE) has completed an all-sky survey for the wavelength region 70 to 760 Angstroms. The shortest wavelength bandpass of this photometric survey is determined by a Lexan/boron filter (Vallerga et al., SPIE Proceedings, 1742, 392; Vedder et al., SPIE Proceedings, 1742, 486). The polycarbonate Lexan filter is known to have an appreciable transmission in a range of wavelengths near 1600 Angstroms and to open up completely longward of 2300 Angstroms. A consequence of a high UV throughput is the detection of bright UV sources such as early O and B stars. To optimize the Lexan bandpass, boron was added to attenuate this UV throughput, as well as the very strong geocoronal Lyman alpha line at 1216 Angstroms. The optimization criteria established was to add enough boron such that B stars dimmer than the fifth magnitude would not be detected by the all-sky survey through the UV leak. This would leave the EUVE Lexan catalog with a finite number (<100) of false EUV detections, easily removed by identifying the sources as early stars. In orbit, it was discovered that the Lexan/boron filters have FUV transmission down to the Lyman edge (912 Angstroms) reaching the detectors, in spatially resolved locations, due to a few pinholes ( ~ 10 microns) in the filter. We present an in-orbit calibration of the filter leak, based on detections of O and B stars, as a function of spectral type and luminosity class, confirming the filters have the expected UV throughput. The FUV response due to pinholes is also calibrated with observations of these stars. We find, as expected, earlier-type stars with a large flux near the Lyman edge preferentially detected. The goal of these efforts is to remove UV contribution from early-type stars from the EUV response. This is particularly important for the case of binary systems where one component is an early star, Algol being a particular system of study. This work has been supported by NASA contract NAS5-29298.