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Kitt Peak Vacuum Telescope Magnetographs (1Sep92) (from NSO, NOAO Newsletter No. 31, 1 September 1992) Early this year, the Kerr cell, which is used to modulate circular polarization for the KPVT magnetographs, failed after about ten years of operation. The failed unit was one of three which were purchased at the same time from a company which no longer exists. The first of the two spare cells failed in early April (thus giving an estimate of shelf life), and the final unit is being used carefully and sparingly. Preparations have been made to recharge the failed units with purified nitrobenzene, and alternative technologies for modulating polarization are being tested. However, until one or both of these tasks produces a working and replaceable modulator, NSO/KP magnetograph operations will remain restricted as discussed below. After a few days of experimentation ending about 10 April 1992, daily full-disk magnetograms have been obtained only with the new NASA/NSO Spectromagnetograph (Jones et al., 1992, Solar Physics 139, 211) in Fe I 5507 (not Fe I 8688 as has been prior standard practice); the lower Kerr voltage at 5507 presumably places less stress on the remaining operable cell. Magnetic observations have been restricted to one full-disk magnetogram per day. The current software does not correct for Land g-factor so that in principle all 5507 flux values should be multiplied by 0.8. However, to prolong the Kerr cell life, we have not performed planned experiments to determine polarization crosstalk and to correlate the new data with the Diode Array Magnetograph. Thus absolute fluxes from the KPVT synoptic data should be treated with more than customary caution from early April until further notice. Full-disk He I 10830 spectroheliograms arecurrently being taken on a daily basis with both instruments, and most cooperative and campaign observations are being obtained in 10830 with the Spectromagnetograph. Harry Jones
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