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Cross-calibration of the NSO/KP Magnetographs (1Dec95) (from NSO, NOAO Newsletter No. 44, December 1995) From the beginning of the NSO/KP full-disk synoptic program until the early spring of 1992, magnetograms were obtained with the 512-channel Diode Array Magnetograph (DM). Since that time, the data have been acquired with the NSO/NASA Spectro-magnetograph (SM)--a term originally coined to describe this specific instrument but which has subsequently been used as a generic label for spectrograph-based magnetographs. The transition between the two data sets was not smooth due to the failure of two of the three available Kerr cells used to modulate circular polarization. The SM, with its two-dimensional detection of long-slit spectra instead of the fixed dual bandpass system of the DM, can operate effectively at a shorter wavelength (550.7 nm instead of 868.8 nm) and hence a lower Kerr-cell voltage. Thus, to preserve the life of the remaining Kerr cell, the SM was pressed into service before the originally planned cross-calibration program could be undertaken. Subsequently, the Kerr cell was replaced with a liquid crystal modulator and the SPM now operates on the original Fe I, 868.8 nm line. A cross-calibration program of closely spaced observations with both instruments was then carried out. The data have now been analyzed and the results are as follows: B(sub)DM(868.8) = (1.46 +/- 0.06) B(sub)SM(868.8) = (1.26 +/- 0.05) B(sub)SM(550.7). The different constants result from the way the calibration procedures account for polarization cross-talk together with some software limitations. Error bars are twice the standard error of the mean averaged over about a month of comparative data. The overall scale of the DM is the better basis for absolute flux estimates, although the SM data are substantially less noisy and do not saturate in sunspots. The magnetogram from 6 April 1992 is the last reasonably reliable DM synoptic observation; data from 7-20 April 1992 do not have a reliable flux scale; data from 21 April-19 November 1992 were obtained with the SM in the 550.7 nm line of Fe I; subsequent magnetograms have been acquired in the original 868.8 nm line with the SM. Disk-integrated flux indices from the magnetograms show continuous and consistent variation when adjusted with the above calibration constants. The file of these daily measurements of flux and absolute flux integrated across the disk has been recently updated and is available by anonymous ftp from argo.tuc.noao.edu, in directory kpvt/daily/stats from the anonymous login. The file includes corrections that put all the measurements onto the DM scale. Harry Jones, Jack Harvey
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