Absolute radiometric calibration of the EUNIS-06 170-205 A channel and calibration update for CDS/NIS

摘要

The Extreme-Ultraviolet Normal-Incidence Spectrograph sounding-rocket payloadwas flown on 2006 April 12 (EUNIS-06), carrying two independent imagingspectrographs covering wave bands of 300-370 A in first order and 170-205 A insecond order, respectively. The absolute radiometric response of the EUNIS-06long-wavelength (LW) channel was directly measured in the same facility used tocalibrate CDS prior to the SOHO launch. Because the absolute calibration of theshort-wavelength (SW) channel could not be obtained from the same labconfiguration, we here present a technique to derive it using a combination ofsolar LW spectra and density- and temperature-insensitive line intensityratios. The first step in this procedure is to use the coordinated, cospatialEUNIS and SOHO/CDS spectra to carry out an intensity calibration update for theCDS NIS-1 waveband, which shows that its efficiency has decreased by a factorabout 1.7 compared to that of the previously implemented calibration. Then,theoretical insensitive line ratios obtained from CHIANTI allow us to determineabsolute intensities of emission lines within the EUNIS SW bandpass from thoseof cospatial CDS/NIS-1 spectra after the EUNIS LW calibration correction. Atotal of 12 ratios derived from intensities of 5 CDS and 12 SW emission linesfrom Fe Fe X - Fe XIII yield an instrumental response curve for the EUNIS-06 SWchannel that matches well to a relative calibration which relied on combiningmeasurements of individual optical components. Taking into account allpotential sources of error, we estimate that the EUNIS-06 SW absolutecalibration is accurate to about 20%.