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

TL;DR

This paper presents a new method for absolute radiometric calibration of the EUNIS-06 EUV spectrograph's short-wavelength channel using solar spectra and line ratios, updating calibration for CDS/NIS and achieving about 20% accuracy.

Contribution

It introduces a technique combining solar spectra and line ratios to calibrate the EUNIS SW channel and updates the CDS/NIS-1 calibration based on coordinated observations.

Findings

CDS/NIS-1 efficiency decreased by a factor of about 1.7.

EUNIS-06 SW calibration accuracy estimated at 20%.

Calibration method validated with multiple emission lines.

Abstract

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