Calibration of the SERTS-95 Spectrograph from Iron Line Intensity Ratios

Abstract

Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph was flown on 1995 May 15 (SERTS-95), carrying a multilayer-coated toroidal diffraction grating which enhanced the instrumental sensitivity within its second-order wave band (170-225 A). Spectra and spectroheliograms of NOAA Active Region 7870 (N09/W22) were obtained in this wave band with a spectral resolution (instrumental FWHM) ~30 mA. We developed and applied a technique for deriving the relative radiometric calibration independent of laboratory calibration measurements by employing a method proposed by Neupert & Kastner for monitoring variations in the sensitivities of orbiting EUV spectrometers by means of density- and temperature-insensitive line intensity ratios. Numerous ratios of emission lines from Fe X-XIV are mutually consistent and yield an instrumental response curve that matches the design characteristics of the multilayer coating. This supports the accuracy of the atomic physics parameters and demonstrates the power of the technique. Many of the ratios employed here can be used to carry out a similar calibration exercise on spectra from the Coronal Diagnostic Spectrometer's Grazing Incidence Spectrograph (CDS/GIS) aboard the Solar and Heliospheric Observatory. Because they are relatively free from blending with nearby strong lines, the following density-sensitive ratios are particularly well suited for analysis with the GIS: Fe X λ175.265/λ174.526, Fe X λ175.265/λ184.534, Fe XII λ186.867/λ195.117, Fe XIII λ203.820/λ202.042, Fe XIII λ200.017/λ202.042, and Fe XIV λ219.121/λ211.317. Densities derived from Fe X, XIII, and XIV yield log ne~9.4±0.2.