EUV variability experiment (EVE); multiple EUV grating spectrographs (MEGS), radiometric calibrations and results

Abstract

The NASA Solar Dynamics Observatory (SDO), scheduled for launch in early 2009, incorporates a suite of instruments including the EUV Variability Experiment (EVE). Two channels of EVE, the Multiple EUV Grating Spectrograph (MEGS) A and B channels use concave reflection gratings to image solar spectra onto CCDs to measure the solar extreme ultraviolet (EUV) irradiance from 5 to 105 nm. MEGS provides these spectra at 0.1nm spectral resolution every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-year mission. The calibration of the MEGS channels in order to convert the instrument counts in to physical units of W/m²/nm was performed at the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility III (SURF III) located in Gaithersburg, Maryland. Although the final post-environmental calibrations have yet to be performed, preliminary results from the pre-environmental calibrations show very good agreement with the theoretical optical design given by Crotser et al. Further analysis is still needed in regards to the higher order contributions to determine the final first order QT for all channels, but two techniques are currently being analyzed and show promising results.