Hinode/EIS Nugget – Sensitivity Performance of the Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode

by George Doschek
NRL SSD Code 7670


Scientists in the Space Science Division (SSD) of the Naval Research Laboratory have been studying the sensitivity of the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Japanese Hinode spacecraft since its launch in late September 2006. This state-of-the-art spectrometer was built by an international consortium that included SSD scientists funded by NASA. The science objective of EIS is to measure spectroscopically physical parameters of the solar atmosphere such as temperature, density, and dynamical properties.

The instrument is 3 meters long and for reasons of weight, thermal design, and mechanical stability, the optical elements were housed in a carbon composite structure. One of the biggest challenges for EUV space instruments is minimizing contamination that degrades the optical sensitivity. Chief among these is carbon itself. The EIS composite structure was designed for cleanliness at every step, and part of the process was a two month vacuum bake at 80 ℃. Keeping the EIS spectrometer "clean" involved a major effort throughout the build of EIS and Hinode.

Since launch, SSD scientists have been checking to see how successful the team was in keeping EIS free of contaminants. Measuring sensitivity loss is accomplished by monitoring the intensities of solar spectral lines in the so-called quiet Sun using observational studies developed for this purpose. Some of these EUV lines are known to have on average constant intensities with time. An example of the data obtained by the monitoring studies is shown below in Figure 1.

Figure 1: The intensity of a solar He II line as a function of time in the quiet Sun.
(Click figure to see full-size image)    

The intensity of the spectral line of He II at 256.32 Å is shown as a function of time between January 2007 and July 2009. A recent analysis of these data, and the data from many other lines observed so far, has shown that the time constant of the EIS sensitivity, or the time over which the sensitivity will drop to 37% of its current value, is about 6.4 years. This e-folding time of 6.4 years is extremely long for an EUV instrument and shows that the international EIS team achieved the goal of producing an exceptionally clean instrument. The team looks forward to many years of fruitful research for EIS, uncomplicated by severe degradation due to contamination.

For more details, please contact: Dr. Lucie Green.

Last Revised: Monday, 04-Jan-2011

Feedback and comments: webmaster

EIS partner logo