The objective of the EIT CalRoc program is to increase the scientific return from the Solar and Heliospheric Observatory (SOHO) spaceflight mission with well calibrated supporting observations from the flight spare EUV Imaging Telescope on a sounding rocket platform. Improvement in the absolute calibration of the SOHO EIT observations is needed because of limitations in pre-flight calibrations imposed by the SOHO instrument integration schedule, and the complex variations in the EIT EUV response observed inflight. Since the observed EUV response variations are a wavelength dependent combination of uniform and localized effects on the SOHO EIT images, only a full disk imaging instrument is capable of providing an adequate calibration.
Details of the SOHO EIT instrument performance on-orbit, including instrument calibration issues, as well as the first science results can be found in a paper currently in press: "EIT Observations of the EUV Sun", Moses et al. 1997, Solar Physics 175: 571. A preprint of this paper is available via anonymous ftp from louis14.nrl.navy.mil in the directory /pub/eit. A postscript version of the text is in the file "paper.ps". The figures are available separately due to the large size of the figure files. Click to download PostScript version of the paper text via anonymous ftp (N.B. this link has only been tested with unix machines).
The response of the EIT backthinned CCD EUV detector has degraded with time during the mission. All imaging EUV detectors currently available for spaceflight applications degrade with EUV exposure (including microchannel plate and phosphor based detectors). Although the level of degradation of the EIT EUV response is much less than any previous solar EUV imaging missions, it is still essential to characterize this degradation.
The average degradation can be estimated be assuming that the solar EUV output is constant, and measuring the total flux observed on the CCD in each waveband. Plots of the average response have been prepared by the EIT Operations Team:
|Fe IX/X 171 Å||Linear||Log-linear|
|Fe XII 195 Å||Linear||Log-linear|
|Fe XV 284 Å||Linear||Log-linear|
|He II 304 Å||Linear||Log-linear|
An indication of the spatial distribution of the degradation can be seen in the ratio of images of the onboard visible light calibration illumination at different times (levels of degradation). To see such a representation, click here. However, the EUV response is significantly different than the visible light response of the EIT detector, so this image can only be used as a relative monitor of the evolution of the detector.