Zone plate EUV Solar Irradiance Monitor

ABSTRACT Measuring the solar extreme ultraviolet (EUV) irradiance is a high priority for space weather and upper atmospheric research. We have designed and fabricated a sensor to meas ure the total solar irradiance in the extremely variable 17.1-19.5 nm spectral band that contains numerous Fe emission lines and we are developing another similar sensor for the 30.4 nm He II spectral line. Each of these sensors uses a 4 mm diameter zone plate (ZP) to focus the in-band solar radiation onto a small pinhole in front of a detector. The focal length of a ZP is inversely proportional to wavelength, and the pinhole’s diameter is sufficient to admit the in-band radiation from the full solar disk and inner corona. A 2 mm diameter central occulting disk minimizes the undiffracted (0-order) and out-of-band radi ation that reaches the pinhole. Two thin Al film filters, one in front of the ZP and one deposited on the detector, are virtually opaque to wavelengths longer than EUV and prevent the detector from responding to most of the solar spectrum. A ZP has a number of advantages over a diffraction grating. Because it focuses in-band radiation, a ZP allows the detector to be smaller than the aperture, reducing both the dark current and the out-of-band response. The circular symmetry of the ZP eliminates both the polarization sensitivity and the shift in the spectral band with field angle that are intrinsic to a linear grating. The optical assembly is contained in a very small volume. We are planning to fly the Fe-band sensor as a secondary payload on NRL’s VERIS sounding rocket solar measurement mission in the near future and are investigating the feasibility of flying both sensors on a future CubeSat mission. Keywords : Zone plate, extreme ultravio let, diffraction, solar, space weather, sounding rocket