  The Miniature X-ray Solar Spectrometer (MinXSS) are twin 3U CubeSats. The
first of the twin CubeSats (MinXSS-1) launched in December 2015 to the
International Space Station for deployment in mid-2016. Both MinXSS CubeSats
utilize a commercial off the shelf (COTS) X-ray spectrometer from Amptek to
measure the solar irradiance from 0.5 to 30 keV with a nominal 0.15 keV FWHM
spectral resolution at 5.9 keV, and a LASP-developed X-ray broadband photometer
with similar spectral sensitivity. MinXSS design and development has involved
over 40 graduate students supervised by professors and professionals at the
University of Colorado at Boulder. The majority of previous solar soft X-ray
measurements have been either at high spectral resolution with a narrow
bandpass or spectrally integrating (broadband) photometers. MinXSS will conduct
unique soft X-ray measurements with moderate spectral resolution over a
relatively large energy range to study solar active region evolution, solar
flares, and the effects of solar soft X-ray emission on Earth's ionosphere.
This paper focuses on the X-ray spectrometer instrument characterization
techniques involving radioactive X-ray sources and the National Institute for
Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility
(SURF). Spectrometer spectral response, spectral resolution, response linearity
are discussed as well as future solar science objectives.
