Calibration of the MaGIXS experiment I: Calibration of the X-ray source at the X-ray and Cryogenic Facility (XRCF).

The Marshall Grazing Incidence Spectrometer {\it MaGIXS} is a sounding rocket experiment that will observe the soft X-ray spectrum of the Sun from 24 - 6.0 A (0.5 - 2.0 keV) and is scheduled for launch in 2021. Component and instrument level calibrations for the {\it MaGIXS} instrument are carried out using the X-ray and Cryogenic Facility (XRCF) at NASA Marshall Space Flight Center. In this paper, we present the calibration of the incident X-ray flux from the electron impact source with different targets at the XRCF using a CCD camera; the photon flux at the CCD was low enough to enable its use as a "photon counter" i.e. the ability to identify individual photon hits and calculate their energy. The goal of this paper is two-fold: 1) to confirm that the flux measured by the XRCF beam normalization detectors is consistent with the values reported in the literature and therefore reliable for {\it MaGIXS} calibration and 2) to develop a method of counting photons in CCD images that best captures their number and energy