Fifth Hinode Science Meeting: Exploring the Active Sun ASP Conference Series, Vol. 456 L. Golub, I. De Moortel, and T. Shimizu, eds. c  2012 Astronomical Society of the Pacific Measuring Uncertainties in the Hinode X-Ray Telescope Adam Kobelski, 1 Steve Saar, 2 David E. McKenzie, 1 , Mark Weber, 2 Katharine Reeves, 2 and Ed DeLuca 2 1 Montana State University, Bozeman, MT, USA 2 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA Abstract. We have developed estimates of the systematic photometric uncertain- ties for the X-Ray Telescope (Kano et al. (2008)) on Hinode (Kosugi et al. (2007)). These estimates are included as optional returns from the standard XRT data reduction software, xrt prep.pro. Included in the software estimates are uncertainties from instru- ment vignetting, dark current subtraction, split bias leveling, fourier filtering and JPEG compression. We show that these uncertainties are generally smaller than the photon counting uncertainty. However, due to the reliance on assumptions of plasma radiation models and elemental abundances, photon counting is not included in the software. 1. Overview Before the data from XRT can be used for photometric analysis, it must be prepped in order to remove instrumental artifacts such as dark current and vignetting. By quan- tifying the accuracy and precision of these corrections, we can make estimates of the photometric uncertainties. The corrections and uncertainties described herein are ap- plied via the xrt prep procedure in SolarSoft (Freeland & Handy (1998)). 2. Systematic Sources We will quantify the contributions to the total uncertainty from a few systematic sources. A primary source of uncertainty is our ability to remove the background instrumental noise to determine the zero point level of the images. This includes the correction for the spilt bias (Nyquist Noise) of the CCD, which offsets the even and odd columns by a few (∼ 8) DN. Also important to the zero point determination is the dark noise, the signal created from within the detector. XRT frequently makes exposures with the shutter closed to quantify the dark noise. By measuring the deviations within these dark images, we can estimate the uncertainty caused by the dark noise and the Nyquist cor- rection. By averaging a large number of corrected darks (a master dark) and measuring the standard deviation we note σ avg . A second deviation is noted between the master dark and the individual darks, yielding 〈σ N 〉. These uncertainties combine into σ dark : σ dark =  σ 2 avg + 〈σ N 〉 2 . (1) We can also use this method to estimate the uncertainty in fourier filtering. To fourier filter, a fourier transform is applied to the image and high frequency signals attributed 241