One of the important goals of MAXI is to discover X-ray transient objects and inform the world of their positions. For follow-up observations with other satellites and telescopes, we must reduce the position uncertainties as small as possible. We therefore performed calibration of alignments of the MAXI/GSC. As a result, we reduced the systematic error in position determination from the initial uncalibrated value of 1.0 degree to 0.2 degree. Calibration of Alignments of MAXI/GSC Kosuke Sugimori, Nobuyuki Kawai, Mikio Morii (Tokyo Tech), Mutsumi Sugizaki, Motoko Serino (RIKEN), and the MAXI team. Calibration Strategy Reference Matsuoka, M. et al., PASJ 61 (2009) 999., M. Morii, et al. Physica E (2010), doi: /j.physe Gas Slit Camera Results As a result, we reduced the systematic error in position determination from the initial uncalibrated value of 1.0 degree to 0.2 degree. Upper panels are histograms of the radial distance between the position of Sco X-1 determined by MAXI and the actual position. Lower panels are cumulative histograms of upper histograms. Figures above are images of bright X-ray objects observed by MAXI. All images are made by using data from September 1, 2009 to September 30, Before the calibration (upper panels), images of the objects are somewhat spreading and their positions shift greatly (green crosses represent actual positions.). But after the calibration (lower panels), they are improved. In September 2009, the attitude of ISS changed greatly because of docking of Space Shuttle. Therefore the shape of X-ray objects in uncalibrated images are very distorted (especially Sco X-1). Before After CrabGRS Cyg X-3, Cyg X-1Sco X-1 BeforeAfter Systematic error ~ 0.2 deg. Systematic error ~ 1.0 deg. Radius [deg] # of Daily Obs. Ratio (Cumulative # of Daily Obs./Total) ISS rot. axis Axis 1: misalignment around the orbital rotation axis. Detection time of all X-ray sources lags uniformly. Scan Actual FoV Ideal FoV Time [sec] Incident Angle [deg] Observed light curve Light curve of ideal alignment Time Axis 3: misalignment around the moving axis. ISS rot. axis Incident angles of X-ray photons shift uniformly. Scan Time [sec] Incident Angle [deg] Position on the GSC counter Incident angle of X-ray photon ISS rot. axis Axis 2: misalignment around the axis drawn in the left figure. Time lags depend on the incident angles. Observed light curve Light curve of ideal alignment Time Star B Star A Scan Time [sec] Incident Angle [deg] Star BStar B Star AStar A ISS rotation axis Scan with ISS rotation Slat Collimator Slit 1-dimensional Position-sensitive gas (Xe) counter MAXI‘s field of view is restricted by the slats collimators to a narrow and long rectangle, which scans the sky in the direction perpendicular to the long dimension. The position of an X-ray source along the scan direction is determined by the time when the X-ray source crosses MAXI’s FoV. To determine the position of the X-ray source perpendicular to the scan direction, we use the position of the X-ray image on the detector. ISS rotation axis