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We present a study of Chandra's optical distortions by examining the positional accuracy of observed sources on the HRC-I. We investigate the Chandra mirror and detector models' ability to reproduce the detector locations of observed sources by simulating ~160 calibration observations of AR Lac, HR 1099, and LMC X-1. To study the optical distortions of the mirrors more directly, we compare a 63 ksec observation of the Orion Nebular Cluster (ONC) with positions based on the well-determined optical astrometry of the cluster. We simulate observations of 100 reasonably bright sources from the Hillenbrand 1997 catalog of the ONC and compare the simulated positions with their observed positions. Offsets between the optical positions and the observed X-ray positions help determine a map of the optical distortions of the Chandra mirrors.

Chandra X-ray Observatory (CXO) the third of NASAs Great Observatories has now been successfullyoperated for four years and has brought us fruitful scienti“c results with many exciting discoveries. The majorachievement comparing to previous X-ray missions lies in the heart of the CXO the High Resolution MirrorAssembly. Its unprecedented spatial resolution and well calibrated performing characteristics are the keys for itssuccess. We discuss the eective area of the CXO mirrors, based on the ground calibration measurements madeat the X-Ray Calibration Facility in Marshall Space Flight Center before launch. We present the derivations ofboth on-axis and o-axis eective areas, which are currently used by Chandra observers.Keywords: Chandra X-ray Observatory, HRMA, X-ray telescopes, X-ray mirrors, calibration, eective area 1. INTRODUCTION NASAs Chandra X-ray Observatory (CXO) is the most powerful X-ray telescope ever built to-date. Its launchin 1999 was a major milestone in the “eld of high energy astrophysics and the world of science. In the past fouryears, it has brought many new discoveries to the general public as well as the scienti“c community, and openeda new window to part of the Universe of which we have never seen before. CXO has unprecedented capabilitiesof high resolution imaging and spectroscopy over the X-ray band of 0.1 keV 10 keV. The success of CXO ismainly due to the design and manufacture of its X-ray mirrors the High Resolution Mirror Assembly (HRMA).At 0.84-m long and 0.6 1.2-m in diameters, the surface area of each mirror ranging from 1.6 to 3.2 squaremeters. They are the largest and the most precise grazing incidence optics ever built. The success of CXO is alsodue to the extensive ground calibration for the HRMA and science instruments, carried out at NASAs MarshallSpace Flight Center before launch. The Eective Area (EA) is one of the most important characteristics of theCXO. It was determined that the CXO eective area should be calibrated to a precision that no X-ray telescopeshave ever achieved before, so it can provide accurate measurement of the ”ux from X-ray sources.