Calibrating the wings of the Chandra PSF T. J. Gaetz, R. J. Edgar, D. Jerius, P. Zhao, and R. K. Smith, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA ABSTRACT We discuss the calibration of the wings of the Chandra point spread function. In order to achieve high resolution imaging, the X-ray mirror surfaces must be extremely smooth in order to suppress the effects of scattering from microroughness. In the Chandra program, surfaces with only 1.3–3 ˚ A roughness were achieved over more than 90% of the mirror length. We describe the current state of the calibration of the Chandra PSF wings, incorporating the results of a deep observation of the X-ray source Her X-1. The galactic Hydrogen column density (N H ) to Her X-1 is small, reducing the amplitude of any astrophysical dust scattering halo which would contaminate the mirror scattering wings. The X-ray data clearly show the shadows of the mirror support struts, confirming that the observed halo is predominantly due to mirror scattering. The extreme brightness of the source allows the energy dependence of the PSF wings to be probed with good statistics. The deep observation (heavily piled up in the core) is combined with a zero order gratings observation (unpiled in the core) to construct an energy-dependent profile. Keywords: X-ray optics, PSF, scattering wings, Chandra 1. INTRODUCTION The Chandra X-ray Observatory is the highest resolution X-ray telescope ever built, achieving subarcsecond imag- ing. In order to exploit fully the capabilities of the telescope, it is necessary to understand the energy-dependent point spread function (PSF) profile over its full angular range. The PSF of the Chandra High Resolution Mirror Assembly (HRMA) includes contributions ranging from nearly specular reflection from low frequency figure errors (the “core” of the PSF) to scattered photons reflecting/diffracting off of surface microroughness on the optics. The scattered photons form a faint diffuse halo extending to large angles. The on-axis, azimuthally-averaged scattering halo is energy dependent, falling approximately as a powerlaw (θ −γ ), where θ is the angle from the center of the specular image, and γ ∼ 2. Extremely smooth mirror surfaces are required in order to attain such high performance. The ground metrol- ogy of the mirrors established that the 1 to 1000 mm −1 surface roughness is ∼1.5–3 ˚ A for the middle 770 (of 832) mm of each optic [1]. The ends of the optics are rougher, and the mean surface roughness including the ends is ∼ 2–6 ˚ A. Ground measurements of the X-ray scattering wings were consistent with these values. Much effort at the ground testing was aimed at calibrating a high fidelity raytrace model for the HRMA performance. However, deficiencies in the scattering model and systematics in the ground testing (e.g., uneven illumination of the rougher ends of the optics) led to substantial uncertainties in the PSF wings. Detailed knowledge of this mirror scattering halo as a function of energy and radius is needed for interpretation of observations with faint structure adjacent to bright sources. Examples include: • X-ray scattering halos from cosmic dust along the line of sight • extracting faint sources adjacent to bright sources • faint structure (e.g., cosmic ray precursors) ahead of shocks in supernova remnants Send correspondence to TJG; E-mail: gaetz@head-cfa.harvard.edu Copyright 2003 Society of Photo-Optical Instrumentation Engineers. This paper will be published in X-ray and Gamma-Ray Instrumentation for Astronomy XIII, Proceedings of SPIE Vol. 5165, and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.