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Astronomical Data Analysis Software and Systems VI ASP Conference Series, Vol. 125, 1997 Gareth Hunt and H. E. Payne, eds.

The AXAF Ground Aspect Determination System Pipeline
M. Karovska, T. Aldcroft, R. A. Cameron, and J. DePonte Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA, E-mail: karovska@cfa.harvard.edu M. Birkinshaw Department of Physics, University of Bristol, Bristol, BS8 1TL, UK, and Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA Abstract. This article describ es the AXAF asp ect determination pip eline and its comp onents, and highlights a procedure for determining centroids-- a crucial step in deriving the p ost-facto asp ect solution.

1.

Introduction

The asp ect determination system (ADS) is resp onsible for the collection of data that allow the p ointing direction and roll angle of the AXAF Observatory to b e reconstructed from the telemetry. The ma jor parts of this subsystem are: 1. the asp ect camera assembly (ACA) with its stray light shade, 2. the gyroscop es (inertial reference units--IRUs), 3. the fiducial light assembly, and 4. the fiducial transfer system. The AXAF asp ect camera is a 0.11 m diameter Ritchey-Chrґ etien telescop e with a 3-element refractive corrector, which images ab out 2 square degrees of sky into one of two red-sensitive 1024в1024 CCDs. The optics of the camera are delib erately defocused, so that the FWHM of star image is ab out 9 , well spread out relative to the CCD scale of 5 /pixel. The asp ect camera directly views the optical sky, and also views fiducial lights arranged around an X-ray detector, imaged via the fiducial transfer system consisting of a retroreflector/collimator and p eriscop e (see Birkinshaw & Karovska 1995). Up to eight images (normally five guide stars and three fiducial lights) are tracked by the asp ect camera, and their astrometry provides a history of the celestial p ointing coordinates and roll angle for each X-ray observation. 2. Asp ect Determination Pip eline

The Ground Asp ect Determination System (GADS) pip eline is a collection of tools which process asp ect telemetry and generate asp ect data products. It 488

© Copyright 1997 Astronomical Society of the Pacific. All rights reserved.


The AXAF Ground Asp ect Determination System Pip eline
asp_f1.2 KALMANDATA Kalman Filter GYROCAL asp_f1.1 asp_f1.8

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Gyro Bias Correction Process Gyro Data GYRODATA GSPROPS GYRODATA GYRODATA

ACACAL

GYROCAL

ACADATA GYRODATA asp_f1.4 ACASIGHT ACACAL Process ACA Data GSPROPS ACADATA Photographic Method asp_f1.5

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FIDPROPS FIDSOL asp_f1.3 asp_f1.6 Combine Star/Fid Light Solutions KALMANDATA

ACASIGHT

Determine Fid Motion

ASPECT

Figure 1.

The Asp ect Process Pip eline and its comp onents.

produces a three-axis asp ect solution in J2000.0 celestial coordinates, to supp ort b oth image reconstruction (with error less than 0.5 ) and celestial location for X-ray data (with error less than 1 ). The asp ect solution is calculated from telemetry data (ACADATA and GYRODATA on Figure 1), generated by the asp ect camera assembly and the inertial reference units (gyroscop es), using their associated calibration and alignment data (GYROCAL, ACACAL, GSPROPS, and FIDPROPS in Figure 1). The ACA telemetry contains pixelated images of stars b etween 10th and 6th magnitude and fiducial lights, nominally at 7th magnitude apparent brightness, at intervals of 1.025, 2.050, or 4.100 s (corresp onding resp ectively to 4в4, 6в6, and 8в8 pixel images). The AXAF IRUs provide four axes of integrated angular rate telemetry at 0.25625 s intervals, from which three orthogonal axis rate data are derived. The asp ect camera data are analyzed to determine the centroid of each star or fiducial light image. The star centroids are combined with the IRU rate data in a Kalman filter to provide a time-varying optimal asp ect solution and associated covariance matrix, assuming known noise models for the asp ect camera and IRU data. The relative offsets b etween the star centroids and the fiducial light centroids are then calculated. Finally, the b oresight calibration is applied to the relative offset, to transform the optical celestial asp ect solution to the X-ray focal plane data. The GADS pip eline also p erforms a simple least-squares simultaneous fit of ACA p osition data and IRU rate data, and generates "photographs" of the ACA guide stars, for diagnostic and simple quality-assurance purp oses. The pip eline also determines intervals during which the asp ect solution is stable, assigns asp ect quality indicators, and up dates databases and the AXAF Optical Star Selection (AOSS) catalog. The asp ect pip eline consists of three comp onents:


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1. Asp ect Interval Separation Pip eline makes one pass through IRU and ACA telemetry data to determine the asp ect intervals during which ma jor parameters (e.g., tracked star, spacecraft attitude) stay invariant or within allowed limits. 2. Asp ect Process Pip eline processes ACA and gyro data, and derives asp ect solution for each asp ect interval. 3. Assign Asp ect Quality Pip eline calculates asp ect stable intervals, given the asp ect solution, and assigns quality indicators to the asp ect solution. 3. Asp ect Process Pip eline

The Asp ect Process Pip eline (Figure 1) is the central part of the asp ect determination pip eline. The functional comp onents of this pip eline are: Process Gyro Data: Filter gyro data, check for internal consistency among the four gyro channels, and convert counts to angles. Gap-fill missing or filtered data, and calculate 3-axis spacecraft b ody angular rate. Process ACA Data: Process asp ect camera data, apply corrections, and calculate centroid coordinates (ACA sightings). Kalman Filter: Optimally combine ACA star centroids and IRU data to determine ACA celestial location and image motion, using a Kalman filter and smoother. The Kalman filter and smoother also provides error estimates for the p osition and rate estimates in each axis, in the form of a covariance matrix. Gyro Bias Correction: Correct bias drift rate using the estimate from the Kalman filter. Photographic Metho d: Use photographic method for solving for the absolute celestial location and roll. Determine Fiducial Motion: Perform a time-averaged solution of the fiducial light p ositions, calculate the fiducial light field centroid, and derive field motion as a function of time. Combine Star/Fid Light Solutions: Calculate offset b etween fiducial light solutions and filtered star solutions, and apply b oresight calibration, to generate image motion and celestial location at the focal plane science instrument. 4. Centroid Determination

We plan to use a PSF-fitting routine to calculate centroids and fluxes for each star and fiducial light image, where the PSFs obtained by interp olating from a database of theoretical PSFs of the optical system. Each model PSF is defined on a pixel grid that is finer than that of the asp ect camera itself, and is calculated using the parameters of the optical system as measured b efore launch or on orbit.


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Using a subset of ACA pixel data from the Asp ect Interval, we identify p oorlyfitting PSFs and find sp oiler stars in star and fiducial light images. (Sp oilers are faint stars in the vicinity of guide stars or fiducial lights). We then create new "effective" PSFs incorp orating any sp oilers, for image centroiding. We apply multidimensional 2 minimization routines (e.g., Powell minimization routine) to derive the optimal values for the coordinates and brightness of the image and the local sky background level. Acknowledgments. AXAF Science Center and SAO staff that contributed to this work include: Daniel Schwartz, Martin Elvis, Jonathan McDowell, Gerald Cardillo, and Peter Daigneau. We thank Nanci Kascak from TRW for providing us with the Kalman filter and gyro model code. This work was supp orted by NASA Contract No. NAS8-39073. References Birkinshaw, M., & Karovska, M. 1995, AXAF News, No. 3