A METHOD FOR CORRECTING ASPECT SOLUTION ERRORS IN ROSAT HRI OBSERVATIONS OF COMPACT SOURCES

X-ray point sources observed with the ROSAT High Resolution Imager (HR I) often appear elongated over scales of approximately 5''-10'' from t he image core. This elongation has been attributed to errors in the at titude correction as the satellite is wobbled during the observations, and affects sources with both soft and hard X-ray spectra. In this pa per, I report the results of an attempt to rid a high signal-to-noise observation of the soft X-ray point source HZ 43 of its characteristic elongation. I divided the observation into 181 separate images, each containing photons from only a small region on the detector through wh ich the source passed during the satellite's wobble. By measuring the positions of the individual image centroids, I found clear evidence fo r systematic offsets from a common mean by up to approximately +/- 3'' in both right ascension and declination as a function of phase in the satellite wobble. Shifting the subimages to a common center and then restacking them into a single image measurably improved the symmetry o f the point-spread function. HRI observations are wobbled primarily to smooth out variations in the pixel-to-pixel sensitivity of the detect or and also to extend the lifetime of the microchannel plates in the d etector since these decay at a given location as a function of the num ber of photons detected. However, the elongations introduced by the as pect errors inhibit the identification of possible extended X-ray emis sion associated with sources such as pulsars and active galactic nucle i. In light of these results, I suggest that until the aspect errors a re understood, observations of compact sources, where this effect may be important, should not be wobbled.