Summary

As explained in the Proposer's Observatory Guide Section 7.2, HRC event positions are determined by collecting the charge emitted from the microchannel plates with a cross-grid charge detector (CGCD) and calculating the centroid of the charge cloud. The centroid calculation uses signals from the six ``taps'' nearest the event, three per axis, where a tap is the output from a group of 8 wires along an axis of the CGCD. Event positions along an axis are determined using the number of the central tap of the triplet (coarse position) and the centroid of the tap signal values to obtain a fine position relative to the coarse position.

Taps are identified according to their ``coarse position'' numbers. In the HRC-S, coarse U (CRSU) runs from 0 to 15 across the short axis and CRSV from 0 to 191 on the long (dispersion) axis. Each tap is subdivided into 256 resolution elements, with square pixels 6.43 μm on each side. Raw position coordinates (RAWX, RAWY) are determined using the 3-tap algorithm for each axis as mentioned above. Because of the extent of the typical charge cloud, not all of an event's charge is collected by the 6 position-determination taps, resulting in a bias in the position toward the center tap. As a result, images of events plotted in RAW coordinates leave gaps between each tap.

A degapping procedure applies corrections to the RAW coordinates to account for the bias, creating CHIP coordinates. These corrected positions are then projected into several detector-, observatory- and sky-centric coordinate systems. More information about the HRC-S degap map and efforts to correct residual errors may be found here. Note that the RAW and CRS columns are not carried from Level 1 to Level 2 event files in standard processing.

Last modified: 02/17/12

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.    Email: cxcweb@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2004. All rights reserved.