Caveat Regarding Off-axis Angle Theta and Phi
All event files processed using a standard data processing version prior to DS6.9 (released on 10 August 2002) have the off-axis angle THETA and rotation angle (azimuth) PHI values switched. This occurs in both ACIS and HRC data. Using dmcoords to find the off-axis angle of your source yields correct results, no matter what processing version was used on your data. However tools such as dmstat when used with data processed before DS6.9 will give results with THETA and PHI switched.
Please see our thread A Note on Processing Versions for instructions on how to check which processing version was used on your data.