INTRODUCTION

The memo OGIP/92-007 (Arnaud, George & Tennant 1992) described the general design criteria and structure of a 'PHA' file, namely that it consisted of a FITS file with a 'null' primary array and the following extensions:

• a Data Extension - containing the PHA dataset
• a Detector extension - containing instrument-specific information
• a Good Time Interval (GTI) extension
• a (Selector) History extension

The memo then gave the detailed format of the Data (or 'Spectrum') Extension. Since the adoption of the format given in Arnaud et al, a number of conventions/rules have been introduced within the OGIP to help clarify & unify a multi-mission, multi-application approach to the design of FITS files. Specifically, the OGIP FITS Working Group (OFWG) was created and started making recommendations. Many of the changes which have been made to the format of the spectrum extension reflect such rulings of the OFWG. In this addendum we list all such format changes along with the corresponding value of the HDUVERS keyword.