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The grade is computed by the BEP (Back-End Processor) by comparing the corrected pulse heights of the eight outer edge pixels of the event to the ``Split Threshold'' specified in the Timed Exposure Parameter Block. Each pixel corresponds to a ``bit'' in the grade code. If a pixel's pulse height is greater than or equal to the threshold, the corresponding bit is marked as a ``1''. If a pixel's pulse height is less than the threshold, the bit is marked as a ``0''. Since there are eight pixels on the edge of a 3x3 event, this leads to an 8-bit grade code. There are 256 possible ``grades'' for a 3x3 event. Figure A.1 illustrates an example 3x3 event, with pixel pulse heights drawn as vertical bars and the split threshold drawn as a grid. The pixels whose pulse height is above the threshold are shown to the right as lightly shaded areas. Since the center pixel has been selected on the basis of being a local maximum, it will always be the pixel with the highest signal. (It is shown using a darker shade in the diagram.)
A complete list of the ACIS grades with a diagram illustrating the split event threshold crossings is given in Table A.1. Please note that ACIS grades and ASCA grades do not use the same numbering convention. ACIS grades run from 0 to 255, while ASCA grades run from 0 to 7. ACIS grade 0 is identical to ASCA grade 0 (a single, isolated event without any neighboring split threshold crossings), but all other ASCA grades are sums of many ACIS grades. Also note that when ACIS operates in ``Graded Mode'' only the total reconstructed energy of the X-ray event is telemetered to the ground. The total energy is computed by summing all pixels which exceed the split event threshold. On ASCA the corresponding mode (`Bright Mode') sums pixels regardless of whether the split threshold is crossed. The result is that some ASCA grade 5 and 7 event total energy values can not be calculated from ACIS grade and energy information alone.
The commonly used ASCA grades are g0 (which are single events and the same as ACIS Grade 0), g0234 (which are the singles plus vertical and horizontal split events, and correspond to the combination of ACIS Grades 0, 2, 8, 16, 64), and g02346 (which corresponds to the preceeding plus ASCA grade 6). ASCA grade 6 has the n=3 and n=4 events which are compact (i.e. form small `L'-shaped or 2x2 square-shaped regions).