The following was taken from a document titled ACIS Grading Scheme Recommendation written by Kenny Glotfelty at the AXAF Science Center, found
at
http://hea-www.harvard.edu/acis/xrcf/grade.html.
Pulse Height
The way the pulse height is computed for ACIS and ASCA are different. The difference comes in the way corner pixels are included in the sum and are explained below. The effect of this difference on the different grades are also discussed. Since the flight software is already written, it would be difficult to adopt a different processing algorithm. This section is provided to show what impact using the ASCA grading scheme would have on subassembly & XRCF data analysis.
--ACIS
The flight software design ( see ACIS Software Requirements Specifications, section 3.2.2.3.14 ) computes the pulse height from the bias and overclock corrected pixels by including the center pixel's pulse height and the pulse height of all edge (non-diagonal) pixel's that are above some threshold (event for center pixel or split for surrounding pixels). Corner pixels are only included in the total event pulse height if they are above the split threshold and at least one of the adjacent edge pixels is also above the split threshold. The entire event island is telemetered in all but the Graded modes; thus allowing reprocessing on the ground.
--ASCA
The pulse height computation for ACIS and ASCA differs only in the treatment of corner pixels. Corner pixels are only included in ASCA grade 6 where both adjacent edge pixels are above threshold (and possibly a "de-touched" corner). In all other geometries, the corner pixels are not included.
ACIS ASCA
- - o - - o
- X - G128 - X - G1
- - - - - -
X X X o X o
X X X G255 X X X G7
X X X o X o
- X X - X X
- X X G208 - X X G6
- - - - - -
- X X - X o
- X - G192 - X - G5
- - - - - -
X = pixel's pulse height included in total pulse height
o = pixel's pulse height not included in total pulse height
The following is a table showing which ACIS flight grades correspond to which ASCA grades and whether the pulse heights are computed using both systems is the same.
ASCA ACIS Comment
----- ---------- ----------
0 0 Same pulse height
1 1,4,5,32,33,36,37,128 Same pulse height
129,132,133,160,161,
164,165
2 2,34,64,65,68,69, Same pulse height
130,162
3* 8,12,136,140 Same pulse height
4* 16,17,48,49 Same pulse height
5 3,6,9,13,20,21,35,38,
40,44,52,53,96,97,100,
101,131,134,137,141, All different pulse heights
144,145,163,166,
168,172,176,177,192,
193,196,197
6 10,11,18,22,50,54,72,
76,80,81,104,108,138, Same pulse height
139,208,209
7 everything else Mostly different pulse heights
* Due to ASCA grades being defined in READ coordinates and ACIS grades
in CHIP coordinates, these two grades will interchange for output
nodes B and D.