XMM-Newton Data Files Handbook

4.1.7.3 OM Tracking History Data Auxiliary File

One OM tracking history file (*OM*THX.FIT) is created for each exposure of an observation if tracking occurred (lack of suitable tracking stars can in principle prevent this from happening). Each file consists of a single binary table, with each row representing one frame. It contains the coordinates, in detector pixels, of the guide stars found in the FOV as well as the offsets found in different tracking frames due to small jitters in the spacecraft pointing. These offsets are corrected by the shift-and-add algorithm on board. The binary table consists of the columns listed in Table 18. A more detailed description can be found in [14].

Table 18: OM Tracking History Data Auxiliary file (*OM*THX.FIT) binary table columns

Name	Type$^a$	Units	Description
FRAME	1J	COUNTER	Tracking number
NGGS	1J	COUNTS	No. of good guide stars found
DX	1J	1/1000th PIXELS	Drift in x direction
DY	1J	1/1000th PIXELS	Drift in y direction
ROLL	1J		Roll drift (in units ${\rm 10^6 \sin(\theta)}$)
QUALITY	1J		Quadratic sum of the x and y residuals
GSX	10J	PIXEL	Guide Star x coordinate
GSY	10J	PIXEL	Guide Star y coordinate
GSCTS	10J	COUNTS	Guide Star counts integrated over PSF

$^a$See Sect. 9 for convention