XMM-Newton Science Analysis System


movecalc (movecalc-1.6) [xmmsas_20230412_1735-21.0.0]

FITS/WCS considerations

Ordinary X/Y sky images have a FITS representation that adheres to the World Coordinate System conventions specified in [1]. In practice this means that the conversion from tangent plane image coordinates $(X/Y)$ to Right Ascension/Declination of the mean geocentric equatorial reference frame and vice versa is uniquely given by the following set of column attributes:

attribute name	meaning	X/Y system	X_MOV/Y_MOV system
TCTYPn	type of projection	RA–TAN/DEC–TAN	RA–TAN/DEC–TAN
TCRVLn	R.A./Dec of tangent point	$ra_{\mbox{ref}}/dec_{\mbox{ref}}$	0/0
TCRPXn	image coordinates of tangent point	$x_{\mbox{ref}}/y_{\mbox{ref}}$	0/0
TCDLTn	coordinate increment at tangent point	$\Delta x/\Delta y$	$\Delta x/\Delta y$

In the X_MOV/Y_MOV system the tangent point is moving with time. In the absence of a true FITS/WCS standard for this situation the point is given the fixed equatorial coordinate $(0/0)$. This allows FITS image viewers like Ds9 to display 'pseudo' sky coordinates for each image pixel which shall at least be useful to estimate extents of structures in the X_MOV/Y_MOV image. Please note that the coordinate increment in both systems is the same.