helplog.20: apall, long slit, dispaxis

Frank Valdes wrote on Feb 04, 2005

NUMBER:	20
KEYWORDS:	apall, long slit, dispaxis
DATE:	Fri Feb  4 13:15:40 MST 2005
FROM:	valdes

Q:	I am attempting to run apall to extract spectra from images that
	have the dispersion axis running horizontally (along x-direction)
	and the spatial axis running vertically (y-direction).  However, it
	seems that this IRAF task is hard-wired to interpret data with the
	oppsite orientation, and I am having a difficult time finding the
	parameter which can be changed so that the program recognizes the
	orientation of my dispersion axis.  Currently, it chooses a
	dispersion line which runs parallel to my dispersion axis...i.e. it
	chooses a line that misses my object spectrum all together! I think
	that the program should be choosing a line that runs perpendicular
	to my dispersion axis (and consequently my object specrum) so that
	eventually the dispersion line will run across my object, from
	which I can define an aperture window from there.


A:	I think you are asking about defining the dispersion axis.  This is
	done either by a header keyword DISPAXIS or the parameter
	"dispaxis" in the parent package parameters.  For example, if you
	load APEXTRACT then use "epar apextract".  If you use KPNOSLIT then
	do "epar kpnoslit".  The values of dispaxis are 1 for dispersion
	increasing/decreasing with the column (first image axis), which is
	horizontal when viewed in an image display, and 2 for the opposite
	orientation.  From what you describe you want dispaxis=1.

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Last post on Feb 04, 2005