iraf.net

NUMBER:	409
MODULE:	dispcor
SYSTEM:	V2.10-V2.11.1
DATE:	Tue Aug 25 14:43:55 MST 1998
FROM:	valdes

BUG:	When users directly add (with HEDIT rather than using REFSPECTRA)
	both REFSPEC1 and REFSPEC2 keywords without weights (where the
	weights are assumed to be 1) or the specified weights do not
	add up to 1, the resulting dispersion is wrong.  The dispersion
	values used are the weighted sum from the reference spectra but
	the bug is that the weights are not normalized to unit sum.
	Thus if both weights are 1 then the computed dispersion value
	is twice what it should be.  The workaround is to make sure
	that the weights have a unit sum.  When only a single reference
	spectrum is used it is fine to simply specify the reference
	spectrum image name since the default weight is 1.  However, if
	two reference spectra are specified then both weights must be
	specified.  The format for the REFSPEC keywords is image name
	followed by weight with a space between; for example,

	    REFSPEC1 = 'arc1 0.5'
	    REFSPEC2 = 'arc2 0.5'


STATUS:	Fixed in the next release.

NUMBER:	414
MODULE:	dispcor
SYSTEM:	V2.10-2.11.1
DATE:	Tue Nov 17 10:20:38 MST 1998
FROM:	valdes

BUG:	If the weights when interpolating between two dispersion reference
	solutions are given to more than three digits in the REFSPEC
	keywords they are internally truncated to three digits.  Using
	DISPCOR to produce non-linear dispersion coefficients in the image
	headers will have the three digit weights which may not add up to 1
	to the level of 0.1%.  This can cause small shifts in the evaluated
	wavelengths.  If DISPCOR is used to linearize the dispersion the
	same effect will apply.  This bug is related to bug 409.  The
	workaround solution is to make sure the weights in the REFSPEC
	keywords add up to 1 within the first three digits.  THIS BUG
	CAN CAUSE SYSTEMATIC ERRORS IN RADIAL VELOCITY MEASUREMENTS
	WHEN USING MULTIPLE WEIGHTED DISPERSION SOLUTIONS.

STATUS:	In future releases all occurences of the weight values in the
	REFSPEC keywords and in the non-linear coefficients will use
	eight significant digits.  Furthermore, the dispersion function
	evaluation for multiple weighted dispersion functions will
	normalize the weights to avoid any systematic error regardless
	of the weight values.