iraf.net

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.