NAME:
	difref
 PURPOSE: (one line)
	Compute amount of atmospheric refraction relative to 5000 Angstroms.
 DESCRIPTION:
	this function is based on the formulas in Filippenko, PASP, v. 94,
	pp. 715-721 for the index of refraction of air (routine REFRAC).
 CATEGORY:
  Miscellaneous
 CALLING SEQUENCE:
	ans = difref(wave,pressure,temp,water,zenith)
 INPUTS:
	wave     - Wavelength of light, in microns.
	pressure - Atmospheric pressure, in mm of Hg.
	temp     - Atmospheric temperature, in degrees C.
	water    - Water vapor pressure, in mm of Hg.
	zenith   - Angle from the zenith, in radians.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD PARAMETERS:
 OUTPUTS:
	Returns the amount of differential refraction in arcseconds.
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
	Written 1991 Feb, by Marc W. Buie, Lowell Observatory