PRO fwhm_idl_snippet, grat
;+
; IDL procedure to plot HETGS FWHM vs wavelength.
; Usage:
;  IDL> fwhm_idl_snippet, 'MEG'
;  IDL> fwhm_idl_snippet, 'HEG'
;-

; Select grating if not specified
if n_elements(grat) EQ 0 then grat = 'MEG'

; Coefficients from http://space.mit.edu/technotes/ede_0102/ede_0102.html
if grat EQ 'MEG' then begin
  c0 = 0.018851868
  c1 = -0.00033946575
  c2 = 2.9781558e-05
  c3 = -4.1097023e-07
  lam_max = 26.0
  fwhm_max = 0.030
end else begin
  c0 = 0.010019662
  c1 = -0.00014101982
  c2 = 1.4591413e-05
  c3 = -1.9632994e-07
  lam_max = 17.0
  fwhm_max = 0.015
end

; Make a wavelength array and calculate the FWHM:
lams = lam_max * FINDGEN(101)/100.0
FWHMs = c0 + c1*lams + c2*lams^2 + c3*lams^3

; and plot it
plot, lams, FWHMs, $
	XTITLE='Wavelength (A)', $
	XRANGE=[0.0,lam_max], /XSTYLE, $
	YTITLE='FWHM (A)', $
	YRANGE=[0.0,fwhm_max], /YSTYLE, $
	TITLE='HETGS/'+grat+ ' Resolution Curve (2/27/01)'

RETURN
END