;-------------------------------------------------------------
;+
; NAME:
;	REDSHIFT
; PURPOSE:
;	Converts between redshift, Recession velocity, and Distance
;
; CALLING SEQUENCE:
;	redshift, [h, HELP = ]
; INPUTS:
;	h = optional Hubble constant (def = 50 km/s/Mpc).      in
; OUTPUTS:
;	Results are displayed at the terminal screen
; NOTES:
;	Note: H may be changed at any time by typing h=new_value.
;	Also displays angular size equivalence and photometric information.
;
; MODIFICATION HISTORY:
;	R. Sterner. 17 July, 1987.
;	Johns Hopkins University Applied Physics Laboratory.
;	RES 7 Jan, 1988 --- added H0.
;
; Copyright (C) 1987, Johns Hopkins University/Applied Physics Laboratory
; This software may be used, copied, or redistributed as long as it is not
; sold and this copyright notice is reproduced on each copy made.  This
; routine is provided as is without any express or implied warranties
; whatsoever.  Other limitations apply as described in the file disclaimer.txt.
;	Converted to IDL V5.0   W. Landsman   September 1997
;-
;-------------------------------------------------------------
 
	PRO REDSHIFT, H0, help=hlp
 
	if keyword_set(hlp) then begin
	  print,' Converts between redshift, Recession velocity, and Distance'
	  print,' redshift, [h]'
	  print,'   h = optional Hubble constant (def = 50 km/s/Mpc).      in'
	  print,' Note: H may be changed at any time by typing h=new_value.'
	  print,'   Also displays angular size equivalence and photometric '+$
	    'information.'
	  return
	endif
 
	C = 2.9979E5	; km/s.
	H = 50		; km/s/Mpc.
	IF N_PARAMS(0) GT 0 THEN H = H0
 
	PRINT,' '
	PRINT,' ---==< Redshift >==---'
	PRINT,' Converts between Redshift, Recession velocity, Distance.'
LOOP:	PRINT,' '
	PRINT,' Enter Redshift as:'
	PRINT,'   Z = xxx'
	PRINT,' Enter Recession Velocity as:'
	PRINT,'   V = xxx (km/s)'
	PRINT,' Enter Distance as:'
	PRINT,'   D = xxx (Mpc)'
	PRINT,' To change Hubble constant (current value = '+STRTRIM(H,2)+'):'
	PRINT,'   H = xxx (km/s/Mpc)'
	PRINT,' '
	TXT = ''
	READ, ' Entry: ', TXT
	IF TXT EQ '' THEN RETURN
	TXT = STRUPCASE(TXT)
	TXT = REPCHR(TXT,'=')
	W = GETWRD(TXT, 0)
	X = GETWRD(TXT, 1)
 
	CASE W OF
'H':	BEGIN
	  H = X + 0.
	  GOTO, LOOP
	END
'Z':	BEGIN
	  Z = X + 0.
	  V = C*((Z+1)^2-1)/((Z+1)^2+1)
	  D = V/H
	END
'V':	BEGIN
	  V = X + 0.
	  Z = SQRT((1+V/C)/(1-V/C)) - 1
	  D = V/H
	END
'D':	BEGIN
	  D = X + 0.
	  V = D*H
	  Z = SQRT((1+V/C)/(1-V/C)) - 1
	END
else:	begin
	  print,' Example:  z=.329'
	  print,' '
	  goto, loop
	end
	ENDCASE
 
	PRINT,' '
	PRINT,' For H = '+STRTRIM(H,2)+' km/s/Mpc:'
	PRINT,' Redshift, Z = ',strtrim(Z,2)
	PRINT,' Recession Velocity, V = '+strtrim(V,2)+' km/s'
	PRINT,' Distance, D = '+strtrim(D,2)+' Mpc ('+strtrim(D*3.258,2)+$
	  ' million lt-yrs)'
	PRINT,' '
	print,' REDSHIFT EFFECTS'
	print,' Angular size/Linear size:'
	print,'   Angular size is increased by a factor of (1+z) = '+$
	  strtrim((1.+z),2)
	print,'     due to recession velocity.'
	t = d*tan((1./60.)/(1.+z)/!radeg)*1000.		; kpc.
	print,"   So 1' in the sky at this distance is a linear distance of "+$
	  strtrim(t,2)+" kpc,"
	print,'   So 1" in the sky at this distance is a linear distance of '+$
	  strtrim(t/60.,2)+' kpc.'
	print,' Photometry:'
	print,'   Distance Modulus = m-M = '+strtrim(5.*alog10(d*1e6/10.), 2)
	print,'   Rest wavelengths increased by (1+z) so for the UBVRI system'
	print,'     U shifts from 360 nm to '+strtrim(360.*(1+z),2)+' nm'
	print,'     B shifts from 420 nm to '+strtrim(420.*(1+z),2)+' nm'
	print,'     V shifts from 520 nm to '+strtrim(520.*(1+z),2)+' nm'
	print,'     R shifts from 680 nm to '+strtrim(680.*(1+z),2)+' nm'
	print,'     I shifts from 825 nm to '+strtrim(825.*(1+z),2)+' nm'
	print,' Press any key to continue'
	k = get_kbrd(1)
	print,' '
	GOTO, LOOP
 
	END