; docformat = 'rst'
;+
;Returns a rotation matrix for a rotation about a given primary axis by a 
;given angle. Performs a right-handed rotation. In other words, if the 
;thumb of the right hand points towards the positive sense of the axis, 
;the fingers curl in the direction of positive rotation. Think of it as 
;transforming the vector, not the coordinate system.
;
; :Author:
;   Chris Jeppesen: 07-07-2005
;   
; :Examples:
;   IDL> print,RotX(3,!dpi/6)##[[1],[0],[0]] ;Rotate (1,0,0) by 30 degrees around the Z axis
;    
;    0.86602540
;    
;    0.50000000
;    
;    0.00000000
;   
;   IDL> print,quat_vect_mult(QuatX(3,!dpi/6),[1,0,0]) ;Rotate (1,0,0) by 30 degrees around the Z axis
;    0.86602540  0.50000000  0.00000000  
;-
;+
; :Params:
;   X : in, required, type=int
;     Axis to rotate around, 1=X-axis, 2=Y-axis, 3=Z-axis.
;   theta : in, required, type=double
;     Angle to rotate, in radians.
;     
; :Returns:
;   A quaternion which will transorm the vector.
;-
function QuatX,X,theta
  axis=[0,0,0]
  axis[X-1]=1;
  return,axis_angle_to_quat(axis,theta)
end