hi everyone,I have a problem with shader.

Shader "Custom/Facula"
{
Properties
{
_MainTex ("Texture", 2D) = "white" {} //main texture
_MainPower ("Texture Power", Range(1.0,3.0)) = 1.6 //facula power
_MainAlpha ("Main Alpha", Range(0.0,1.0)) = 1 //contorl transparency
_TextureFac ("Texture Facula (Alpha)", 2D) = "white" {} //alpha texture,influence facula
_RimColor ("Rim Color", Color) = (1,0,0,0) //rim color
_RimPower ("Rim Power", Range(0.1,5.0)) = 0.1 //rim power
_FaculaColor ("Facula Color", Color) = (1,1,1,0) //facula color
_FaculaPower ("Facula Power", Range(0.0,2.0)) = 0.4 //facula power
}
SubShader
{
Tags {"Queue" = "Transparent"}
LOD 250
ZWrite Off
ZTest Always
CGPROGRAM
#include "UnityCG.cginc"
#pragma surface surf Lambert alpha:fade interpolateview noshadow nofog noforwardadd exclude_path:deferred
struct Input
{
float2 uv_MainTex;
float2 uv2_TextureFac;
float3 viewDir;
//float4 screenPos;
//float3 worldRefl;
};
sampler2D _MainTex;
float _MainPower;
float _MainAlpha;
sampler2D _TextureFac;
float4 _RimColor;
float _RimPower;
float4 _FaculaColor;
float _FaculaPower;

void surf (Input IN, inout SurfaceOutput o)
{
o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb*_MainPower; //main color
fixed alpha=tex2D(_TextureFac,IN.uv2_TextureFac).a; //sampler alpha texture
o.Emission = _FaculaColor*_FaculaPower*alpha; //if alpha not equal to zero,so emission facula
fixed dotValue=dot (normalize(IN.viewDir), o.Normal);
if(dotValue<=0.3) //control rim width
{
half rim = 1.0 - saturate(dotValue);
o.Emission = _RimColor.rgb * pow (rim, _RimPower); //rim color
}
o.Alpha=_MainAlpha; //control model alpha
}
ENDCG
}
Fallback "Diffuse"
}

this shader，how alter can let facula always face to camera.
all help and advise will be very thanks.