How do I add alpha writes to this shader?

I draw the camera output to a RenderTexture with sprites drawn with the following shader (2D normal mapped sprite). The sprites appear when drawn to the screen, but are invisible when drawn to the RenderTexture. What do I need to change to write opacity to the RenderTexture? Thanks!

 Shader "SG/_ShipSurfaceShader" {
 Properties {
     _MainTex ("Main texture", 2D) = "white" {}
     _NormalTex ("Normal map", 2D) = "bump" {}
     _SpecColor ("Specular colour", Color) = (1,1,1,1)
     _SpecPower ("Specular power", Range (0.0,2.0)) = 0.5
 }
 SubShader {
     Tags { "Queue"="Transparent" }
     Blend SrcAlpha OneMinusSrcAlpha
     LOD 200
     
     CGPROGRAM
     #pragma surface surf BlinnPhongTransparent alpha

     inline fixed4 LightingBlinnPhongTransparent (SurfaceOutput s, fixed3 lightDir, half3 viewDir, fixed atten) {
         half3 h = normalize (lightDir + viewDir);
         fixed diff = max (0, dot (s.Normal, lightDir));
         float nh = max (0, dot (s.Normal, h));
         float spec = pow (nh, s.Specular*32.0) * s.Gloss;
         fixed4 c;
         c.rgb = (s.Albedo * _LightColor0.rgb * diff + _LightColor0.rgb * _SpecColor.rgb * spec) * (atten);
         c.a = s.Alpha;
         return c;
     }

     sampler2D _MainTex;
     sampler2D _NormalTex;
     half _SpecPower;

     struct Input {
         float2 uv_MainTex;
     };

     void surf (Input IN, inout SurfaceOutput o) {
         fixed4 c = tex2D (_MainTex, IN.uv_MainTex);
         o.Albedo = c.rgb;
         o.Alpha = c.a;
         o.Normal = UnpackNormal(tex2D (_NormalTex, IN.uv_MainTex));
         o.Specular = _SpecPower;
         o.Gloss = 1;
     }
     ENDCG
 } 
 FallBack "Diffuse"
 }