RenderTexture is black, nothing rendered

Ok this seems to be working though I get an occasional flicker. Had to add a OnRenderImage and also play with the Offset values in my RenderTexture material.

Would appreciate ANY feedback or tweaks that might improve this! Thanks again!

 using UnityEngine;
 [ExecuteInEditMode]
 [RequireComponent(typeof(Camera))]
 public class PhysicalLogical_Blit : MonoBehaviour
 {
     public RenderTexture top;
     public RenderTexture bottom;
     public RenderTexture intermediateRT;
 
     RenderTexture rt;
     RenderTextureDescriptor mainrtdesc;
 
     Camera myCam;
     void Start()
     {
         myCam = GetComponent<Camera>();
     }
 
    void OnPreRender()
     {
          mainrtdesc = new RenderTextureDescriptor(3840,720, RenderTextureFormat.ARGB32);
         rt = RenderTexture.GetTemporary(mainrtdesc);
         myCam.targetTexture = rt; // This makes the camera render to the rt RenderTexture
     }
 
 private void OnRenderImage(RenderTexture source, RenderTexture destination)
     {
         Graphics.Blit(source, rt);
     }
 
     void OnPostRender()
     {       
         myCam.targetTexture = null;
        Graphics.Blit(rt, top);       
         Graphics.Blit(rt,null as RenderTexture);
        RenderTexture.ReleaseTemporary(rt);
     }
 }