Taking snapshots of a image in Unity - unity3d

Question

I am trying to take snapshots of materials I used in my application in Unity. I simply add a directional light and a camera and in a perspective mode. Then I render the result to a texture and save it as a .png file. The result is good but there is a strange gizmo like figure in the middle of image. Here it is :
Camera and light is far enough from the object. Also I disabled light to see if it is caused by directional light. But didn't solve. Anyone knows what cause this elliptic figure? Thanks in advance.
Edit. Here is the code
public static Texture2D CreateThumbnailFromMaterial(Material _material, string _name, string _path)
{
GameObject sphereObj = GameObject.CreatePrimitive(PrimitiveType.Sphere);
sphereObj.name = _name;
sphereObj.GetComponent<Renderer>().material = _material;
Texture2D thumbnailTexture = CreateThumbnailFromModel(sphereObj, _path);
sphereObj.GetComponent<Renderer>().material = null;
Object.DestroyImmediate(sphereObj.gameObject);
return thumbnailTexture;
}
public static Texture2D CreateThumbnailFromModel(GameObject _gameObject, string _path)
{
Texture2D thumbnailTexture = new Texture2D(textureSize, textureSize);
thumbnailTexture.name = _gameObject.name.Simplify();
GameObject cameraObject = Object.Instantiate(Resources.Load("SceneComponent/SnapshotCamera") as GameObject);
Camera snapshotCamera = cameraObject.GetComponent<Camera>();
if (snapshotCamera)
{
GameObject sceneObject = GameObject.Instantiate(_gameObject) as GameObject;
sceneObject.transform.Reset();
sceneObject.transform.position = new Vector3(1000, 0, -1000);
sceneObject.hideFlags = HideFlags.HideAndDontSave;
// Create render texture
snapshotCamera.targetTexture = RenderTexture.GetTemporary(textureSize, textureSize, 24);
RenderTexture.active = snapshotCamera.targetTexture;
// Set layer
foreach (Transform child in sceneObject.GetComponentsInChildren<Transform>(true))
{
child.gameObject.layer = LayerMask.NameToLayer("ObjectSnapshot");
}
// Calculate bounding box
Bounds bounds = sceneObject.GetWorldSpaceAABB();
float maxBoundValue = 0f;
if (bounds.IsValid())
{
maxBoundValue = Mathf.Max(bounds.size.x, bounds.size.y, bounds.size.z);
}
double fov = Mathf.Deg2Rad * snapshotCamera.GetComponent<Camera>().fieldOfView;
float distanceToCenter = (maxBoundValue) / (float)System.Math.Tan(fov);
cameraObject.transform.LookAt(bounds.center);
cameraObject.transform.position = bounds.center - (snapshotCamera.transform.forward * distanceToCenter);
cameraObject.transform.SetParent(sceneObject.transform);
snapshotCamera.Render();
thumbnailTexture.ReadPixels(new Rect(0, 0, textureSize, textureSize), 0, 0);
thumbnailTexture.Apply();
sceneObject.transform.Reset();
snapshotCamera.transform.SetParent(null);
RenderTexture.active = null;
GameObject.DestroyImmediate(sceneObject);
GameObject.DestroyImmediate(cameraObject);
// Save as .png
IO.IOManager.Instance.SaveAsPNG(_path + thumbnailTexture.name, thumbnailTexture);
}
return thumbnailTexture;
}
And here is my camera properties