only want the Admin to be able to post to.Authors not even be able to see it in the category checklist when composing a new post

gistfile1.aw

<?php
// add_filter('get_terms', 'restrict_categories');
function restrict_categories($categories) {	
 
	// If we are in the new/edit post page and not an admin, then restrict the categories
	$onPostPage = (strpos($_SERVER['PHP_SELF'], 'post.php') || strpos($_SERVER['PHP_SELF'], 'post-new.php'));	
	if (is_admin() && $onPostPage && !current_user_can('level_10')) {
		$size = count($categories);
		for ($i = 0; $i < $size; $i++) {			
			if ($categories[$i]->slug != 'site_news')
				 unset($categories[$i]);
		}
	}
 
	return $categories;
}
?>