Author archive only for custom post type

According to this blogpost you just need to add the following code to your themes functions.php or in a plugin:

/* Add CPTs to author archives */
function custom_post_author_archive($query) {
    if ($query->is_author)
        $query->set( 'post_type', array('custom_type', 'post') );
    remove_action( 'pre_get_posts', 'custom_post_author_archive' );
}
add_action('pre_get_posts', 'custom_post_author_archive');