Fed. R. Evid. 614

(a) Calling.

The court may call a witness on its own or at a party’s request. Each party is entitled to cross-examine the witness.

(b) Examining.

The court may examine a witness regardless of who calls the witness.

(c) Objections.

A party may object to the court’s calling or examining a witness either at that time or at the next opportunity when the jury is not present.