What is the meaning of the three letters in the halo of the Acheiropoieta?

In many renditions of the **Acheiropoieta**, three letters are present in the halo around Christ's face: **omega, omicron, eta**. (This is the usual clockwise, left-to-right order; but in other versions the same letters are present in the order omicron, omega, eta.) **What is the meaning of these letters?** I heard **in a tour of a Greek Orthodox church ** that the letters refer to **God's revelation of his name in Exodus 3**. However, the crucial words in the Septuagint - the historic Greek-language version of the Old Testament - are ἐγώ εἰμι ὁ ὤν ("I am who I am"). None of these words includes the letter eta. Is it that case that the three letters are actually omicron, omega, **nu** - which is to say that they spell out ὁ ὤν ("the one who is")?