Exegesis:

ōphthē de autō aggelos ap’ ouranou ‘there appeared to him an angel from heaven,’ cf. on 1.11. ap’ ouranou may go with ōphthē, adding an idea of ‘coming’ to ‘appearing,’ or with aggelos ‘an angel from heaven,’ preferably the former.

enischuōn auton ‘strengthening him,’ denoting the purpose of the appearing.

enischuō ‘to give, or inspire strength,’ hence ‘to strengthen,’ of moral strength.

Translation:

There appeared to him an angel from heaven, or, ‘coming from heaven,’ cf. on 1.11.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.