A.T. Robertson Commentary Acts 1:10

"And while they were looking stedfastly into heaven as he went, behold, two men stood by them in white apparel;" — Acts 1:10 (ASV)

Were looking steadfastly (ατενιζοντες ησαν). Periphrastic imperfect active of ατενιζω, a late intensive verb (intensive α and τεινω, to stretch). Common in Acts and also in Lu 4:20; 22:56 as well as Ac 10:4, which see.

As he went (πορευομενου αυτου). Genitive absolute of present middle participle. They saw him slipping away from their eyes as the cloud bore him away.

Stood by them (παρειστηκεισαν αυτοις). Past perfect active indicative of παριστημ and intransitive (note in B instead of ε for augment, mere itacism).