He missed Fajr prayer and found the congregation offering the eclipse prayer
Question

                If a man comes late for Fajr prayer, can he join the imam whilst he is offering the eclipse prayer?.
Praise be to Allah.
      If he knows that he is offering the eclipse prayer, then he 
      should not join him, because of the difference in nature between the two 
      prayers. Although the eclipse prayer is two rak'ahs, each rak’ah includes 
      two bowings, two standings and two recitations. So whoever does not 
      realize that the imam is offering the eclipse prayers and join the imam 
      with the intention of praying Fajr should form the intention of praying 
      separately from the imam when he realizes that the imam is offering the 
      eclipse prayer. Then he should complete his prayer as Fajr and then if he 
      wants he may join the imam for whatever is left of the eclipse prayer. 

    And Allaah is the Source of strength.