Can someone late for Fajr prayer join the Imam in the Eclipse prayer?

If the worshiper knows that the Imam is praying the eclipse prayer, he should not join him due to the difference in the nature of the two prayers. However, if he is ignorant and thought it was the Fajr prayer, he must intend to separate from the Imam as soon as he realizes it is the eclipse prayer. He should then complete his Fajr prayer, and if he wishes, he may join the Imam for the remainder of the eclipse prayer.