An interesting alternative route to the Ibon de Astanes is the following: on the old road to the Col du Somport ( DO NOT GO THROUGH THE TUNNEL ), coming from Oloron, stop at the " Parking du Sansanet " ( elevation: 1330m ). After crossing the Gave d'Aspe, a one hour-walk on a good trail ( half in the forest ) will lead you to the Ibon. No difficulty.
You should mention that this route requires more than 1,600m elvation gain: don't forget that from the Circo de Olibon, you go down 200m to the Llano Mistresas; these 200m need to be climbed on the way back. So, from the Parking du Sansanet, it is a 1,600m elevation hike, but from the Electric Plant, it is a 2,000m elevation hike!
Thank you, edouet. I have already incorporated your information to the route page