[quote=@Sophrus]
I suppose you have never heard of contingencies? Yes, the Eldar can mask their presence, Yes the astropath probably wouldnt notice them until it was too late. However, the captain probably isn't a psyker and doesnt know that. Even if he did know he certainly may order somthing that has a very low chance of success but could just as easily catch something they otherwise wouldnt have. like, i dont know an Eldar who didnt hide themselves on time, or well. Or give the ship a once over periodically in search of Chaos cults (although this would probably be a daily or weekly thing rather than something they would do in combat) 
[/quote]

To add to this, many Eldar don't even use their psychic powers beyond basic needs such as creating war masks. In fact, they're encouraged not to, I believe? Thus, it might be that many Eldar couldn't cloak their own psychic presence without the help of one of their more psychically-inclined allies. I'm not sure about that, it's just a theory. [s]A GAAAAAME THEORYYY[/s]