Concealment takes away dice from perception tests. One player says that works for matrix perception and astral perception also. Ruling?

I'd say yes to matrix (I assume for someone veiwing the scene from a camera) and no to astral as the power is a physical one and thus has no effect on the astral plane, this goes some way to explain why hell hounds and such like are popular guard animals. Obviously it wouldn't work on someone who is viewing stuff wholey in the matrix, though that is one for our philosophers.

if you're talking about spotting someone's icon on the Matrix, the answer is no, because you'd have to use the concealment power on the icon itself. it's impossible to get physical LOS on an icon. if you're talking about spotting someone on a camera, the answer is yes; the concealment power physically conceals the target.

Aside from the very valid points about "No line of sight to icons" and "Physical effects can't work on the astral," it's also important to note that the Concealment power works only on Perception checks. That's the Perception skill, even if you're defaulting on it.

Matrix perception, despite the terminology, doesn't use the Perception at all; it's Computer + Analyze. Astral perception likewise uses Assensing + Intuition instead. So the Concealment power wouldn't impact either situation.

*sniff* Now you can conceal a katana on your person if you're an adept? AWESOME!

You could always attempt to conceal a katana, Emo, it's just hard even with the appropriate trenchcoat.

If you're talking about using a spirit's Concealment power to help you out, just remember that the katana needs to be in the spirit's LOS and the spirit needs to be physically manifest the whole time.