[忍]

Is there an action or metamagic that allows you to make less obvious spells on the astral plane. IE improved invisibility.

[Rosa]

Yes, the metamagic Extended Masking, which allows you to extend your masking to quickened, anchored and sustained spells. You can mask up to your initiation grade in spells, though the individual spells force cannot exceed your magic.  ( SM pg. 149 )

[adzling]

also the spell Manascape, I think.

[Jack_Spade]

Especially Manascape.
You can't actually get invisible, but you can appear as the most boring aura you can imagine.

[firebug]

There is no way to make you harder to see on the astral beyond increasing background count, which isn't easy to do if you're trying to avoid drawing attention to yourself in the first place.  There's no way to just make yourself astrally invisible, sadly.

This can make a mage the bane of stealth builds as astral perception is not a visual sense, and so isn't affected by common modifiers like darkness, meaning it's tougher to sneak past them.  However, the book does say an Assensing test is needed if someone is trying to actively hide from you, so it doesn't mean they automatically find you.

Also worth noting, Manascape changes what your aura looks like, but you're still clearly a living thing--  Manascape can't remove your aura, and objects like drones don't have auras (so you can't pretend to be a cardboard box on the astral).

[Rosa]

The most stealthy result you could get would probably be a combination of Improved invisibility  plus manascape to change your aura to something like a fly or something, then use extended masking to mask both spells.  That way even if a mage or a spirit assenses the area you're in, they may simply just discount you as a harmless little gnat.  There's bound to be countless auras like that when you assense generally except in the most sterile environments.

[Slipperychicken]

I'm away from my books, but doesn't Flexible Signature do exactly this? I recall that if your initiate grade is equal to the spell's force it removes the signature completely.

[Kiirnodel]

Signature isn't the same as the aura. The aura of a spell could be detected as long as the spell is being sustained. The signature is sort of like the residue from the casting (like gun shot residue). For instantaneous spells, you mostly just have to worry about the signature (leaving behind evidence), whereas with sustained spells, the aura giving away your presence while trying to be stealthy is more concerning.