In the description of sight of the active magic, there is a warning that the auras seen could be false as they could be influenced by vim or imaginem spells. . My question is what would that imaginem spell look like?
W
All the thinking power of this forum and no answer!
Sodalis, need your help here 
Or is it my question that is somehow flawed ...
W
I think what it is saying is that another magus could do a CrIm effect that could mask the true magic on whatever...
So Imaginem can dupe InVi? what is the mecanic behind this?
W
CrIm(Vi) perhaps?
I'm speculating, nothing is mentioned anywhere else. But the idea is that this spell alter's the magus to detect things he ordinarily cannot, altering his sight R:Per T:Vision. An illusionist could cast a CrIm in response to just emit the desired species.
Pretty much what Jonathan said, although it would be a fairly specialized spell, I mean, it would have to be designed to affect species that wouldn't normally be produced.
Sounds like MuIm(Vi) indeed, and too much trouble IMO.
Here there be Legacy?
Indeed. That note goes back to the 3rd edition.
Really, just use MuVi on your effects to change their appearant forms instead.
How about:
PeIm 25 - Per | Conc | Ind | - Vanish
This spell allows to completely vanish from all senses. when cast by Taj the magnificent, he disappears in a puff of sparkly smoke as his sigil always makes an impressive effect. Not only will it render you imperceptible to the common senses but it will also obfuscate you from most, if not all, exotic means of perception save if one sees directly into your soul! You can interact with your environment so long as you can maintain concentration on the spell.
(Per +0 | Conc+1 | Ind +0 | Base 5 (all senses except touch), +1 Changing Image, +1 no Heat Species, +1 no Magical Species)
Feels okay...
W
No objections after a few days ... this is strange. Must be okay then 
W
I think you got it wrong here. It's simplier than that.
You seem to forget by what means Sight of the Active Magic allow you to detect magic.
It allows you to "see spells as "auras" around things and people" (p. 159).
The color let you know the Form, the aura's form, the Technique. There is always some intermediary means by which you detect things. If your Troupe finds it meaningfull to use the 'Species Theory' as fact in your Saga, I would say that that means is not some "Magical Species", but normal species that only magical means can detect. A bit like high pitch sounds that only dogs can hear.
I wouldn't allow a fail proof Imaginem spell with "+1 Magical Species" to fool all InVi spells of lower level. Instead, I would allow an Invisibility spell to fool InVi spells that rely on sight, and so on for the other senses.
The answer to your question "My question is what would that imaginem spell look like?" is:
Any spell that can change shapes and/or color of things like 'magical auras'. For exemple, if you use a CrIm spell to create a changing red aura around a door, you could fool a mage using SotAM that there is a Muto Ignem spell on that door. But such a spell would not fool InVi spells using Scent or Hearing, and maybe any spell that has a Target such as Individual or Room.
If I understand properly, you say that normal invisibility would make you invisible from InVi that use the vision. Good point. Simplifies the mechanics...
Thanks for the insight.
W
I disagree.
For exemple, Second Sight allow you to see invisible things. Because it does not rely on species. You only interpret it that way.
IIRC, Vision target spells do just that: They perceive the information, and translate it to you using one of your senses. They DO NO rely on species.
To give a more modern exemple: Imagine a hand-held radar linked to someone's brain, allowing a blind man to "see" the rough shapes of objects. If this does not yet exist, it is not impossible at all, given the existence of devices that do similar things. Putting that man in a dark room (depriving him of species) would not impede his sight at all.
=> You might make imaginem illusions that resemble what one could see with InVi (for exemple, create the illusion of a magical aura around someone), and fool him that way, but you won't be able to affect his magical senses with Imaginem: It affects species, not the senses, and InVi does not perceive/create species.
Now a mentem spell might do the trick, maybe (mentem is not my strong point), because it affects the mind and the senses (IIRC, not@home parma)
To be honest I'd prefer using MuVi for this purpose, but the original point was that the spell specifically states that it can be deceived via Imaginem magics.
Which I find odd, but can explain as legacy.
What I like about species theory is that it is all wrong! species do not exist in our reality but they are real in Mythic Europe and they are one of the few strange things that survive in Mythic Europe. Some campaigns could errata this is simply say it is a flaw in hermetic theory but I think that core setting sets species as the real thing. Species is a fancy way of stating the particle that travels to your sense and allows you to perceive what you perceive.
Second sight allows you to see things differently not just things that aren't visible. Your perception could even be altered by God/Angels/Demons/Feys/magical creatures or whatever that is from the realm from which your second sight is aligned with. So maybe you see the odor species of what that is invisible or the divine aura species that tickles the invisible magus that stands on holy ground. I admit this is an interpretation of species and second sight and the works of them but it is logical and much more prone to interesting stories.
W
SFB can see "things that aren't visible" aka don't emits species.
Perfection, such as perfect invisibility, removes stories.
Not sure what your point is here. Only visible species are visible. The others are all invisible.
And yes, perfect invisibility would in many cases remove stories.
W
Ok, I'll try it differently.
You can cast an intellego spell at an individual target, in order to gain information on it. If it succeeds, you just know what you sought.
For exemple, if wondering if a given horseshoe countains Vis, you may cast an InVi spell, Target individual, on it.
Does this relies on species? Of course not.
Now, you can cast the same spell with a sense target, in order to gain that information, not on a given individual, but on all suitable targets in range of the sense. The information is then displayed through that sense. For exemple, casting that same InVi spell with a T: Vision would enable you to perceive all the vis in range of your sight. Same with T: Smell.
Does this relies on species? No more.
YMMV, but if you decree that all perception, including that coming from Second Sight and Intellego spells (targeting or not the senses) uses species, you're amping up considerably the already powerful imaginem, since it then does allow perfect invisibility (everything is species, and imaginem affect species). This is something that, IMO, you should consider. It also tends to break down when using non-sensory targets, like the first spell above.