In the Stargate SG-1 TV series it is shown prominently many times that the stargate in the SGC has a retractable iris. This iris is used to prevent matter from exiting the wormhole.
When the iris retracts it appears to pull back into the stargate itself and is completely hidden from view.
My question is, when the iris is retracted where is it stored?
Each individual "petal" of the iris looks thin enough that, once rotated about 90 degrees from the closed position, it could fit within the ring of the gate.
Of course the big problem with this is that the Stargate would have to have been designed with a hollow slot around the edge for basically exactly this purpose. A secondary problem is that the petals aren't completely flat (otherwise the "flower" formation in the center couldn't pop out), which would require said slots to be kind of thick.
So I think we have to chalk this up to aesthetic physics. The only possible place to hide the iris would be such that it covers the dialing ring and the chevrons, which is no good.
This concept-art slide (auctioned as part of the series closure) clearly indicates how the iris works and where the petals are stored when the iris isn't in use.
I've taken the liberty of animating the effect with a single petal which, as you can see fits neatly inside the bezel of the Stargate's cover:
I'm no expert on apertures, but in studying many a camera body, it appears that some apertures are made up of a lot of thinner petals, and can retract into a fairly small area. The actual movement of aperture petals is a rotation inward from the edge, they are designed in such a way that they can fit into small spaces. Think about camera bodies, they aren't very big compared to the lens mount, those petals don't "pull out" towards the edge, they rotate into conveniently compact rings.
I originally thought that the inner "spiking" of the petals wasn't physically real, but inserted to look sinister. However, it appears this is a fairly common aperture design, and quite a few Canon bodies use the technique of spiking the petals after the aperture is stopped up beyond a certain point, though for what purpose I don't know, maybe for a smoother hole and better bokah? Anyway, I just looked at the animation frame-by-frame, and sure enough, it's a continuous model, those spikes aren't coming out from nowhere, they are the actual petals spikes. Of course, it's actually computer generated, but as for being a realistic physical geometry, it works.
No question that those petals could fit fine inside the ring, they're just slightly thinner at their widest point, which is all they would need to be. For an aperture to work, as long as there is a slight overlap of petals at the largest stop, they will work, so you don't have to imagine the petals going much beyond the thickness of their widest point when completely closed. And since the "iris" is behind the rotator ring and chevrons, there's really no concern of interference. their overlap would be such that maybe 4 or so petals would overlap at any given point. If the thickness of each petals is 1 inch thick (they look pretty thin), then you need a 4 inch gap, which is fairly believable.
The biggest problem is machinery and cost. This kind of machined action is requires extreme precision and would be very expensive. Since the liquid is already computer modeled (probably a lot of it is just flat planer effect, ie: After Effects), it makes sense to just throw in a model of an iris and forgo having to make the real thing.
But from what I can see, there is really no physical reason that the iris couldn't be physically built.
I always assumed that it retracted behind the gate, but, of course, that makes no sense because it is is always described as being just above the event horizon of the wormhole generated by the gate.
So I think the answer is, basically, into a production goof/or oversight (intentional or unintentional).
While it seems implausible the iris must rectract into some small gap between the inner spinning ring and the outer ring. It's even harder to swallow that this doesn't interfer with the inner ring but the iris was based off of back engineered serpent guard helmets, several of which were in their possession after ep1. I'm pretty sure that's mentioned when it's installed in the first couple episodes after the premier. So however exactly those helmets are supposed to fold down to a stylish collar is how the iris fits into whatever small amount of space there is.
