Flase wrote:Is that a planetary nebula just to the left of the star near the centre, or is that just an artifact of refection in the lens?
My guess is that it isn't a planetary nebula.
This is a starforming region dominated by young stars. There are some extremely massive stars here, particularly Eta Carina itself, but there are other heavyweights too. They have not gone supernova yet.
A star that is massive enogh to go supernova will not leave a planetary nebula behind, but a supernova remnant along with a neutron star or a black hole. If that ring-like object is really a planetary nebula, it must have been made by a star that was considerably less massive than the dominant stars of the Carina Nebula. But the more massive a star is, the faster it will burn out and die. It the ring-like object is a planetary nebula, it must have been made by a star that was much older than the dominant stars of the Carina Nebula as well as less massive.
In short, the Carina Nebula with its bright stellar behemoths is not the place where we would expect to find many planetary nebulae. Of course we can't disregard the possibility that the object is indeed a planetary nebula and that it is a member of an older population in the Carina Nebula.
But there is another thing that seems slightly wrong to me if the ring-shaped object is a plantery , and that is the combination of its color and structure. If the object is a planetary, then the blue color almost certainly means blue-green OIII emission. However, OIII emission is chiefly found in very rarefied conditions, and that ring seems pretty "thick".
So I would guess that the ring is an artifact of some kind.