According to my notes, the "ring" at the center of this picture is the
structure to which Hubble gets affixed during a servicing mission. As such, I
believe this is the "flight support system" (FSS) scaffold. The FSS is
described as providing "structural, mechanical and electrical interfaces
between Hubble and the space shuttle. It serves as a maintenance platform,
holding Hubble securely in place as astronauts work on the telescope. After
servicing is complete, the support system enables astronauts to safely return
Hubble to its orbit."
dsc35693.jpg shows a
mockup of the aft end of the HST which mates with the FSS.
Page 4 of STS-109
Hubble Space Telescope Servicing Mission-3B flight Readiness
Review [direct link to 252K PDF] shows the location the FSS took
in the cargo bay on the previous servicing mission. Page 5-32 and 5-33 of the
Space Telescope Servicing Mission 3A Media Reference Guide [direct link
to 685K PDF] (p. 32-33 of the PDF) contains additional information and shows
additional diagrams of the FSS.