dsc35573.jpg

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 Hubble 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.