What “improvements in technology” allowed the primary mirror of RST (WFIRST) to be less than 1/4 the weight of Hubble's?

The recent NASA press release Primary Mirror for NASA’s Roman Space Telescope Completed states:

While it’s the same size as the Hubble Space Telescope’s main mirror, it is less than one-fourth the weight. Roman’s mirror weighs only 410 pounds (186 kilograms) thanks to major improvements in technology.

What are the "major improvements in technology" that would allow for such a drastic reduction in weight?

• Interesting question! I was going to guess that they used silicon carbide like these instead of glass, but the article says they used glass. – uhoh Sep 5 '20 at 22:14
• Every article I read parrots that exact same phrase, but never gives any detail. Annoying. – Organic Marble Sep 6 '20 at 19:12
• @OrganicMarble there are some images here. It's a complicated Three-mirror anastigmat and so 1) doesn't look at all like Hubble, 2) has two more pretty big mirrors, 3) has a giant hole in the primary, and (4?) I don't know but there might be actuators in one of the other elements to actively compensate for small thermal drifts. Until recently it was called WFIRST so that may be a helpful search term. – uhoh Sep 7 '20 at 0:52
• A more fair, or at least interesting comparison would be the payload mass of RTS vs HST. RTS's telescope itself may not be lighter than HST's telescope for example. – uhoh Sep 7 '20 at 0:56
• @OrganicMarble - That this satellite is an NRO hand-me-down may well explain why details are so hard to find. As I've said before, the US Department of Defense would classify $F=ma$ as TS/NOFORN if they had their druthers. – David Hammen Sep 9 '20 at 7:44