How can Webbs primary mirror be six times the size of Hubbles but be less massive?
There has been a lot of progress in technology since Hubble was built. The best example of weight reduction is the primary mirror, which takes up a considerable fraction of the total mass budget. The mirror has to be very accurately shaped. Any variations from the perfect shape of the mirror have to be less than a fraction of the observing wavelengths, which start at about 0.1 micrometer (in the ultraviolet) for Hubble and 0.6 micrometer (gold light) for Webb. To keep the mirror in such a perfect shape, Hubble has a thick, solid glass mirror with a mass around 1000 kg (2200 lbs on Earth). Webb’s mirror will consist of 18 thin, lightweight beryllium mirror segments, which will be kept in the right shape and place by a large number of adjustors attached to a stiff backing frame. Including the backing frame, the 18 segments of the Webb primary mirror total about 625 kg (1375 lb on Earth). These kinds of technologies, which were not available at the time Hubble was built, will be used thro
