How must astronomers dress in their job?

Clothing is generally required.

Most observatories are on mountain tops, and are kept cold to prevent condensation from fogging the mirrors and lenses. Back in the days when astronomers had to actually LOOK through the telescopes, this meant that they generally dressed warmly.

Nowadays, all the telescope sensors are electronic, and astronomers no longer live at their observatories. Everything except the physical maintenance is done remotely, via computer. So they can dress any way they like.