Well, in this age of COVID-19 how we define essential is taking a very different shape. Essential is no longer “how do we keep our timelines intact”. Essential is now, “what is necessary to prevent Planet of the Apes from becoming our new reality.” Zookeepers are self-quarantining with their animals to keep themselves safe and keep their animals fed and cared for. Spacecraft and telescopes can – if shut down properly – be left alone, but if you stop construction… things can go sideways.
Different facilities are taking different approaches. AURA, the organization that runs many of the telescopes on Kitt Peak, in putting out daily updates, and continuing to observe where possible in the Hawaii and Arizona facilities, but the Vera Rubin Observatory, which we still think of as the Large Synoptic Survey Telescope (LSST), is being locked up and left abandoned, partially built, on its mountain in Chile, and all scientific operations are suspended at Cerro Tololo, SOAR, and Gemini South, all in Chile. The Space Telescope Science Institute in Baltimore continues to operate Hubble, but is otherwise in telework mode only, where “Essential personal” include those that keep space craft functioning. As for JWST, well, they pushed back the submission date for observing proposals, and that is never a good sign.