Abstract Simultaneous observations from two spatially well-separated telescopes can lead to measurements of the microlensing parallax parameter, an important quantity toward the determinations of the lens mass. The separation between Earth and Sun–Earth L2 point, ∼0.01 au, is ideal for parallax measurements of short and ultra-short (∼1 hr to 10 days) microlensing events, which are candidates of free-floating planet (FFP) events. In this work, we study the potential of doing so in the context of two proposed space-based missions, the Chinese Space Station Telescope (CSST) in a low-Earth orbit (LEO) and the Nancy Grace Roman Space Telescope (Roman) at L2. We show that joint observations of the two can directly measure the microlensing parallax of nearly all FFP events with timescales \(t_{\rm E}\lesssim 10\) days as well as planetary (and stellar binary) events that show caustic crossing features. The potential of using CSST alone in measuring microlensing parallax is also discussed.

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