X
- XCS – (observing program) XMM Cluster Survey
- XIS – (instrumentation) X-ray imaging spectrometer, an instrument on the Suzaku space telescope
- XMM – (telescope) X-ray Multi Mirror, part of the name of the XMM-Newton X-ray space telescope
- XN – (celestial object) X-ray Nova
- XRF – (celestial object) X-Ray Flash
Read more about this topic: List Of Astronomy Acronyms