ALMA can point at the Sun. Small grooves or etchings in the antenna surface are designed to prevent optical and infrared sunlight from being reflected to the secondary subreflector, where extreme heating could cause permanent damage. At the same time, the antenna surface is optimized so that millimeter and submillimeter radiation from the Sun is not scattered away, enabling scientific observations of the Sun.
When the target is not the Sun itself, however, observing close to the Sun can seriously affect calibration. When the target is close to the Sun, solar radiation contaminates the system temperature (Tsys) measurements taken with the offset positions, which can have a severe impact on the amplitude calibration. Even alternative strategies for choosing the off-source Tsys position would still compromise the data. If the science target Tsys measurements are corrupted, a manual calibration would be needed to use the most suitable Tsys measurements instead (from one of the calibrators included in the observation), but this can still degrade the amplitude calibration, especially depending on the elevation difference between the target and the calibrator used, and potentially more strongly for bright sources or near strong atmospheric features.
For this reason, ALMA observations of non-solar targets are not allowed while the target is within 15° of the Sun, unless such observations are explicitly requested and approved.
When the target is not the Sun itself, however, observing close to the Sun can seriously affect calibration. When the target is close to the Sun, solar radiation contaminates the system temperature (Tsys) measurements taken with the offset positions, which can have a severe impact on the amplitude calibration. Even alternative strategies for choosing the off-source Tsys position would still compromise the data. If the science target Tsys measurements are corrupted, a manual calibration would be needed to use the most suitable Tsys measurements instead (from one of the calibrators included in the observation), but this can still degrade the amplitude calibration, especially depending on the elevation difference between the target and the calibrator used, and potentially more strongly for bright sources or near strong atmospheric features.
For this reason, ALMA observations of non-solar targets are not allowed while the target is within 15° of the Sun, unless such observations are explicitly requested and approved.
Observations of sources within 15deg of the Sun may be made in exceptional circumstances and must be requested via the submission of a Major Change Request. See Appendix D of the ALMA Users' Policies for details and other important considerations. Proposers should also refer to the ALMA Technical Handbook, Sections 8.10 and 10.4.2, for more information on solar observations and calibration considerations near the Sun.