A friend asked me a very thoughtful question the other day: “Why do you observe at night?” I had been telling her about how I had been observing at night recently, explaining one of the reasons I hadn’t been around much. At first, she accepted that without much thought because of course you observe at night, right? Later, though, she asked me why we observe at night since radio telescopes can observe all the time.

Indeed, one of the advantages of radio astronomy is that observations can take place all the time. While the Sun is a radio source, it is not bright enough to prevent other observations from occuring during the day. In fact, observations are scheduled on Arecibo around the clock. You can visit this link to see a pictorial representation of the Arecibo schedule. Downtime for maintennance of the telescope preferentially occurs during the daytime, as makes sense both for daylight and worker’s schedules.

While observations can occur around the clock, ALFALFA observations occur at night (or only slightly before/after sunrise/sunset). There are two main reasons for this. The first is that the Sun can affect our observations. While it doesn’t drown the sky at radio wavelengths like it does in the optical, it is still a strong radio source. Since we observe with the telescope in a fixed position, the Sun moves relative to the receiver throughout the day. While we would avoid pointing the receiver somewhere it would cross the Sun’s path, it would come near the receiver at some point and a little bit of power would be registered by the very edges of the beam (the sidelobes). This would change the base power level (baseline) that we look for signals against. Since we are looking for very faint signals, having the baseline vary would make it much harder to find the galaxies we are interested in. The other reason for observing at night is that typically there is less RFI (radio frequency interference). RFI is generated because people use radio frequencies for communication. Since it is the result of human activitity, RFI is also often less of a problem at night.

So, we will continue to observe at night in order to help us detect those very faint galaxies. And while I may complain about the late nights and daytime sleeping, I have to admit it wouldn’t feel like observing if it happened during the day. I guess that’s the optical astronomer in me.

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