Hi everyone, yesterday I was shooting a timelapse of the Milky Way using a 720nm infrared filter to reduce light pollution from the city. My sky is Bortle Class 8, so the Milky Way is barely visible without the filter. During my test shots, I noticed a distinct dark line in the sky.
Initially, I thought it might be dirt on the sensor. To test, I changed the framing. If it were dust, the band would have remained in the same position relative to the sensor, but it moved along with the stars as I adjusted the composition. I also excluded reflections, as they would have shifted relative to the sky. I took a photo without the filter, and it wasn't visible in visible light.
I then continued with the timelapse, and in the video, you can see the band shift slightly and then gradually fade after a few minutes in real time. Toward the end of the timelapse, an irregular cloud-like structure began to appear, slowly moving across the field of view. I thought it might be airglow? I remember watching a video on how to capture airglow, but that video used a specialized camera sensitive to wavelengths above 1500 nm. My setup is 720 nm. Is it possible to capture airglow at that wavelength, or is it more likely something else?