Contamination of spectroscopic observations by satellite constellations

Abstract

Context. With the onset of large telecommunication constellations, the number of satellites on low orbits has dramatically increased over the past years, raising concerns among the astronomical community about their impact on observations. Aims. Since spectroscopic observations represent a large fraction of professional astronomical observations and given that spectro-graphs lack spatial information that can reveal the presence of a satellite, this paper is focused on how satellites affect spectroscopic optical observations. Methods. We simulated how often satellites contaminate spectrograph observations by using realistic constellations with over 400 000 objects. We counted the satellites that crossed a representative 10 × 1" slit during a 1000 s exposure, for different solar elevations and observation directions. We also measured how a satellite affects a spectrum by using real data from different scientific targets and a scaled solar analog as the satellite. We then used standard tools to measure astrophysical parameters and compare them with the clean spectrum. Results. As expected, the fraction of affected spectra varies dramatically with the direction of the observation and the elevation of the sun, with a maximum of 10% at twilight down to no contamination when the sun reaches −30°, with a nightly average of ~2%. The probability of a satellite crossing over the astronomical target will be further reduced depending on the target’s apparent size. Because of the fast motion of the satellites and the limiting magnitude of the spectrographs, high-resolution instruments are essentially blind to most satellites. For lower resolution spectrographs, the effect on the measured astrophysical parameters depends strongly on the signal-to-noise ratio of the exposure, longer exposures on brighter targets being the least affected at ≲1%. Satellites that are brighter and/or higher than the constellation satellites, while less numerous, can also contaminate the spectra. Conclusions. Satellites (either coming from a large constellation or not) have the potential to contaminate spectra. While the fraction of affected spectra is likely to remain low even with a number of satellites about 100× larger than today, some of these contaminated spectra will be difficult to identify, as is already the case with existing satellites and asteroids. The best mitigation is to ensure that their brightness is fainter than V = 7 and that the satellites’ absolute magnitude V1000 km is also fainter than 7, and whenever possible to shoot multiple exposures.