Thread by @astro_jcm

I'm reading some comments saying that, as Starlink satellites are getting fainter to the naked eye, astronomers should stop complaining. Apart from the fact that they still flare, here's a short thread explaining why even faint satellites are an issue for us.

1/ Under dark conditions your pupil is, at most, 8 mm wide. Telescopes like VLT, Keck, Subaru, GTC or LSST are 8-10 m wide. The light collecting area goes as the diameter squared, so these telescopes collect 1 million times more light than your pupil in the same amount of time.

2/ Not every photon that hits a detector triggers a signal. Your eye registers only ~10% or less of the photons it receives. The efficiency of professional astronomical detectors can reach ~80%-90%.

3/ Your eye collects light for only 0.1-0.2 secs before sending it to your brain. Astronomical exposures are several seconds, minutes or even hours long. Satellites move fast, so they don't stay on the same pixel, but this is still a concern as you can have many long streaks.

4/ Satellites look faint if you're not in a truly dark spot. Even low light pollution will wipe out 1000s of faint stars, so satellites will look faint compared to the bright stars that are left. But they'll still be much brighter than most stars we see from astronomical sites.

5.a/ Then there's the issue of Radio Frequency Interference, which affects radiotelescopes regardless of whether the satellites are illuminated or not. Check this link for more info on RFI: public.nrao.edu/telescopes/rad…

5.b/ Image from the previous link (G.B. Taylor, NRAO/AUI/NSF). Left: image of a star taken with the Very Large Array. Right: same star, when a satellite was passing 25 degrees away on the sky (that's 50 full moons!). Note the dramatic increase in the background level and noise.

6/ So don't underestimate the impact of satellites on astronomical observations. We use really, really large "eyes" to observe the sky, and some of those "eyes" see radiation that our normal eyes don't. What's faint to you is ridiculously bright to us!

7/ Finally, don't buy arguments along the lines of "some more satellites in the sky are an acceptable price in exchange for <whatever>". Similar arguments have led us to more plastic in the oceans and more CO2 in the atmosphere.