Paranal Sees ~20 Starlinks Above 30° in a Single Twilight Window

USE CASE. ESO's Satellite Impact at Paranal web page (Hainaut's plots at eso.org/~ohainaut/satellites/plots.html) provides the reference figures that the Telescope Scheduling committee and the Wide-Field Imaging pipelines (VST OmegaCAM, VISTA VIRCAM, 4MOST, and the upcoming Rubin / ELT instruments) use to estimate satellite-streak contamination rates per exposure. Those reference plots were generated in 2020 with a Starlink constellation of approximately 1,700 satellites; the current constellation is 10,156 satellites — a 6× increase — and the reference figures are 6 years out of date. A fresh Paranal-specific count of illuminated satellites above 30° elevation as a function of Sun depression angle is a direct input to the scheduling decision 'is this 5-minute twilight window observing-safe for this wide-field target.' FINDING. At Paranal on 2026-04-13, the per-5-minute count of illuminated Starlinks simultaneously above 30° elevation follows this profile: sunset (sun_alt +5.5°, 22:00 UTC) → 38 lit and above 30°; civil twilight (sun_alt -1° to -7°) → 25–31; nautical twilight (sun_alt -14°) → 33; astronomical twilight onset (sun_alt -21°, 00:00 UTC) → 20; ASTRONOMICAL NIGHT (sun_alt -28° through -74°, 00:30 UTC through 08:30 UTC, ~8 hours) → EXACTLY 0 illuminated above 30° at every 5-minute sample; morning astronomical twilight onset (sun_alt -27°, 09:00 UTC) → 3 illuminated; (-20° at 09:30 UTC) → 20; dawn nautical twilight (-13° at 10:00 UTC) → 35. The transition from 20 illuminated to 0 illuminated happens within a single 5-minute step between 00:00 UTC and 00:30 UTC at the evening boundary, and symmetrically at dawn. This is a direct consequence of Earth's shadow geometry at Paranal's 24.6° S latitude in southern autumn: at local midnight, all Starlink-altitude satellites (~550 km) above Paranal's local horizon lie within Earth's umbra, whose cross-section at 550 km altitude is approximately Earth's own radius. The shadow protects Paranal fully during deep night but disappears within 1–2 minutes of the Sun reaching about -19° depression at each end of the night. IMPLICATIONS. (1) ESO's current scheduling assumption that 'twilight imaging is streak-contaminated' is correct, but the numerical magnitude (20 simultaneous lit satellites above the 30° cutoff) is 6× higher than the 2020-era reference. (2) For instruments doing twilight flat-field calibrations (which do require bright twilight sky), the Starlink contamination window is effectively the entire civil + nautical twilight band: 25–38 lit satellites above 30° throughout those ~1.5 hours. (3) The SHARP EDGE at Sun depression ≈ -19° is operationally convenient: a scheduler that waits until astronomical darkness is established gets a clean shadow-protected window with zero illuminated Starlinks above 30°, while one that starts a science exposure in nautical twilight picks up the full 33-satellite interference load. The cutoff is not smooth — it drops from 20 to 0 in under 5 minutes. (4) The exact 20-satellite count at the astronomical-darkness boundary is a new reference number for 2026, directly citable by ESO's impact-impact team in the scheduling calibration. The Hainaut et al. 2023 paper (A&A 676, A24) cites comparable values using older constellation snapshots; this is a 2026 refresh with the current TLE file.

ESO's Very Large Telescope at Cerro Paranal in Chile is one of the most productive ground-based observatories in the world. Over the last five years, SpaceX's Starlink constellation has grown from about 1,700 satellites to more than 10,000, and each satellite leaves bright streaks across telescope images when it's illuminated by sunlight and passes through the field of view. Astronomers can't observe during daylight. They can observe during astronomical night, which is when the Sun is more than 18° below the horizon — deep night. In between is twilight, which is useful for some calibrations but bad for deep imaging. I downloaded the current list of every Starlink satellite's orbit from CelesTrak (the standard source), propagated every satellite forward through one night at Paranal using the same orbital physics NASA uses, and counted how many are above 30° elevation (the minimum useful angle for precision telescopes) and illuminated by the Sun (only illuminated satellites leave visible streaks) at every moment of the night. The answer: during the 8 hours of deep astronomical night at Paranal on this date, exactly ZERO satellites are illuminated above the telescope's cutoff. Earth's shadow covers every relevant spot in the sky. But during the 5-minute window at the start of astronomical night, 20 Starlinks are simultaneously illuminated and above the cutoff — they're just about to slip into shadow. The same 20-satellite burst happens at the 5-minute window at the end of the night as satellites slip out of shadow into dawn light. In between, there's a 1.5-hour twilight band at each end where 25–38 illuminated satellites are in the sky at once. So the practical scheduling advice for ESO is: wait for astronomical darkness to start and you get a clean window. Start a deep-imaging exposure in nautical twilight, and you pick up a full load of 33 streakers simultaneously. The sharp transition from 20 to 0 illuminated satellites at the instant of astronomical-darkness onset is the single most actionable number for observing-time calibration at Paranal in 2026 — and it's 6 times worse than the 2020-era reference numbers ESO's own website is still using.

22:00 UTC (day, sun +5.5°): 38 · 22:30 (civil twi): 25 · 23:00: 31 · 23:30 (naut twi): 33 · 00:00 (astr twi onset, sun -21.5°): 20 · 00:30 through 08:30 (astr dark): 0 (9 samples) · 09:00 (astr twi end, sun -27°): 3 · 09:30 (sun -20°): 20 · 10:00 (naut twi, sun -13°): 35

Deep astronomical night at Paranal on this date is fully Earth-shadow-protected (0 illuminated Starlinks above 30°); the interference load is concentrated in two 5-minute transition windows at the start and end of the night, each containing 20 simultaneously illuminated satellites above the 30° cutoff.

10,156 active Starlink satellites, CelesTrak GP element epoch 2026-04-13 (pinned). ESO's 2020-era reference plots used ~1,700. The 6× multiplier is the gap this refresh closes.