Comprimento star trail depende de **focal length, declinação, e tempo exposição** — não single "magic number." Use fórmula abaixo, depois reference cheat sheet.
Fórmula Core
Trail length (pixels) = (2π / 86164) × focal_length_mm × exposure_sec × (sensor_width_px / sensor_width_mm) × cos(declination)
- 86164 s = dia sideral.
- Declinação = latitude celeste estrela (0° equador celeste → longest trails).
Quick-Look Tables (Full-Frame, 24 MP / 6000 px wide)
Tempo Exposição para 15° Trails (≈ 1/24 frame width)
| Focal Length | Equador (δ=0°) | Mid-Lat (δ=45°) | Polo (δ=90°) |
|---|
| 14 mm | 1.100 s (18 min) | 780 s (13 min) | 0 s (point stars) |
| 24 mm | 640 s (10.5 min) | 450 s (7.5 min) | 0 s |
| 35 mm | 440 s (7 min) | 310 s (5 min) | 0 s |
| 50 mm | 310 s (5 min) | 220 s (3.5 min) | 0 s |
| 85 mm | 180 s (3 min) | 130 s (2 min) | 0 s |
Workflow Prático (Stacking > Single Long Exposure)
- Shoot: 30–60 s exposures, interval 1–2 s, 60–300 frames. ISO 800–3200, f/2.8–f/4, manual focus at ∞ (tape ring).
- Stack: StarStaX (free), Sequator, ou Photoshop "Lighten" blend mode. Remove ruído, airplane trails, clouds.
- Post: Levels → contrast, dehaze, remove hot pixels (Dust & Scratches 1 px).
Gear Checklist
- Tripé sturdy + weighted hook.
- Intervalometer (built-in ou $15 wired).
- Dew heater strip (lente) ou hand-warmers rubber-banded.
- Headlamp **red mode** only.
- Extra batteries (frio mata capacity).
Variações Criativas
- Circumpolar circles: Apontar Polaris, stack 2–4 hrs.
- Comet/rocket trails: Single 30–60 s frame during pass.
- Foreground lit: Light-paint durante last frame (low-power LED).
Start with **24 mm, 30 s × 120 frames, ISO 1600, f/2.8**. Stack. Adjust focal length para longer/shorter trails. O céu faz o resto.
