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AstroBrainCalculators
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Plan

  • Tonight
  • Targets
    • Saved targets
  • Plan a night
  • Night plans
    • Logbook

Tools

  • Sky atlas
  • Field of View
  • Night Outlook
    • Weather
  • Exposure Lab
  • Calculators
  • Horizon Studio
  • Sequence Studio
    • Nights
    • Projects

Setup

  • Settings
  • About & Support
  • What’s new

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Imaging

Focal ratio calculator

AllField of viewSamplingImage scaleFocal ratioFocal length for seeingEyepieceBinocularsResolving powerDust shadowGuide scope

Telescope

mm
mm
Reducer / Barlow
×
Medium
Focal ratio
f/5.00
Effective focal length
600 mm
Time vs f/4
1.56×

Your optics work at f/5.0. For the same signal per pixel on an extended object they need 1.56× the time of f/4.

F = f / D = 600 / 120 = f/5.00 · t / t(f/4) = (F / 4)² = 1.56

Same camera, binning and throughput; extended objects; not a full signal-to-noise prediction.