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Plan

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

Tools

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

Setup

  • Settings
  • About & Support
  • What’s new

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SharpnessNew

Focus tolerance calculator

How exact must my focus be?

All 16 calculators
SamplingFocal length for seeingGuide scopeFocus tolerance

Telescope

mm
mm
Reducer / Barlow
×

Focuser

µm

Telescope, camera and sky are shared by every calculator in this visit and kept in the link.

Your focuser lands inside the zoneDrawn to scale

Critical focus zone

±34µm

At f/5.0 focus must sit within ±34 µm of best focus — 16.8 focuser steps fit across that zone. Halve the focal ratio and the zone shrinks four times.

Full zone depth
67 µm
Focuser steps inside the zone
16.8
Focal ratio
f/5.00

With your numbers

CFZ = ±2.44·λ·F² (λ = 0.55 µm; Wodaski)

= ±2.44·0.55·5.00² = ±33.6 µm

67.1 µm / 4.0 µm = 16.8 steps

Diffraction-limited depth of focus at 550 nm; in poor seeing the usable zone is wider. The step thresholds are a rule of thumb.