Drift Method TFOV Calculator

Determine True Field of View using Star Drift Timing

Input Measurements

Required for Apparent Field of View calculation
Calculation Formulas
TFOV = Average Time / 240
TFOV = Average Time × 0.00417

Explanation: The Earth rotates 360° in 86400 seconds (24 hours), giving 240 arcseconds per second of time.

Drift Method Guide

Background

When looking through a telescope, knowing the field of view helps compare what you're seeing with star charts. The drift method provides an accurate way to measure this.

Observation Procedure
  1. Select a bright star near the celestial equator (within ~20°)
  2. Turn off the motor drive - the star will drift due west
  3. Adjust declination so the star passes through the center of the eyepiece
  4. Adjust right ascension so the star is just outside the east edge
  5. Time from when the star appears to when it disappears off the west edge
  6. Repeat 2-3 times for accuracy
Method Illustration
Drift Method Diagram

The star drifts across the field of view due to Earth's rotation.

Note: Use this method with various eyepieces to determine their true field of view. It can also verify manufacturer claims for binocular field of view.