Advanced Drift Method TFOV Calculator
Determine True Field of View with Declination Adjustment
Input Measurements
Where:
- Time = Average drift time (seconds)
- Declination = Star's declination in decimal degrees
- 86164 = Sidereal day in seconds (23h 56m 4s)
Declination Conversion: Decimal Degrees = Degrees + (Minutes/60) + (Seconds/3600)
Advanced Drift Method Guide
Stars travel westward through the sky at the rate of one revolution every 23 hours 56 minutes (86,164 seconds). Stars near the celestial equator (declination near zero) move most rapidly across the sky. Stars further from the equator move more slowly.
The adjustment factor is the cosine of the star's declination. This advanced method provides more accurate results for stars away from the celestial equator.
- Select a bright star (within ~20° of celestial equator recommended)
- Look up the star's declination in astronomical tables
- Turn off the motor drive - the star will drift due west
- Adjust declination so the star passes through the center of the eyepiece
- Adjust right ascension so the star is just outside the east edge
- Time from when the star appears to when it disappears off the west edge
- Repeat 2-3 times for accuracy and calculate average
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The star drifts across the field of view due to Earth's rotation. The rate depends on the star's declination.
Stars at the celestial equator (0° declination) move at 15 arcseconds per second. Stars at higher declinations move slower:
- 30° declination: 13 arcseconds/second
- 45° declination: 10.6 arcseconds/second
- 60° declination: 7.5 arcseconds/second
The cosine adjustment accounts for this variation in apparent motion.