TFOV using Drift Method
DRIFT METHOD (find TFOV of an eyepiece)
Drift Time 1 (Seconds)
Drift Time 2 (Seconds)       Average: 0 Seconds
Drift Time 3 (Seconds)

Declination  (degrees)    DECLINATION TABLE (open in new TAB or Window)
Declination (Minutes)
Declination (Seconds)



True Field of View of an Eyepiece=0.00°


Use Various Eyepiece to find True field of view of an eyepiece by using the above method. Can also find the binocular field of view to see if the manufacturer claim is true. Click Here for known star DECLINATION which will open in a new window


Measuring the field of view of an eyepiece and telescope.
When looking through a telescope, it is useful to know how wide or narrow a view you are seeing. This can help you compare what you are seeing to your star chart.
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 cos of the star’s declination. These facts can be used to calculate the field of view (angle of view) through any combination of eyepiece and telescope. It is known as the drift method.

Select any bright star which is near (within about 20° of ) the celestial equator.
Look up the declination of the star in an appropriate table.
Observe the star through the eyepiece-telescope combination you want to measure.
Turn off the motor drive. The star will drift due west (on the celestial sphere).
Adjust the declination of the telescope so that the star passes right through the centre of the eyepiece (the widest part).

Adjust the right-ascension of the telescope so the star is just out of view on the east edge.
Let the star drift into view.
Begin timing when the star first appears.
Stop timing when the star disappears off the other (west) edge of the field of view.
Calculate the elapsed time (drift time).
Repeat this measurement 2 more times.
Compare the 3 times to be sure no errors have been made.