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.



Find the average time for the 3 measurements. Convert the average time to an angle of view as follows:
	            (drift time) x cos(star’s declination) x 360°
Field of view = ---------------------------------------------
	  	   	               86,164 seconds
Be sure to use the same units for all like measurements (usually seconds for time, degrees for angles).


Aldebaran (brightest star in Taurus) has a declination of +16°30"

cos(16°30") = 0.9588

Drift timings:

  1. 121 seconds
  2. 119 seconds
  3. 120 seconds
Average 120 seconds
                 120 seconds x 0.9588 x 360
Field of View =  --------------------------
                       86164 seconds

              =  0.481°  =   28’ 51" 
This is about the size of the Moon.
Every combination of eyepiece and telescope will be different, but in general: