Measuring the field of view of an eyepiece and telescope.

Background

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.

Observations

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.

Calculations

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).

Example

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

cos(16°30") = 0.9588

Drift timings:

121 seconds
119 seconds
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:

The longer the focal length of the eyepiece, the larger the field-of-view (with the same telescope).
The longer the focal length of the telescope, the smaller the field of view (with the same eyepiece).

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