TRUE FIELD OF VIEW IN A TELESCOPE
Eyepieces have a characteristic called Apparent Field¡ªthis is essentially
the width of the field of view as seen by the observer. When you look
in an eyepiece, there is an edge to what can be seen and it is somewhere
off to the side. Narrowfield eyepieces have apparent fields from 30 to
52 degrees. Widefield eyepieces have apparent fields from 60 to 75 degrees.
And Ultrawidefield eyepieces have 78 to90 degree fields of view. This
last field of view is so large, it is often necessary to rock the head
from side to side to see the edge of the field when looking through the
eyepiece¡ªjust as you would have to do when looking out a ¡°porthole into
The actual field of view seen will depend on this apparent field AND
the magnification of the eyepiece. Low power eyepieces typically have
larger true fields (the real size of the field in the sky) of view. There
is a simple relationship, and it is expressed by the following formula:
AF (Apparent Field Width)
TF (True Field Width) = --------------------------------
If you know two of those numbers, you can easily derive the third.
Or, you can use our Calculator.
How important is it to know the True Field of a given eyepiece in your
scope? Objects in the sky come in many different sizes, and you want to
be sure the true field can accommodate a view of the entire object.
Every book or observing guide will list a size for each object that can
be seen. This size will determine how high a magnification you can use
and still see the entire object, preferably with some surrounding dark
sky in the image to ¡°frame¡± the object viewed.
If you don¡¯t know the Apparent Field of your eyepieces, there is another
way to measure it. You can determine the actual apparent field using the
formula by entering a measured True Field. Take a star on the Celestial
Equator (look at a planisphere or Atlas to find a relatively bright star
on the equator) and, with drive motors in your telescope turned off, time
how long it takes for a star to go from one edge in the eyepiece to the
other edge of the field:
The True Field, in degrees, is the number of minutes timed (in tenths)
divided by 4 (i.e.4 minutes timed = one degree), or the number of seconds
timed divided by 240 (240 seconds timed = one degree of arc).
The True Field, in minutes, is the number of seconds timed divided by
4 (60 seconds timed = 15' of arc), or the number of minutes timed times
15 (1 minute timed = 15' of arc)