Field of view

✅   Field of view


The field of view which can be achieved with an eyepiece is a crucial factor. If you look at the eyepieces available today, you will find fields of view ranging from 45° to 110°.

Here, this means the 'apparent field of view' (AFOV) of the eyepiece - that is the angle that can be seen by means of the eyepiece. But these large fields of view can be misleading. This is because the AFOV is very far from the field of view that you can actually see in the sky.

A very important criterion here is the telescope used. Different actual or true fields of view (TFOV) will be achieved, depending on the magnification used. If you know the AFOV of the eyepiece, then you can relatively easily calculate TFOV in the sky.

The magnification of the eyepiece in the telescope:

M = focal length of telescope / focal length of eyepiece

Example: You use a telescope with a 1000mm focal length and a 10mm eyepiece.

1000mm/10mm = 100X magnification

Calculation of the true field of view (TFOV):

TFOV = AFOV / magnification

As an example we take a Super Ploessl eyepiece with 52° AFOV:

TFOV = 52°/100X = 0.52° = ~30'

The field of view in the sky now has a size of 0.5° or *30 arc-minutes.
For comparison, the moon has a diameter of ~30 arc-minutes.

For comparison, here is a table with the different fields of view:

How doyou calculate the AFOV of an eyepiece if it is not provided?

Measure the diameter of the field stop at the bottom of the eyepiece. To do this, unscrew the eyepiece barrel, allowing you to easily determine the diameter of the free passage of light. The second value you need is the focal length, which can be found printed on the eyepiece. The following inverse tangent function allows you to calculate the AFOV:

AFOV = half the field stop diameter / eyepiece focal length tan-1

Not the whole field stop, but only half is used.
The result is then multiplied by 2.


Example:

I measure a field stop of 12mm with a 12.5mm focal length Ploessl eyepiece. These two pieces of information are now put into the formula with, however, only half the diameter of the field stop, i.e. 6mm:

6mm/12.5mm tan-1 = 25.6 x 2 = 51°