AFOV vs TFOV - I don't understand!

[Planetesimal]

I'm trying to work out the difference between apparent field of view and true field of view.

I think I understand TFOV: since I got the telrad I've been getting more of a feel for degrees of arc. For example I read that the moon is 0.5^o, so I positioned the moon in the telrad reticle and sure enough, it fits into the central 1/2 degree circle. So much for TFOV.

Say I look at the spec on an EP or a finderscope, they always quote both AFOV and TFOV - I'm thinking AFOV must mean that if you were to see the same area of sky that you're looking at in the EP without magnification, it would equate to say 40 degrees of sky. Have I got it right, or am I wrestling with the wrong end of the stick?!

[DirkSteele]

You are pretty much spot on. Apparent field of view is the angular diameter of a circle of light the eye would see when looking through the EP on its own. The true field of view is the amount of sky, once again in degrees that the EP shows when used with a telescope.

The true field is calculated as the Apparent Field of View divided by the magnification that the EP being used has in the telescope. So for example, a standard Plossl EP with a 50 degree apparent field of view with a focal length of 20mm in a telescope of focal length 1000mm would give a magnification of 50x, and so 50/50 = 1 degree of sky.

Clear skies,

[Ags]

The apparent field of view is how wide the image circle shown by the eyepiece is. So an SR4 eyepiece has a 30 degree AFOV, and it is like looking down a narrow drinking straw. An Ethos has a 100 degree AFOV and is like looking out a window - much wider.

But both a 13mm Ethos and 21mm Ethos have a 100 degree AFOV, so what's the difference? The 13mm has nearly twice the maginifaction so it shows the same objects nearly twice the size. As the image circle formed by both EPs is the same (100 degrees) that means the 13mm can only fit about half the amount of sky into that image circle, because it is giving more magnification. So the true field of view (the amount of actual sky shown) is much smaller with the 13mm.

[Planetesimal]

Thanks both - two different ways of looking at it, I think I've got it now!