One of the things that puzzled me at first when I bought my telescope was the case of apparent field of view and true (or actual) field of view.

If you look through the telescope, is the area of the sky you can see how the real (or real) field of view known. The field of view (FOV) of the telescope is determined by the design of the eyepiece of the telescope.

To calculate the FoV, you must first value of the apparent FOV. Each eye has its own value of the apparent FOV, which is provided by the manufacturer of the eye.

Then work on the value of the magnification. To do this, divide the value of the focal length of the telescope by the focal length of the eyepiece. For example, if your telescope has a focal length of 650mm (that’s the case for a Sky-Watcher Explorer 130P), and your eye has a focal length of 10mm, the magnification of the telescope will be 650/10 = 65

Finally, the apparent FOV divided by the magnification, the FoV true / real For example, if the eyepiece has an apparent FOV of 50 degrees and a magnification of 65X, then the actual FOV is 50 degrees / 65X = 0.77 degrees .

If the eyepiece has a visible FoV of 55 degrees and a magnification of 65X, then the actual FoV is equal to 55 degrees / 65X = 0.85 degrees.

So, the clearer the FoV, the more FoV real / true. Also note that the lower magnification, you get more FoV at higher magnifications.

If you do not know the FOV is obtained from the eyepiece, you can true a rough estimate for the real FoV / with the following formula:

42/Magnification

So for the example above, we would get 42/65 = 0.65 degrees.

This is only an approximate figure, but it is good enough to give you an idea of ??how the sky will see, you can if you want a specific eyepiece of the telescope.