[bojan]

Magnification is pretty irrelevant term in photography...
We are actually dealing here with "scale" : pixels/arcsec or arcsec/pixel or equivalent (in the times of film photography it was mm/arcsec or arcsec/micron).
You already know the FOV, so it is not hard to calculate the scale you have with your camera and your telescope.
Magnification is meaningful term in case you are comparing two values of the same dimension - in case of visual observation, it is the ratio between apparent angular size of the object in the sky (visible by the naked eye) and apparent angular size visible in eyepiece, and it is given by the ratio of focal lengths between objective lens (or mirror) and eyepiece.