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Message: <blockquote data-quote="John Russell" data-source="post: 2760173" data-attributes="member: 22179">As Henry mentioned above, it's really quite simple. One only has to mount the bin on a tripod with a calibrated panorama head and view a vertical line through the objective at the left and right edges of the field stop.However, I should point out that as it will be difficult to align the exit pupil with the axis of rotation, it is preferable to choose a distant object like the edge of a building to reduce parallax errors. Nevertheless, one should be able to attain an accuracy of +/-1°.An alternative method is to mount the bin perpendicular to a wall with its eyepieces facing the wall. A laser (ideally green) is then shone through an objective and the extremities of its reach, left and right, are marked on the wall.The AFOV is then 2.arctan D/2d, where D is the measured distance on the wall and d is the distance from the exit pupil to the wall.John</blockquote>