As I understand it now Jon, the 4.4 degrees is not a limit set by the TV85, but just by the eyepieces available at the market today, that's also what I originally thought, got just confused when I saw the max. FOV was smaller than on my NP101. So am I right that the only difference according to wide field view between NP101 and TV85 will be possible vignetting in TV85, if the future will bring eyepieces with even wider angles, so maybe more than 5 degrees FOV becomes a possibility?
That is not how it works.
The field of view is determined by two things, the focal length of the telescope and field stop of the eyepiece.
- The field stop is a ring you see as the edge of the field of view looking through the eyepiece. If you turn the eyepiece over, with many eyepieces you can see the field stop. With the 41mm Panoptic, you can see it as very narrow ring (1mm wide) near the top of the barrel.
For a 2 inch eyepiece, the inner diameter of the barrel is 48mm so 46mm is just about the largest field stop possible. If the field stop is smaller in diameter, then the field of view is reduced.
The objective produces an image at the focal plane and the field stop is at the focal plane of the eyepiece and determines how much of the image you see in the eyepiece.
- The objective produces the image at the focal plane and the image scale, the size of the image, depends on the focal length of the telescope. The longer the focal length, the larger the image is, this is why magnification is proportional to focal length. The larger image means a smaller field of view.
The difference between the TV-85 and the NP-101 is the focal length. The 600mm focal length of the TV-85 means that with the same eyepiece, the magnification will be:
600mm/540mm = 1.11x greater than the NP-101 and the field of view will be:
540mm/600 = 0.90x as wide.
- This is relationship between focal length, field stop and TFOV is expressed in the equation:
TFoV = Field Stop / Focal length telescope. The result is in radians so to convert to degrees, it much be multipled by 180 degrees/Pi Radians = 57.3 deg/radian.
TFoV = (57.3 deg) x (Field Stop)/(Focal length telescope)
- So, the difference in field of view is related to inherent limitations in the 2 inch eyepiece format. With a 3 inch focuser and a 3 inch diagonal, wider fields of view are possible,