I know how to calculate the FOV for an eyepiece but every so often I see a written specification for a telescope for a maximum FOV. How is this determined? By the eyepiece calculation I get 4.95 degrees with my Paragon 40mm in my 80mm f/550mm refractor. Am I actually seeing this wide a FOV or something less?
How to Determine Refractor FOV
Started by mte1955, 12/05/2007 01:57PM
Posted 12/05/2007 01:57PM
Opening Post
Posted 12/05/2007 05:45PM
| Edited 12/05/2007 05:49PM
#1
I think the maximum FOV of a scope is usually calculated with the same eyepiece FOV calculation that you mentioned, assuming a 7mm exit pupil - the average maximum for most peoples eyes. Refracting telescopes have no upper limits on exit pupil sizes due to central obstruction issues.
To answer the obvious question, what if you don't care about exit pupil size? from
http://www.mapug-astronomy.net/astrodesigns/MAPUG/MaxField.htm :
"... The actual field of view of the sky is determined only by the focal length of the telescope and the diameter of the most restrictive field stop in the optical system. The field stop is usually at or near the real aerial image position in the system. For a telescope this is just outside the back plate of the telescope at a position called the back focus focal plane.
The ultimate field stop is the hole in the back plane of the telescope. The practical field stop is that of the eyepiece. The size of this field stop is limited by the diameter of the eyepiece tube. On a given telescope a 2 inch eyepiece has greater maximum field of view of the sky than a 1 1/4 inch because it can intercept more of the real aerial image that the telescope objective produces.
The purpose of the eyepiece is similar to that of a fancy magnifying glass. It enables the human eye to inspect the real aerial image in fine detail. The focal length of the eyepiece determines the magnifying power of the whole system. A so called wide field eyepiece is designed in such a way that for a given focal length of the eyepiece the field stop can be made larger. The field stop can never be larger than the diameter of the eyepiece tube. (obviously) The larger diameter field stop of the wide field eyepiece actually intercepts a larger portion of the real aerial image and thus gives a view of a larger circle of the sky. It also gives what is called a larger apparent field of view. This is the size of the circle seen when one looks into the eyepiece."
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K. Michael M.
To answer the obvious question, what if you don't care about exit pupil size? from
http://www.mapug-astronomy.net/astrodesigns/MAPUG/MaxField.htm :
"... The actual field of view of the sky is determined only by the focal length of the telescope and the diameter of the most restrictive field stop in the optical system. The field stop is usually at or near the real aerial image position in the system. For a telescope this is just outside the back plate of the telescope at a position called the back focus focal plane.
The ultimate field stop is the hole in the back plane of the telescope. The practical field stop is that of the eyepiece. The size of this field stop is limited by the diameter of the eyepiece tube. On a given telescope a 2 inch eyepiece has greater maximum field of view of the sky than a 1 1/4 inch because it can intercept more of the real aerial image that the telescope objective produces.
The purpose of the eyepiece is similar to that of a fancy magnifying glass. It enables the human eye to inspect the real aerial image in fine detail. The focal length of the eyepiece determines the magnifying power of the whole system. A so called wide field eyepiece is designed in such a way that for a given focal length of the eyepiece the field stop can be made larger. The field stop can never be larger than the diameter of the eyepiece tube. (obviously) The larger diameter field stop of the wide field eyepiece actually intercepts a larger portion of the real aerial image and thus gives a view of a larger circle of the sky. It also gives what is called a larger apparent field of view. This is the size of the circle seen when one looks into the eyepiece."
------
K. Michael M.
Michael Tencza said:
I know how to calculate the FOV for an eyepiece but every so often I see a written specification for a telescope for a maximum FOV. How is this determined? By the eyepiece calculation I get 4.95 degrees with my Paragon 40mm in my 80mm f/550mm refractor. Am I actually seeing this wide a FOV or something less?
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