1. ## AFOV

How does one calculate AFOV knowing the stop diameter of an EP?

2. ## AFOV

Mike Thomas wrote:

AFOV = FS/FL_eyepiece * 180/Pi

Plus add up to ~5% extra AFOV for good measure if the design is a
ultrawide type. This accounts for the extra real AFOV that barrel or
pincushion distortion will contribute.

Lawrence Sayre

--
My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')

Hi Mick

4. ## AFOV

Mike Thomas wrote:

Well, there is only an approximate method (the best way is to just measure the
apparent field in a simple bench test). The old TF = AFOV/Mag formula for
true field is the one most often quoted, although it isn't quite as accurate
as the Field Stop formula. The Field Stop formula for true field is:
TF = (180/Pi)*EFSD/Fl, where EFSD is the eyepiece field stop diameter and Fl
is the focal length of the telescope. You can set the two formulae equal to
each other and then solve for AFOV:

AFOV = (180/Pi)*(EFSD/Efl), where Efl is the eyepiece focal length.

For two examples, my 27mm Kellner has a field stop diameter of 25.38mm.
Using the above formula, its apparent field calculates out to be 53.86
degrees. Its measured true field is 52.5 degrees, so its just a bit off the
mark. My Meade 14mm Ultrawide has a field stop diameter of 20.2mm. Using the
above formula, the apparent field should be 82.7 degrees. The actual
*measured* field of view of my 14mm Ultrawide is 83.1 degrees, which is
closer, but not quite dead-on. Thus the formula for calculating apparent
field of view of an eyepiece is only approximate. Clear skies to you.

--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
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5. ## AFOV

"Lawrence Sayre" <lsayre@neo.rr.com> wrote in message
news:r0GEc.8938\$2T2.6105@fe2.columbus.rr.com...

Ok but 57.3*FS / FL(ep) is the same thing and is the CFOV (calculatedFOV).
Also, you forgot to divide that value by the magnification of the EP in
question. so CFOVorAFOV / Mag = TFOV

6. ## AFOV

Mike Thomas wrote:
There is clearly no mistake here. Please check your math again. Here's
an example:

TeleVue 13mm Nagler Type 6, FS = 17.6mm

AFOV = 17.6/13 * 57.296

AFOV = 77.57 degrees (if distortion free, which it is not)

77.57 * 1.05 = 81.45 degrees AFOV (with pincushion distortion added)

Lawrence Sayre

--
My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')

7. ## AFOV

How on earth do you "measure" the measured FOV?

Happy trails to you

8. ## AFOV

Martin R. Howell wrote:
That would be my guess.
Heh.

Regards,
Etok

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10. ## AFOV

This is the exchange in question:

This is what Lawrence replied:

This is the correct formula for estimating the AFOV from the FS (Field Stop)
and the focal length of the eyepiece.

However you then wrote:

You did ask how to calculate the FOV from the FS and the FL of the eyepiece....

question. so CFOVorAFOV / Mag = TFOV

jon

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