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Originally Posted by astro_south
My guess is that you see the hole as the aperture of the telescope, when in reality the hole is the field stop diameter of the eyepiece. Of course I might be wrong about your thought processes, but I hope this helps anyway
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Andrew,
Thats exactly right.
The "maximum possible TFOV of any eyepiece is given by the formula
field stop diameter (in mm) x 57.3 (1 radian) / focal length of objective.
This is simple physics and geometry. You can't exceeed this True field size. Some manufacturers falsely exaggerate the AFOV of their eyepieces. The only accurate way to calculate the TFOV of an eyepiece is to use the "drift timing method" or where possible measure the internal field stop.
For those interested in calculating the TFOV of some of their eyepieces using the drift timing method, just too see how
much "horse _ _ _ _" some manufacturers come out with, here is how you do it.
Select a star as close as possible to the celestial equator. Position the star so that it is just "outside" the FOV but set to pass across the center of the FOV of the eyepiece (if using an eq mount turn the RA drive off) using a watch or stopwatch measure the time it takes for the star to drift across the center of the FOV from edge to edge.
A star on the celestial equator will drift 15.04 arc minutes per minute of time. This translates to a simple "approximation formula"
You take the drift time in seconds and divide it by 4 to get the TFOV in arcminutes. Multiply this by 60 to get TFOV in degrees. You then back calculate to the eyepieces AFOV by multiplying the TFOV in degrees as a decimal by the magnification the eyepiece gives in that particular telescope. This will give the eyepieces AFOV in degrees.
Televue are the only company that "publish" all of their eyepiece field stop data and you will find calculating TFOV based on the field stop data closely approximates the actual field of view measured using the drift timing method. Not all eyepiece manufacturers can support the same claim and accuracy as Televue.
CS-John B