|
Today most "standard" EPs that come with scopes are plossl's. These have a typical AFOV of 50 to 52deg in the 1.25" format like Merlin mentioned, but not always.
There is a way of actually determining both the AFOV if it is not mentioned on the EP, or to verify the actual value. Aim at a star that sits along the equatorial line in the sky, or close to it. What you want to do is time its drift takes to span the FOV of the EP. You will end up with a figure in seconds (1min 20sec is 80seconds). Then divide this time value by the magnification given by the EP. This will give you the TFOV in decimal value. The AFOV is then given by manipulating the equation Merlin gave:
AFOV = TFOV X magnification
Not all EPs in a given series will have the exact AFOV mentioned. As an example a series of EP that states 68deg could have EPs that vary plus or minus a few degree, but the average is 68. The timing method is a quick & easy way to determine this.
|