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Hi all,
After a lot of analysing, I've realised this is a little bit more complicated than I originally thought. For a 12 inch (300mm) DOB with focal length 1500mm, the current maximum true FOV is about 1.9 degrees.
I've set a maximum Exit Pupil of 7mm. As you get older, Exit Pupil will reduce to 6mm or 5mm. Low power 2 inch eyepieces are now pushing 80 degrees apparent FOV. The theoretical limit for apparent FOV is 180 degrees. The human eye has an Apparent and also True FOV nearly 180 degrees (magnification = 1). Sharpness, however, diminishes rapidly away from viewed centre.
Now go to attached Excel Spreadsheet for your own telescope. Was zipped on a Mac but should open as an Excel Workbook.
AFOV is the Apparent FOV of the eyepiece in degrees. TFOV is the True FOV in degrees, as seen through the eyepiece in the telescope. Eyepiece focal lengths have been round off to the nearest 5mm for convenience.
Enter your telescope's aperture diameter and aperture focal length. All the rest is calculated automatically. You can compare different eyepiece combos mms/Mag/Exit Pupil against AFOV.
Numbers in blue are the TFOV for each eyepiece setting.
If you want to enter a specific eyepiece focal length, say 12mm, simply change the 50mm to 12mm.
The maximum TFOV for your telescope will occur for Exit Pupil=7 and the highest AFOV for that eyepiece focal length. I've gone all the way to AFOV 180 degrees even though eyepieces are still generally way below an AFOV 120 degrees.
Hope you find it of some use.
Regards, Rob.
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