The following table shows the eyepieces that I currently use with my 11″ Starmaster (f/5.4). Also shown in the table are the eyepieces’ apparent field of view, the magnification when used in the 11″ Starmaster, the calculated true field of view, and the measured true field of view:
| Eyepiece | Apparent FOV | Magnification | Calc. True FOV | Meas. True FOV |
|---|---|---|---|---|
| TMB Optical 3.2mm | 60o | x472 | 0.13o | 0.125o |
| TMB Optical 6mm | 60o | x252 | 0.24o | 0.23o |
| Nagler Type 6 9mm | 82o | x168 | 0.49o | 0.50o |
| Nagler Type 5 16mm | 82o | x94 | 0.87o | 0.88o |
| Panoptic 27mm | 68o | x56 | 1.21o | 1.17o |
11″ Starmaster focal length = 11in. x 25.4mm/in. x 5.4 = 1509mm.
Magnification = (scope focal length) / (eyepiece focal length)
Eyepiece calculated field of view = (Apparent Field of View) / Magnfication
The eyepiece’s calculated field of view is found in a non-tracking scope by centering a star in a given eyepiece then timing how many seconds the star takes to leave the field of view. Divide this number of seconds by 120 to get the true size of the field of view in degrees. (Repeat several times for each eyepiece to be sure of the reading.)