Posted By Andreas Braun
all of us know about the importance of the diameter of the field stop of a particular eypiece since it determines together with the focal length of the telescope the true field of view (TFOV). One proven method is the star-drift-method which is based on counting the time a star near the celestial equator takes to cross the field stop.
My method can be performed at daylight and even indoors.
The measurement setup is as follows:
You place a measuring rod (yardstick or alike) at a convenient distance of your telescope's objective and note the length of the part of the yardstick to be seen in the eyepiece. That's all you need. The evaluation is quite straightforward.
If you name
DT = distance of the target (the measuring rod) to the objective,
WT = width of that part of the measuring rod which is seen with the particular eypiece under test,
WE = field stop diameter to be determined,
FL = nominal focal length of the telescope,
DP = actual distance of the focal plane with the telescope focused at DT,
then the simple lens formula
1/FL – 1/DT = 1/DP
will give you the value of DP.
You now can derive your desired field stop diameter WE as
WE = WT(DP/DT)
Example: Eyepiece Nagler T5 26 mm, Telescope TEC 140 with FL = 980 mm
DT = 9.35 m ; WT = 298 mm; FL = 980 mm; DP = 1/(1/FL – 1/DT) = 1.095 m
(the short distance DT = 9.35 m has been made possible through the use of a 2"-extension of 80 mm length)
yielding WE = 298 mm(1.095 m/9.35 m) = 34.9 mm
This result is pretty much in accordance with the value of 35 mm published by Tele Vue for the Nag26. As can be seen the ratio DP/DT is nothing else but a scaling factor between the observed width of the target and the field stop diameter.
The advantage of this method is that it doesn't matter whether the field stop is accessible to direct measurement with a caliper or hidden within the eypiece.