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[ATM] Eyepiece in question

Dear ATMers,

I'm trying to restore a 30 years old ATM scope (Newton 8").
I got an Erfle eyepiece of focal length f=20mm;
I measured the field stop : d = 18mm.
The telescope has a focal length F=1103mm.

I compute the angular field of view :
   FOV = 2 atan(d/2F) = 0.93 deg
so the apparent field of view :
   AFOV = F/f * FOV = 51 deg !!

An Erfle has rather 70 deg of field. Is there
a problem with my equations, or this eyepiece
will not suit well with that scope ?

Thanks for help,

--
Fabrice MAINGOT
Strasbourg - FRANCE


--
Fabrice MAINGOT
IPCMS - ULP
03.88.10.70.89
maingot@ipcms.u-strasbg.fr

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