APM will soon offer a new 30 mm wide angle eyepiece. In contrast to other long focal length eyepieces (e.g. Nagler 31 mm) it is rather slim (~ 55 mm maximum diameter) and can be used for binoviewing:
It is part of the Ultra-flat field series and the optical layout is available on the the APM web page:
According to this drawing and the description we can expect 70 degree AFOV, according to Marcus Ludes it is designed by an optical designer active here at CN. The eyepieces looks very interesting it will be an excellent choice for binoviewing.
Now my question:
1. what true field of view can be expected?
2. I don not understand the optical design, why is the field stop (diameter 30.4 mm) indicated in the figure so small?
My understanding of super wide angle eyepiecs with a smyth lens is the following:
The smyth lens as the first element acts as a Barlow, the image will be magnified and as the result the field stop diameter (the diameter of the aperture) is larger than the effective field stop diameter which can be used to calcute the true field of view (TFOV).
So the effective field stop diameter will be smaller than the 30,4 mm given in the drawing, but that does not fit to a wide angle 30 mm eyepieces. Then, the two optical elements following the field stop are rather small in diameter, that makes the eyepiece slim, but their diameter is even smaller than that of the smyth lens. This seems to me very strange because the smyth lens magnifies the image. This is by the way the reason why the long focal length Naglers are so fat, the first lenses following the smyth lens must be large in diameter.
Can anybody help me out? May be the disigner of the eyepiece can explain?
with many thanks in advance
edit: correct link to the APM web page
Edited by ThomasM, 16 April 2017 - 07:06 PM.