The 100mm APM ED with 90 deg. deviation seems to have Amici II prisms,( from the plans shown from Binomania and linked by JCB recently) ,in which the roof line is outside the light cone. If the light cone were large, to feed a large focal plane, such as for 2 inch eyepieces or the Kasai big one (2inch?), then an Amici II would be very large and heavy. But since the cone does not intersect the roof line, the roof angle tolerance is very relaxable. So, the prism cost for Amici II is not as high as one might expect.
And because the 90 deg. APM 100mm. ED only uses 1 1/4 eyepieces, with their consequent in this case reduction of apparent field and/or exit pupil diameter and/or eye relief, depending upon the designer's tradeoffs and compromises, the Amici II size is not excessive.
So there is a cost saving for the erecting and deviating prism. It seems that a further cost saving is from the absence of ghost/fingernail or false pupil eliminating grooves in the non-reflecting sides of the rhomboid prisms which swing for interpupillary adjustment.
The WW II Schneider- Goettingen 25 x 105 , with 45 deg. deviation of the sight line, by Albert Tronnier, ( see my correspondence with him in Kuhne's section of the Seeger book) has big Schmidt roof prisms, offset away from the centerline of the light cone. The objectives are tele-objectives (Petzval ?).
The overall F/ratio is high for the body length. So long focal length, large diameter eyepieces, with their consequent improved imagery and eye relief, and wide apparent field, are used. There are none of the optomechanical inefficiencies which would come from interchangeable eyepieces.
There are no rhomboid prisms there. IPD change is via changing the distance between the telescopes. Admittedly, this would cost more to implement than simple swinging rhomboids. Images are very good at 25x.
The field corrections are superior to the WW II Zeiss 25 x 100 with aspheric eyelens Orthoscopic oculars due to Albert Koenig. For IPD change, that one translates one roof prism laterally by half the lateral translation of one of the eyepieces. That system was also used in the WW II Zeiss 12 x 60. Both of those designs have 60 deg. inclination of the sight line.
The case against rhomboid side grooves has been overstated . "enormous", was used to described the required rhomb size to be groovable. WW II Japanese 20 x 120 x 3 deg Nikko or Toko 45 deg. deviation , and their US Navy copies use Schmidt roof prisms followed by rhombloids which are not "enormous".
Again ,what is the ' fingernail' or false pupil presence or absence situation in the Fuji 45 deg deviation 25 x 150mm? Externally, they look to be Schmidt roof prism followed by rhomboid prism. We know that there is a roofline in their light cones, from testimony by owners when asked about roofline spike images, including Markus Ludes of APM.
Incidentally, that Fuji model is less than the 20,000 Euros mentioned above in #23 for a binocular telescope, at least in the USA or Japan.
Edited by Gordon Rayner, 11 February 2016 - 04:14 PM.