The idiot designers have made the GPC/Barlow lens 31mm in diameter instead of 31.8mm. Just to be extraordinarily difficult, you ask? Heaven knows, but once again, T-S trashes its potential to be fit for [perfectly normal] purposes.
You cannot use a Teleskop-Service binoviewer with its matching T-S GPC/Barlow with a 1.25" star diagonal. How crazy is that? The protruding 23mm x 31mm Ø "proboscis" of a GPC/Barlow stops all use of my 90° star diagonals and my Baader 45° erecting diagonal. Even if it entered to a greater depth it would probably crush the diagonal's precision mirror with its delicate, front surface coatings. Fortunately there is a stop ring built into most, if not all, 1.25" diagonals.
Once fitted with its Teleskop-Service GPC/Barlow/OCS, the Teleskop-Service binoviewer can only be used for "straight through" viewing. Let's remember that this is quite an expensive accessory. Mostly intended for use by amateur astronomers. Many of whom may well have noticed that most of the sky is well above eye level. Particularly if higher magnification is desired. The magnification provided by the GPC/Barlow can only be used at higher pointing angles demanding a star diagonal be used to avoid neck strain. Otherwise the poor "seeing" at lower pointing altitudes make high magnifications completely pointless.
So now we have the heavy Teleskop-Service binoviewer, already ridiculously cantilevered out of a flimsy 1.25" tubular fitting. Probably one sporting only a single, tiny thumbscrew for security. With its 23mm x 31mm Ø GPC/Barlow extension the T-S binoviewer cannot even reach its flimsy supporting tube with its single silly little locking screw.
The expensive binoviewer rises to "half mast" on its matching but now UNDERSIZED GPC/Barlow. Being UNDERSIZED it cannot possibly support the already tottering binoviewer. It was never designed to support the binoviewer so they deliberately made it UNDERSIZED so nobody [sensible] ever tried! Had they made it 31.8mm in diameter it could have supported the binoviewer. Even when it was lifted by the 23mm length of the GPC/Barlow.
One wonders whether the drooling designers, or far more likely, the factory managers [all obviously working well above their natural pay grade] have ever used the binoviewer, which they, themselves manufacture. Had they actually tried, they would probably have dived off the factory roof to end their shame.
Can you imagine a car manufacturer offering exclusive alloy wheels for their products? But made the wheels undersized so they wouldn't fit their own cars? Can you imagine them being made with ridiculous extra spacers so they rubbed on the bodywork and caused their cars to crash on the very fist drive? No, I can't see it happening either.
A Glass Path Correcter [GPC] or Optical Correcter [OC] is a form of modified Barlow. It uses a negative and positive lens, in addition to the normal negative "Barlow" lens. This helps to shorten the very long, glass path of the attached binoviewer. A normal Barlow is a simpler, purely negative lens designed to boost magnification alone. It cannot alter the glass path length. The distance from the Barlow to the eyepiece alters the magnification. Because the binoviewer often has 100mm, or more, in glass path length the GPC or OC makes good sense to keep magnification factors within reasonable limits. Despite the claim of 1.6x the GPC often provides 2.5-2.8x increase in magnification at the eyepiece[s].
The 1.6x refers to the glass path shortening effect. A 110mm glass path binoviewer would be shortened by 1.6x or about 65mm. This usually saves sawing lengths from expensive telescope tubes just to be able to fit a binoviewer. Without the optical correction [for glass path length] the telescope would fail to reach focus even when the focuser is fully racked in. A star diagonal adds its own glass path length. Making life even more difficult for the telescope user if they cannot reach [sharp] focus at infinity.
Teleskop-Express: TS Optics 1.6x Glass Path Corrector for binoviewers
Teleskop-Service have informed me that they are considering an update to the T-S GPC but with no date available. There is an obvious communications problem with some of their staff despite their English website service. I have now ordered a 1.25 dielectric, star diagonal with compression ring from a UK dealer. [Fingers crossed, but the long Easter weekend intervenes.]
The original binoviewer dealer has now responded to my safety concerns with an offer on a good quality, 1.25" compression ring diagonal. I hope it will solve the insecurity problem when the undersized GPC is required to support the entire binoviewer and its two eyepieces. More on this, on arrival.