Photographic lens mounting standards always define both thread or bayonet attachment and a distance of such attachment from the focal plane. This was not the case for eyepiece or camera mounting for astronomical telescopes, as telescope focusers have significantly longer travel range compared to photographic lenses, so it was generally possible to reach focus. Situation changed when low-profile, photography-optimized focusers with shorter travel appeared and especially when various optical elements, designed to suppress telescope optical aberrations (field-flatteners for refractors and RCs and coma-correctors for Newtonians), started to be used to cover wider field of view of large-detector cameras. These correctors are designed for particular distance from the focal plane and if the distance is not optimal, their performance is more or less compromised. So the back focal distance (BFD) of the camera attachment become important also in astronomy.

Attachments used of DSLR cameras (typically Canon EOS and Nikon bayonets) also define BFD and all adapters, using these standards, are mechanically designed to preserve defined distance to focal plane. Somewhat more complicated situation is with various threaded attachments. Some standards do not define BFD at all (e.g. Zeiss M44 × 1 thread) and some are used in both ways—with preserved BFD and as a thread standard only, without keeping any particular BFD. The most common representative of the second case is the Tamron thread (know also as T-thread or T2 thread etc.). The thread itself has dimensions M42 × 0.75 and if the BFD should be preserved, it has to be 55 mm.

But 42 mm diameter of the T-thread is rather limiting, somewhat wasting the available diameter of the 2-inch focusers and correctors. This is why a new standard thread M48 × 0.75 becomes more and more popular. It really nearly fills the 2-inch diameter of the focusers and limits vignetting to minimum possible with 2-inch focuser. Kind of problem is the M48 × 0.75 emerged without any standard BFD. But more and more manufactures simply switch from T-thread to M48 thread and keep all other properties (like BFD) unchanged. So many new correctors with M48 thread also require 55 mm focal distance. We decided to support this newly emerging de-facto standard and introduce adapter for G2 and G3 cameras with M48 × 0.75 thread, which at the same time preserves 55 mm BFD.

So similarly to T-thread adapters, which are available in two lengths (one offering just the M42 × 0.75 thread standard, the second longer one, also keeping the 55 mm BFD), also the M48 × 0.75 adapters are now available with exactly the same lengths.

Please note all adapters are designed to keep the BFD with Gx CCD camera with external filter wheel attached. If the camera with internal filter wheel is used, a thin spacer, compensating different BFD of camera with EFW and IFW, has to be used. Similarly for Gx camera without filter wheel at all a thick spacer, replacing the thickness of the EFW shell, must be used to preserve BFD.