Back Focal Distance of Gx Mark II cameras

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Gx Mark II cameras are available in many variants — individual G2, G3 and G4 series offer camera integration with internal or external filter wheels, Off-Axis Guider adapters, various threaded adapters as well as standard Canon EOS and Nikon lens bayonet adapters etc. One of the distinguishing feature of Mark II cameras is the telescope/lens adapter interface allowing precise adjustment of optical axis (sensor tilt). Despite the vast variability and many options, the Gx Mark II camera system was designed to maintain adapter-specified back focal distance of adjustable adapters on all camera variants.

This article summarizes back focal distances of various variants of Gx Mark II cameras. Back focal distances stated here are so-called optical, not mechanical dimensions. This means the distance includes prolonging of the back focal distance by plan-parallel glass in the optical path. However, the thickness of used filters is not included, because various filter providers use different thickness and cameras can be used without filters at all.

Design of adjustable adapters

Adjustable telescope/lens adapters are attached slightly differently depending if the adapter is attached directly to the camera head (e.g. when camera is equipped with internal filter wheel) or to the External filter wheel case.

Mark II adapters are not mounted directly on the camera head. Instead a tilting adapter base, holding the circular spring, is always used.
If the External filter wheel is used, the adapted base is not necessary, as the Mark II External filter wheel front plate is already designed to hold the spring and it also contains threads to fix respective adapters.

Adjustable adapters are mounted either on adapter base, attached to camera head, (left) or directly to External filter wheel adapter interface (right)

The necessity to use the adapter base on the camera head eliminates the need of inserting of spacers between camera head and adapter itself if the particular BFD has to be maintained. Adapter bases are manufactured in two variants, differing in thickness, to compensate BFD among camera with external filter wheel, internal filter wheel and camera without filter wheel at all.

Thin adapter base is 4.5 mm thick. It compensates the difference in BFD between camera with External filter (33.5 mm) and camera with Internal filter wheel (29 mm).
Thick adapter base measures 17 mm, which is the thickness of External filter wheel housing. If the thick adapter base is attached to camera without filter wheel (with 16.5 mm BFD), it provides mounting interface for adjustable adapters in the same 33.5 mm BFD like camera with External filter wheel.

Hint:

All documentation state Gx Mark II camera BFD 33.5 mm, measured to adjustable adapter mounting interface. This dimension can be achieved regardless if internal or external filter wheel or no filter wheel is used. However, if the camera without filter wheel is combined with the thin adapter base, resulting BFD is only 21 mm.

This combination cannot be used if particular adapter requires some back focal distance (for instance OAG or Canon EOS and Nikon bayonets etc.).

G2 Mark II cameras

G2 cameras use only “S” (small) adjustable adapters.

Back Focus Distance between sensor and adjustable adapter mounting interface:

G2 camera with External filter wheel G2 camera with Internal filter wheel G2 camera without filter wheel

Back Focus Distance between sensor and thin adjustable adapter mounting interface on the camera without filter wheel:

G2 camera without filter wheel

Camera with M42 × 0.75 (T-thread) or M48 × 0.75 threaded adapters with 55 mm BFDBFD

G2 camera with External filter wheel G2 camera with Internal filter wheel G2 camera without filter wheel

Camera with M42 × 0.75 or M48 × 0.75 threaded “short” adapters

G2 camera with External filter wheel G2 camera with Internal filter wheel G2 camera without filter wheel

Camera with Canon EOS bayonet adapter (EOS adapter is available in both S and L adapter sizes, EOS adapter is available in both “S” and “L” adapter sizes, “S” size adapter is required for G2)

G2 camera with External filter wheel G2 camera with Internal filter wheel G2 camera without filter wheel

Camera with Nikon bayonet adapter

G2 camera with External filter wheel G2 camera with Internal filter wheel G2 camera without filter wheel

Camera with Off-Axis Guider Adapter with M42 × 0.75 (T-thread) or M48 × 0.75 thread

G2 camera with External filter wheel G2 camera with Internal filter wheel G2 camera without filter wheel

G3 Mark II cameras

G3 cameras can use both “S” (small) and “L” (large) adjustable adapters. The size (diameter) of the adjustable adapter interface depends on the adapter used. For instance Nikon bayonet, M42 × 0.75 (T-thread) or M48 × 0.75 threaded adapters are available in “S” variant only, G3-OAG or M68 × 1 threaded adapter require “L” variant of adapter base.

Adjustable adapters are mounted on adapter base when camera with internal filter wheel or camera without any filter wheel is used or directly on the external filter wheel front surface. This means both “S” and “L” adapter bases can be mounted on any G3 camera, but external filter wheels are made for one particular adapter size only:

“S” external filter wheels are compatible with “S” adapters
“M” and “L” external filter wheels are compatible with “L” adapters

Back Focus Distance between sensor and adjustable adapter mounting interface:

G3 camera with External filter wheel G3 camera with Internal filter wheel G3 camera without filter wheel

Back Focus Distance between sensor and thin adjustable adapter mounting interface on the camera without filter wheel:

G3 camera without filter wheel

Camera with M42 × 0.75 (T-thread) or M48 × 0.75 threaded adapters with 55 mm BFD (requires “S” adapter base or “S” External filter wheel)

G3 camera with External filter wheel G3 camera with Internal filter wheel G3 camera without filter wheel

Camera with M42 × 0.75 or M48 × 0.75 threaded “short” adapters (requires “S” adapter base or “S” External filter wheel)

G3 camera with External filter wheel G3 camera with Internal filter wheel G3 camera without filter wheel

Camera with M68 × 1 threaded adapters (requires “L” adapter base or “M” or “L” External filter wheel)

G3 camera with External filter wheel G3 camera with Internal filter wheel G3 camera without filter wheel

Camera with Canon EOS bayonet adapter (EOS adapter is available in both “S” and “L” adapter sizes, “S” External filter wheel needs “S” variant, “M” and “L” External filter wheels need “L” variant)

G3 camera with External filter wheel G3 camera with Internal filter wheel G3 camera without filter wheel

Camera with Nikon bayonet adapter (requires “S” adapter base or “S” External filter wheel)

G3 camera with External filter wheel G3 camera with Internal filter wheel G3 camera without filter wheel

Camera with Off-Axis Guider Adapter with M68 × 1 thread (requires “L” adapter base or “M” or “L” External filter wheel)

G3 camera with External filter wheel G3 camera with Internal filter wheel G3 camera without filter wheel

Remark:

Technically the G2-OAG (small size) adapter can be mounted on G3 camera. But the G2-OAG mirror distance from optical axis is small and it may partially cover large sensors used in G3 cameras. This is why G2-OAG is not supplied with G3 cameras.

G4 Mark II cameras

G4 cameras use only “L” (large) adjustable adapters.

Remark:

Please note the G4 camera is not available with internal filter wheel. Large 37 × 37 mm sensors, used in G4 cameras, require 50 × 50 mm square filters, which are too big for internal filter wheel.

Back Focus Distance between sensor and adjustable adapter mounting interface:

G4 camera with External filter wheel G4 camera without filter wheel

Back Focus Distance between sensor and thin adjustable adapter mounting interface on the camera without filter wheel:

G4 camera without filter wheel

Camera with M68 × 1 threaded adapters

G4 camera with External filter wheel G4 camera without filter wheel

Camera with Canon EOS bayonet adapter (EOS adapter is available in both “S” and “L” adapter sizes, “L” size EOS adapter is required for G4)

G4 camera with External filter wheel G4 camera without filter wheel

Camera with Off-Axis Guider Adapter with M68 × 1 thread (requires “L” adapter base or “M” or “L” External filter wheel)

G4 camera with External filter wheel G4 camera without filter wheel