Back focal distances of Gx cameras

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Gx CCD cameras are available in many variants. There are small guiding and planetary G0 and G1 cameras, cooled G2 series, large format G3 and G4 series. G2, G3 and G4 cameras are provided without filter wheel and with internal or external filter wheel. These variants have different mechanical dimensions and naturally offer different back focal distance.

This article summarizes back focal distances of various variants of Gx CCD cameras. Back focal distances stated here are so-called optical, not mechanical dimensions. This means the distance calculates with prolonging of the back focal distance by plan parallel glass in the optical path.

G0 cameras

G0 CCD cameras are equipped with standard 1.25" (31.7 mm) eyepiece adapter, so they can be used instead of any 1.25" standard eyepiece. The adapter is a part of the camera body and cannot be removed or replaced.

G0 CCD camera

A = 22 mm

Back focal distances for G0 cameras

G1 cameras

G1 CCD cameras are equipped with CS-mount standard mount (1" thread with 32 TPI). Back focal distance for CS adapter is standardized on 12.5 mm. CS-mount adapter can be easily converted to C-mount adapter using 5 mm ring.

Another variant of G1 cameras with low-profile T-thread (M42 × 0.75) adapter is available. Note the back focal distance of this variant is only 14 mm, not 55 mm required by T-thread mount.

Special adapter for G1 cameras called “G1-to-G2 adapter” is available allows usage of any adapter intended for G2/G3 cameras on G1 models. This adapter prolongs back focal distance of G1 camera to 29 mm, which corresponds to G2 camera with internal filter wheel, and offers four M3 threaded holes for adapter attachment. All adapters for G2 cameras (Nikon bayonet, Canon EOS bayonet including clip filter variant, M48 × 1 thread, T-thread, etc.) then could be used with G1 cameras and with proper back focal distance required by particular mounting standard.

G1 camera with CS-mount adapter G1 camera with low-profile T-thread adapter G1 camera with “G1-to-G2” adapter

A = 12.5 mm D = 14 mm A = 29 mm

Back focal distances for G1 cameras

Remark:

Individual G1 adapters (CS-mount, low-profile T-thread and G1-to-G2 adapter) cannot be exchanged by the user. Camera must be ordered with required adapter and if another adapter is required, G1 camera must be sent to the manufacturer.

G2 and G3 cameras

Remark:

Back focal distances are demonstrated on G2 camera body, but G3 camera models differ only by front-side outline, camera body height and back focal distances are exactly same like in the case of G2 cameras. So all dimensions state here are valid also for G3 cameras.

G2 and G3 camera back focal distance depends on the particular variant.

Gx camera without filter wheel Gx camera with internal filter wheel Gx camera with External filter wheel attached

A = 16.5 mm A = 29 mm A = 33.5 mm

Back focal distances for G2 and G3 camera head variants

Set of spacers are available to unify back focal distance of all Gx camera models with the variant with external filter wheel.

Gx camera without filter wheel and long spacer Gx camera with internal filter wheel and short spacer Gx camera with External filter wheel attached

A = 33.5 mm A = 33.5 mm A = 33.5 mm

Back focal distances for G2 and G3 camera head with spacers unifying back focal distance

Using the spacers, single set of adapters can be used to maintain desired back focal distance for various Gx camera variants

Gx camera without filter wheel + Canon EOS adapter Gx camera with internal filter + Canon EOS adapter Gx camera with External filter wheel attached + Canon EOS adapter

B = 44 mm B = 44 mm B = 44 mm

Back focal distances for G2 and G3 camera head variants with Canon EOS lens adapter

Remark:

Spacers for Canon EOS adapters are round, not rectangular like spacers for other adapters to reflect large diameter of EOS bayonet.

Gx camera without filter wheel + Canon EOS adapter for Clip filters

B = 44 mm

Back focal distances for G2 camera head with Canon EOS lens adapter for Clip filters

Remark:

Canon EOS adapters for Clip filters is designed for G2 cameras with internal filter wheel only. It is not possible to use it on cameras with external filter wheel due to height limitations. This should cause no problem, because external filter wheel offers enough positions for filters and there is no need to use clip filters in such case.

It is also not possible to use it with G3 cameras, because the Clip filter is too small for large detectors used in G3 cameras.

Gx camera without filter wheel + Nikon adapter Gx camera with internal filter + Nikon adapter Gx camera with External filter wheel attached + Nikon adapter

C = 46.5 mm C = 46.5 mm C = 46.5 mm

Back focal distances for Gx camera head variants with Nikon lens adapter

Gx camera without filter wheel + T-thread adapter Gx camera with internal filter + T-thread adapter Gx camera with External filter wheel attached + T-thread adapter

D = 55 mm D = 55 mm D = 55 mm

Back focal distances for Gx camera head variants with Tamron T-thread adapter

G3 cameras with M68 × 1 adapter

When a “photographic full frame” sensor (24 × 36 mm) is used in the G3 camera, many telescope adapters compatible with the G2 cameras cannot be used due to vignetting. So the G3 cameras are by default shipped with M68 × 1 inner threaded adapter.

G3 CCD camera without filter wheel + M68 × 1 adapter G3 CCD camera with internal filter wheel + M68 × 1 adapter G3 CCD camera with external filter wheel + M68 × 1 adapter

E = 30.5 mm E = 43 mm E = 47.5 mm

Back focal distances for G3 cameras with M68 × 1 inner thread adapter

G4 cameras

Large-detector G4 CCD cameras cannot be equipped with internal filter wheel, because large 50 × 50 mm square filters must be used not to cause vignetting. Also common adapters, typically limited to 2 inches diameter (50.8 mm), like T-thread M42 × 0.75, M48 × 1 thread, Nikon bayonet etc. cannot be used with G4. This is why G4 cameras are by default shipped with M68 × 1 inner threaded adapter.

The back focal distance from the camera front plane to the detector is the same like in the case of G2 and G3 cameras without filter wheel and with external filter wheel (16.5 mm and 33.5 mm for EFW variant). The M68 × 1 thread adapter, shipped as a default mounting adapter of all G3 and G4 cameras, is 14 mm thick.

G4 CCD camera without filter wheel + M68 × 1 adapter G4 CCD camera with external filter wheel + M68 × 1 adapter

E = 30.5 mm E = 47.5 mm

Back focal distances for G4 cameras with M68 × 1 inner thread adapter

A different version of the M68 × 1 thread adapter was manufactured, but it is now discontinued due to its complexity (it consisted of two pieces — an adapter attached to the camera body with outer thread and a nut, which screwed on the adapter and because the nut was twice as long as the adapter itself, it created inner thread).

G4 CCD camera without filter wheel + M68 × 1 combined adapter G4 CCD camera with external filter wheel + M68 × 1 combined adapter

E = 24 mm, F = 31.5 mm E = 41 mm, F = 48.5 mm

Back focal distances for G4 cameras with double-piece M68 × 1 thread adapter

Another alternative is Canon EOS photographic lens adapter, which has enough diameter not to cause significant vignetting. This adapter is compatible with External Filter Wheel and preserves 44 mm back focal distance.

Off-Axis Guider adapters

OAG adapters exist in two variants, one for G2 cameras (with T-thread M42 × 0.75 and M48 × 0.75 thread interface) and the second one for G3 and G4 cameras with M68 × 1 thread interface (the difference between G3 and G4 model is in the distance of threaded holes, intended for fixing of OAG to camera body, and in the reflecting mirror distance from the optical axis).

G2 OAG with M42 × 0.75 (T-thread) or M48 × 0.75 thread G3 and G4 OAG with M68 × 1 thread

BFD = 55 mm BFD = 61.5 mm

Back focal distances for OAG adapters for G2 and G3/G4 cameras

Filter wheel distances

Gx series cameras can be equipped with internal (G2 and G3 series) or external (all G2, G3 and G4 series) filter wheel. Regardless on camera model and filter dimensions or number of filters in the wheel, distanced between filter wheel and CCD detector are always the same. Only difference is whether the filter wheel is internal or external (internal wheels are naturally located closed to the detector, it is not necessary to include the thickness of the camera shell and external filter wheel housing).

G2 and G3 cameras with internal filter wheel G2, G3 and G4 cameras with external filter wheel

A = 21 mm B = 25 mm

Distances of the CCD from the back side of filter wheels for internal and external filter wheel models

Distance between filter wheel back side (defined on images above) and the filter front side itself depends on the wheel type and how the filter is fixed in the wheel:

Filters in wheels without threads, intended for filters without cells (including 50 × 50 mm square filters), are 2 mm above the filter wheel back site (closer to the lens).
Filters in wheels with threads, but used with filter without cells (e.g. 31 mm filters in wheel for 1.25" cells or 50 mm filters in wheel for 2" cells), are 1 mm above the filter wheel back site (closer to the lens).
The distance of filters in threaded cells, screwed into wheels with threads, depends on the construction of the filter cells. The filter wheel is 3 mm thick and offers 2 mm of thread, located on the front filter wheel side (further from the CCD), so typically the filter front side is approximately in the same distance as the filter wheel back side.