Moravian instruments, Inc., source: https://www.gxccd.com/art?id=396&lang=409, printed: 16.12.2018 7:35:13
Main page Products CCD & CMOS cameras Articles
|Back focal distances of Gx cameras||Updated: 29.8.2016|
|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 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.
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.
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.
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.
Set of spacers are available to unify back focal distance of all Gx camera models with the variant with external filter wheel.
Using the spacers, single set of adapters can be used to maintain desired back focal distance for various Gx camera variants
Spacers for Canon EOS adapters are round, not rectangular like spacers for other adapters to reflect large diameter of EOS bayonet.
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.
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.
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.
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).
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.
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).
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).