Moravian G3-16200C1 monochrome CCD camera with EFW-3S-7 filter wheel package

Moravian EFW-3S-7 external filter wheel for Moravian G3 Series ccd cameras. Included carousel carries 7 x 50 mm round unmounted or 7 x 50.8 mm mounted filters.

Moravian filter wheels

The ability to capture individual exposures through various filters is one of the key features of the cooled CCD cameras, regardless if being used for scientific research or for astronomical photography. This is why the Gx CCD cameras were designed to include the filter wheel inside the camera head and to integrate filter wheel control into camera control from the beginning. When more than 5 or 6 filters are necessary or the filter wheel has to be used with G4 camera, the filter wheel is too large to be placed inside the camera head. External filter wheel is then the only option.

When there is no filter wheel inside the camera head, all electronics and firmware, intended to control it, stays idle. These components can be utilized to control external filter wheel with only little changes. Also the camera front shell can be manufactured thinner, the space for filter wheel is superfluous. The G2 and G3 camera design, allowing attaching of the external filter wheel, differs from the variant with internal filter wheel in two points:

• Digital electronics PCB is not equipped with internal connectors for filter wheel control, but all necessary signals are routed to single 8-pin RJ-45 connector. This connector is placed between power and USB connectors on the bottom side of the camera shell.
• Camera front shell is 12.5 mm thinner compared to shell for cameras with internal filter wheel. Back focal distance measured from camera front plane is lowered from 29 mm to 16.5 mm.

Controlling of the external filter wheel with electronics already present in the camera head eliminates the necessity to connect the external filter wheel to the host PC (with separate USB or serial cable) as well as to plug in a separate power supply. Only single short 8-wire cable connects the external filter wheel with the camera head.

Another advantage of this design is full compatibility with all software for Gx camera control. External and internal filter wheels are indistinguishable from the software point of view (besides the fact, that external wheel often offers more positions). The SIPS camera control software and also other programs using supplied drivers (The Sky, MaxIm DL, AstroArt or ASCOM compatible programs) control external and internal wheels the same way using the same drivers. When the camera is connected over the Ethernet interface, naturally it is possible to control also the external filter wheel through the TCP/IP protocol.

External filter wheel types

External filter wheels are manufactured in several sizes, depending on the camera series and also on the number and size of filters. All external filter wheels has the same 17 mm thickness (the distance between the shell back side and the front plate, to which the telescope adapters are mounted). So the distance from the front plate to detector is 33.5 mm.

Sometimes the thickness of the front side of filter wheel shell is increased from the mechanical reasons and the resulting shell thickness is then 19 mm. But 17 mm distance between the telescope adapter plate and the back shell side remains the same.

The shell diameter depends on the external filter wheel type:

• “S” series (for G2 and G3 cameras) wheel diameter is 212 mm.
• “M” series (for G4 cameras) wheel diameter is 216 mm.
• “L” series (for G4 cameras) wheel diameter is 252 mm.

Mounting external filter wheels

External filter wheels are attached to the camera head the same way like telescope or lens adapters—using four M3 screws in the corners of a square around the light entrance to the camera head. Distances of the threaded holes depend on the camera type:

    G2 and G3 cameras use four M3 screws in the corners of the 44 mm square. The single external filter wheels type can be used on both camera models.

    G3 cameras use four M3 screws in the corners of the 52 mm square.

Remark: although external filter wheels for G2 and G3 cameras are compatible, in reality the different filter size is the limiting factor. CCD detectors up to 24 × 36 mm are used in G3 cameras and any other filters than Ø50 mm (or 2 inches) cover corner parts of the chip. On the other side using such large filters for relatively small detectors, used in G2 cameras, is superfluous and it is more effective to use cheaper Ø36 mm or Ø31 mm filters. What's more, more positions are available for smaller filters.

The external filter wheel housing thickness is always the same regardless of the number of positions, filter size and camera type.

The distance of the filter wheel housing front plate from the detector is 4.5 mm greater than in the case of camera with internal filter wheel. (33.5 mm compared to 29 mm). All available adapters for telescopes and lenses, which require preserving of the back focal distance, are designed to reflect this distance. Adapters keep the defined back focal distance if mounted 33.5 mm from the CCD. When these adapters have to be used on camera with internal filter wheel, it is necessary to use 4.5 mm thick spacer.

Also Off-Axis Guider (OAG) for G2 cameras is designed to preserve 55 mm back focal distance when mounted to external filter wheel housing. It it used on camera with internal filter wheel, spacer supplied with the OAG must be used.

Installation procedure is as follows:

    1. External filter wheel has to be attached directly to the camera head. If there is some camera adapter mounted to the head, it must be removed first.

    2. The filter wheel housing front side must be unscrewed first. The front side is held on place with 8 M3 screws close to the outer edge and one central screw. So 9 screws must be removed in total. If there is an adapter on the external filter wheel housing front side, it is not necessary to remove it.

    3. The filter wheel itself, located inside the housing, can be freely removed after removing of the front side. The wheel is fixed in place by the central screw, going through the front side.

    Warning: the filter wheel position is determined by the optical bar on the wheel side. One sensor of a pair is located above the filter wheel and care must be taken when removing the wheel not to damage this sensor. Wheel must be slightly pulled and then slid. Opposite movement must be applied when mounting the wheel back.

    4. There are four countersunk holes in the wheel housing back shell. The filter wheel is fixed to the camera head using for M3 screws going through these holes.

    Warning: always use the screws supplied with the filter wheel. (M3×6 screw with countersink head). Longer screws may extend inside the camera head and may interfere with the camera shutter.

It is also necessary to connect the filter wheel with the camera head using the supplied short 8-wire cable with RJ45 connectors.