Filterwheels

For colour imaging with the first of my CCD-camera's I used a small MYC-filterwheel (Magenta, Yellow and Cyan), built inside of a 1.25" (31.75mm) manual focusing unit. See here for an impression of this first focuser. It also contains a shutter (it's shown without the neat cover!).
When the chipsize substantially increased with my FTT1010-M camera (with a 17mm diagonal, requiring at least ~25mm clear filter aperture at f/D=5.7), I switched to a larger design, 4-position LRGB-filterwheel. It contains 50mm diameter RGB-filters from Edmund Optics. Since the focusing range of my focusers is limited and I (historically) chose for a focal plane close to the side of the telescope tube the extra pathlength introduced with the hardware of the filterwheel is as small as possible. I managed to construct the filterwheel such that the extra pathlength is below 5mm. The RGB-filters do not block infra-red light and as such an IR-blocking filter is used in front of the CCD-camera.
With a growing interest in SHO-imaging and, also for the large FTF2020-M chip (35mm diagonal) I constructed a 7-position (i.e. LRGBSHO) filterwheel. The Baader Planetarium 2" SHO narrow-band filters were taken from their holder and have a diameter of 47-48mm. The S-II and O-III filters are glued into the pockets of the filterwheel. For the H-alpha filter I embedded two pieces of Neodymium magnet to the side of the pocket, keeping the filter in place with two small pieces of sheet metal. It can easily be taken out and used for other H-alpha photographing purposes (for instance with standard SLR-camera objectives).

Sketch of 4-position filterwheel
Photograph of 4-pos. filterwheel without cover
Sketch of 7-position filterwheel
Photograph of 7-pos. filterwheel without cover
Filterwheel front and back
The FTT1010-M camera with the 7-pos. filterwheel
Manual IR-filter
Drawings and images above show the designs of the 4- and 7-position filterwheels. From left to right:
  • This drawing shows some constructional details of the 4-position filterwheel that can also be found in the design of the 7-position version: The shaft of a small DC-motor is coupled to the rim of the filterwheel through a rubber pressure roll (both taken from a cassette recorder). A microswitch switches between powering the motor in between the notches in the rim of the filterwheel, and braking the motor when it hits a notch. Braking means switching off power and short-circuiting the motor. While in a notch the motor starts turning the wheel when the microswitch is temporarily overruled by an external (transistor) switch. See the schematics.
  • A photograph of the 4-position LRGB-filterwheel without its cover. Opposed to the drawing, the notches are actually dents in the lower rim of the wheel and not visible. This is to have an uninterrupted surface for driving the wheel.
  • Drawing of the 7-position filterwheel. The bearing of the filterwheel consists of three grooved wheels with ball bearings, that are attached to the 2mm thick 'back pane' of the housing. The top wheel has an excentric pillar to enable adjusting the play. Notches, again, are dents along the back rim of the filterwheel.
  • Photograph of the 7-position, LSHORGB filterwheel. The H-alpha filter (top, right) is held in its pocket with small metal parts and glued-in magnets.
    The simple schematics of both filterwheels is shown here. Care should be taken that the micro-switch not only interrupts the supply to the motor but also (nearly) short-circuits the motor in order to force it to a quick hold. Otherwise inertia of the motor and wheel may be enough to keep it rotating during the short interruptions of the micro-switch!
  • A view of the 7-position filterwheel from different angles. Since there is no feedback regarding the actual filter position the system control and image acquisition program, upon closing down, always returns the filterwheel to the L-position (i.e. the empty filter position). Also a small peek-hole in the top of the housing allows to visually check what filter is in position.
  • The 7-position filterwheel mounted on the FTT1010-M camera.
  • Changing from narrow band filters to LRGB-imaging requires inserting an IR-filter. Currently the IR-filter is held in a manually operate slide attached to the inside of the focuser interface. Instead of the IR-filter, a 10x ND-filter can be mounted when photographing the moon.