Field of invention

The present invention relates to a Fiber Positioning Unit (FPU) for telescopes. It more precisely concerns a FPU of the SCARA type.

State of the art

A FPU of the SCARA type uses two parallel rotary joints to position a fiber free end within an annular workspace (see figure 1).

Examples of existing FPU are shown in figure 2, 5 and 6.

The FPU are fixed to a focal plate. Figure 3 represents a front view of a focal plate.

Nowadays, a maximum densification of FPU on the focal plate is desired, to increase the instrument throughput. Thus, the pitch between two adjacent FPU (Pitch = distance center to center), and therefore the FPU diameter, should be minimized.

The graph of figure 4 shows the number of FPU for a focal plate of 800mm in diameter with respect to the positioner pitch.

Figure 5 shows an FPU according to the state of the art. The FPU design is made of a single part. The optical fiber and the beta motor wires must be bent to pass on the side of the alpha motor. Any bending of the optical fiber reduces the system performance from an optical point of view.

Figure 6 shows a tentative to miniaturize an FPU, with a diameter of approx. 7 mm. Such a relatively small object requires the use of an extremely small motor, typically 2 mm in diameter. This affects, i.e. reduces, the system performances (torque, precision, repeatability and robustness).

A FPU of the SCARA type usually comprises an alpha and a beta mechanism; the alpha mechanism successively comprising a motor, a driving shaft and a rotatable shaft (often a hollow shaft), the rotation of the rotatable shaft being carried out by the alpha motor via the driving shaft. The beta mechanism comprises a motor and a fiber holding element that may be rotated by the beta motor 4. Both mechanisms are mechanically connected in a way to allow a rotation of the beta mechanism by the rotatable shaft. The rotatable shaft is located within or above a focal plate.

The FPU disclosed in Chinese patent documents CN1095085C and CN2344786Y notably reduces the bending of the optical effect (see figure 7).

Until now, FPU have been designed in a single part, that integrates the two motors. The FPU is mounted in one step on the focal plate (see figure 8).

Prior art FPU, in particular the previous ones cited in the previous cited Chinese patent documents, do however pose some challenges and/or problems, such as:

- A focal plate needs to be precise on both sides in order to position the ball bearings.

- A complex assembly has to be done directly on and within the focal plate: 1) Press fitting of the ball bearings from both sides. 2) Insertion of the FPU. 3) Insertion and fixation of a gear (by gluing or laser welding) on the rotatable shaft.

4) fixation of the motor. Such an assembly cannot be made for a high FPU density configuration.

- The rotatable shaft is in the focal plate, which means that a relatively large bore has to be drilled through the focal plate reducing its rigidity (which is critical with the mass of thousands of FPUs)

- It is impossible to test the performances of the FPU before its fixation to the focal plate. The assembly process into the focal plate being irreversible, if bad performances of a positioner are detected at this stage, it compromises the complete telescope performances. This simply prohibits the use of such approach for high density FPU telescopes.

There is therefore a need to improve the existing FPU to obtain a higher FPU density and a stronger stability of the general structure. Description of the invention

The problems mentioned in the previous chapter are solved with the FPU of present invention, as defined in the claims.

The FPU according to the invention is essentially made in two separated parts and may be mounted on the focal plate in two distinct and independent steps. The central axis of the FPU and the alpha motor are not concentric. With such a configuration, the optical fiber may be maintained essentially straight along the FPU body. This configuration also allows the use of motors with a diameter very close to the diameter of the pitch for the rotatable shaft, typically of 6mm, which results in an increase of the FPU density on the focal plate. The use of an alpha motor and gearbox with a diameter very close to the pitch makes the complete design more robust and precise for a given pitch.

List of figures

Figure 1 : Prior art FPU of the SCARRA type

Figure 2 : Other example of a prior art FPU

Figure 3 : Front view of a prior art focal plate

Figure 4 : Graph showing the density of FPU on a focal plate

Figure 5 : Other example of a prior art FPU

Figure 6 : Other example of a prior art FPU

Figure 7 : Other example of a prior art FPU

Figure 8 : Mounting of a FPU on a focal plate

Figure 9 : Example of a FPU according to the invention

Figure 10 : Front view of a focal plate according to the invention

Figure 11 : Other view of the FPU of figure 9

Figure 12 : Other view of the FPU of figure 9 Numerical references used in the figures 9 to 12

1. Alpha motor

2. Driving shaft

3. Hollow rotatable shaft

4. Beta motor

5. Fiber holding element

6. Focal plate

7. Driving shaft passage

8. Optic fiber passage

9. Electric wire passage

10. Optical fiber

11 . Beta motor electric wire

Figure 9 shows an example of a FPU according to the invention with the optical fiber 10 being straight and placed between two adjacent alpha motors 1.

The FPU according to the present invention provides the following advantages:

- The focal plate 6 may be more easily manufactured.

- Assembling the FPUs on the focal plate 6 does not require any complex process.

- FPUs are preferably screwed, the alpha motor 1 being inserted on one side of the focal plate 6 and held in position by friction and the beta mechanism 4,11 being positioned and fixed on the other side of the focal plate 6. A press fitted pin may be advantageously used to avoid rotation of the motor. (No glue or laser welding are required). - The alpha rotatable shaft 3 and the beta mechanism 4,11 are on the same side of the focal plate 6, the passages 7,8,6 within the focal plate 6 are therefore relatively small, which results in a more robust structure.

- It is possible to test the performances of each positioner before assembling it on the focal plate 6.