Fiber positioner system "Echidna"

Introduction

In order to populate a large number of fibers in the relativly small physical area of the Subaru prime focal plane, the FMOS fiber positioner system employs the "Echidna"-type design and technique: Each fiber is assembled into "spine" unit and the tilt of this determines the position of the fiber tip on the focal plane. Fig. 1 shows an illustration of the dynamics of a spine unit. The spine tilt is driven by sending a saw-tooth voltage pulse to the quadratic tube piezo actuator with a counterweight mass, which overcomes the magnetic friction holding the spine static and makes a "stick-and-slip" motion.

Fig. 1 - Illustraction of the dynamics of a spine unit.

The 400 fibers are populated in the 30-arcmin diameter field of view (FOV) by assembling twelve modules each of which consists of two rows of twenty spine units. A photo of the Echidna spines and a sketch of the distribution of the spines in the field of view are presented in Fig. 2. There are 14 guide fiber bundles at the edge of the FOV (7 at one side), which are not connected to the spectrographs. Note that the modules of the fibers connected to IRS1 are interlaced with those to IRS2. Each fiber (including guid fiber bundle) has an ID number and, in planning observations, the spine allocation software (see this page for details) records the one-to-one correspondence between the fiber ID numbers and science targets/guide stars in the output file (*.s2o).

Fig. 2 - (Left) A photo of Echidna spines. The spines are distributed within approximately a ~15cm x 15cm square. (Right) A sketch of spine distribution in the field of view.

Restriction to spine motion

Due to the design, the position of each spine is restricted in the patrol area around its home position (i.e. pointing vertical to the focal plane) as shown in Fig. 3. Note that the patrol area of a spine is overlapped with that of the next one (cf. the fiber spacing is ~84 arcsec when the spines are at home positions), so there is no inaccessible area in the FOV. The minimal fiber spacing is 12 arcsec, which is due to the thickness of the spine tip: The fiber is inside a double tube made by silica inside and carbon fiber outside.

Fig. 3 - Patrol field of a single spine (left) and minimal spacing between neighboring spines (right).

Fiber configuration

In front of the focal plane where the Echidna fiber array exists, the Focal Plane Imager (FPI) can be inserted to measure the current fiber positions. When doing this, the back illumination in the fiber connectors is turned on and the tips of the Echidna fibers are lit. FPI then scans the FOV with taking an image of a block of several neighboring fibers using a small CCD camera inside ("spine camera") and measures all the fiber positions. The actual fiber configuration is an iterative process of measuring the fiber positions and move the spines towards the requested positions (a short movie of spine and FPI movements is available here). To achieve the positional accuracy of ~10 um (i.e. ~0.1 arcsec on sky), this measure-and-move process needs to be repeated seven times and this takes ~13 minutes.

Fig. 3 - The FPI system around the Echidna spines.

Readers can also refer to the paper as listed below and references therein for more details of this system:

"Performance of Echidna fiber positioner for FMOS on Subaru"
Akiyama, M., et al. 2008, Proc. of SPIE, Vol. 7018, 94
"Echidna: the engineering challenges"
Brzeski, J., et al. 2004, Proc. of SPIE, Vol. 5492, 1228

Last updated: January 31, 2012



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