1. Introduction. OzPoz is the fiber positioner for FLAMES, a multi-object spectroscopic facility on the VLT. It can feed light from as many as 140 objects within a 25 arcmin diameter field to two spectrographs. OzPoz was designed and built by the Anglo-Australian Observatory (AAO) under contract with ESO and commissioned during 2002. Science verification observations were made successfully in Jan./Feb. 2003 and regular observing began following that. The only subsequent attention by the AAO has been a 10 day visit by one of the authors in Nov. 2003. OzPoz mechanical design features and the performance achieved part-way through commissioning were described in previous SPIE proceedings. This paper deals with the performance in regular observational use since Feb. 2003. Peter Gillingham 1 *, Jonathan Stoker 2, Matthew Colless 1, Andreas Kaufer 2 Operational performance of OzPoz the multi-fiber positioner on the VLT The bars show the hours allocated for observing using OzPoz. Times lost to OzPoz problems are colored red. 2003 2004 The gripper about to lift a parked button Reliability. In the OzPoz design, priority was given to attaining high reliability with minimal need for maintenance. The robot and gripper motions are constrained by air bearings which will maintain accuracy through a long wear-free life. The exchanger structure and its drives were designed conservatively with careful regard to earthquake safety. The operational record thus far has been very satisfactory, as indicated by the small percentage of observing time lost. From early Dec. 2003 to Apr. ‘04, over 32000 button placements were made with no problems. A typical array of fibers on a (spherically curved) field plate. gripper collet CCD enclosure IFU bundle single fibers plate center fiber retractors The robot positioned below a plate ready for engagement. kinematic locators With 20 min set as the minimum exposure time, re-configurations are always done by the time an exposure is completed on the other plate. Logging initial placement errors of particular buttons gives a useful indication of those that may require cleaning or other attention. The plot below identifies three buttons that require iteration in placement much more often than is typical. error (  m) fiber number on plate 1 Record for ~16000 button placements. Error on first placement v fiber no. Errors in first placement of buttons in a full configuration. Those outside inner circle were placed a second time. Re-configuration speed. The mean time to reposition a button is 5.5 sec. This includes an allowance for occasionally placing a second time if the button is not within 40  m of the intended position (0.07 arcsec on the sky). The plot on the left shows a typical distribution of first placements, with a few requiring correction. An additional allowance of ~15% is needed to allow for buttons that have to be moved twice during a re-configuration. This is needed to uncross fibers. With all factors included, the mean time needed per button for a full re- configuration is 6.3 sec. 1 arcsec Accuracy of alignment on celestial targets. The placement accuracy discussed so far refers to how well buttons are placed in relation to where the robot believes they should be. Errors in modeling the robot geometry and atmospheric refraction, etc. could lead to fibers not collecting light efficiently from the intended targets. With collection diameters for single fibers ~1.2 arcsec, the required accuracy is high – the specification was < 0.15 arcsec radial error. Demonstrating full compliance was difficult; the figure to the right shows apparent angular errors from a test during the commissioning year – it gave an rms of 0.15 arcsec. This was short of the aim but included errors from the test method and from the astrometry of the target stars. In scientific use there have been no concerns about alignment accuracy. Sky subtraction. Sky subtraction can rely on accurate knowledge of the relative transmission of all fibers, including those allocated to sky measurement. The routine method for calibrating relative spectral transmission of fibers is to feed each of the fibers with a nominally identical exposure from a projector carried on the robot and swept several times over the fiber buttons arrayed in a spiral as indicated to the right. Calibration consistency of ~0.3% rms has been demonstrated. 1 Anglo-Australian Observatory 2 European Southern Observatory * pg@aao.gov.au http://www.aao.gov.au Plot of error vectors derived from raster scanning of telescope indication of how fibers are calibrated for relative transmission by spiral scan of robot carrying a projector start of sweep reverse here Illustration of spiral scan used to calibrate fiber transmission R error (µm) -80 -40 0 40 80 -80-4004080  error (µm)