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The F2 On-Instrument Wavefront Sensor (OIWFS) consists of a 2x2 lenslet array on a moveable arm that patrols an area in front of the mask plane. Signals from the OIWFS are fed to the secondary for fast tip-tilt guiding and to the primary for astigmatism correction. The OIWFS is close to optimally commissioned, but does not yet deliver uniform image quality across the full field of view.

July 2017: The OIWFS is unavailable due to a mechanical fault at least until September. 

  • Limiting magnitude and availability of guide stars: Guide stars for the OIWFS need to be fainter than V=9.5mag. Detailed limiting magnitude measurements for the OIWFS under different conditions are still pending. But the overall performance is expected to be similar to the OIWFS in GMOS-S.
  • For the purpose of planning observations for the current semester we suggest more conservative limits than for GMOS-S. Therefore, it is required that guide stars be brighter than V=14.5 mag for guiding on dark sky (BG=50% or BG=20%) in seeing conditions of IQ=70% or IQ=20%. For guiding in moon light under the same seeing conditions (BG=Any), guide stars need to be brighter than V=14.0 mag. For programs that can executed in poorer seeing conditions, guide stars that are 1-2 magnitudes brighter are needed. Users are encouraged to select guide stars as bright as possible.
  • Patrol field: The patrol area has thle red shape on the schematic diagram below. The patrol area is effectively doubled since the field can be rotated 180 degrees. The red shaded are shows also the vignetting caused by the OIWFS probe arm.

Schematic diagram of the patrol area of the OIWFS (red) relative to the F2 FOV.

    OIWFS detector parameters
    Array EEV CCD-39
    Pixel format 80x80
    Pixel size 24 micron pixels
    Spectral Response 0.65 at 450nm, 0.85 at 700nm, 0.42 at 900nm
    Readout time 4.71ms for full frame
    Readnoise 2.9e-/pix
    Total noise (on telescope during observations) pending
    Dark current (uncooled) 30 e-/s/unbinned pixel