The GMOS 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.

• Filter: GG495 (longpass filter with a blue cut-off at 495nm)
  
• Limiting magnitude and availability of guide stars: Guide stars for the OIWFS need to be fainter than V=9.5mag. Limiting magnitude measurements for the OIWFS have been made in photometric, low wind conditions, and are summarized in the tables below. An unexpected difference in performance between the GMOS-N and GMOS-S OIWFSs exists, presumably due to significantly higher noise on the GMOS-S OIWFS. Attempts to reduce the noise on the GMOS-S OIWFS are on-going; in the meantime PIs must ensure their guide stars are valid for the appropriate GMOS. Bear in mind that programs that can be carried out in non-photometric conditions require sufficiently bright guide stars to enable guiding through clouds.
  
  The effective limiting magnitude for the OIWFS also depends on the seeing and the guide frequency. Fainter guide stars can be utilized if the guide frequency is lowered, but this will degrade the image quality and cannot be employed in high wind situations.
  
• GMOS-N: It is required that guide stars be brighter than V=16 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=15.0 mag. For programs that can be 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.
  
  

  GMOS-N OIWFS limiting (V) magnitudes
  Seeing	Dark sky (BG=50% or BG=20%)	Full moon (BG=Any)
  < 0.75arcsec (IQ=70% or IQ=20%)	200Hz 15.5 mag 100Hz 16.2 mag 50Hz 17.1 mag	200Hz 15.0 mag 100Hz 15.3 mag 50Hz 15.5 mag
  1.5arcsec (IQ=Any)	200Hz 13.8 mag 100Hz 14.9 mag 50Hz 15.5 mag	Estimate 100Hz 13.0 mag

  
  
• GMOS-S: It is required that the guide stars be brighter than V=15.5 mag for guiding on dark sky in seeing conditions of 70-percentile or better. For guiding in moon light under the same seeing conditions, guide stars need to be brighter than V=14.5 mag. For programs that can be executed in poorer seeing conditions, guide stars that are 1-2 magnitudes brighter are imperatively needed. Users are encouraged to select guide stars as bright as possible.
  
  

  GMOS-S OIWFS limiting (V) magnitudes
  Seeing	Dark sky	Full moon
  < 0.75arcsec	V < 15.5 mag	V < 14.5 mag
  1.5arcsec	V < 15.0 mag	V < 14.0 mag

  
  
  REMARK: These GMOS-S limit numbers are usually attainable only at 50Hz, the lowest guiding frequency value.
  
• Patrol field: 3.54 arcmin x 4.15 arcmin = = 14.7 sq arcmin, overlapping one corner of the imaging field with some area outside the imaging field. The patrol area is the red box on the schematic diagram below. The patrol area is effectively doubled since the field can be rotated 180 degrees. The OIWFS cannot patrol outside a circle (the green one in the diagram below) with radius 4.8 arcmin. Thus a small area in the lower right of the patrol area shown on the schematic cannot be reached.

   

  Schematic diagram of the patrol area of the OIWFS (red box) relative to the GMOS CCD array (which consists of three chips). The imaging field is shown by the dashed line. The OIWFS cannot patrol outside a circle with radius 4.8 arcmin (the green circle).

   

• Vignetting of GMOS science field by OIWFS: An image showing the vignetting of the GMOS science field by the OIWFS is shown below. The OIWFS on that image is in the center of the field, just for illustration. The OIWFS can be placed anywhere in the patrol field outlined on the diagram above, including outside the imaging field.

   

   

  

  Vignetting of GMOS science field by OIWFS, if the OIWFS is in the center of the imaging field. The imaging field of view of 5.5 arcmin is shown. The width of the OIWFS arm as seen by the GMOS camera is about 20 arcsec.