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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 otimally commissioned, imaging quality is good across the whole FOV and in good seeing, 80% EED is within 0.6" at all radial range. The use of the OIWFS has a large nightime overhead, therefore we recommend its use only when there are no suitable PWFS2 guide stars.
- 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|
|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|
|Total noise (on telescope during observations)||pending|
|Dark current (uncooled)||30 e-/s/unbinned pixel|