Summary of Tip-Tilt Corrected Image Quality Models at J (1.2µm)

Important caveat: the simulations presented here make as yet unverified assumptions about the telescope and wavefront sensor performance and long-term site conditions. The values are therefore subject to change and we anticipate improvement as the facility is optimised.

The tables below show 50% encircled energy diameter (EED) as a function of wind speed and seeing conditions and indicate the dependence on zenith distance assuming use of an on-instrument wavefront sensor. See the specific instrument pages for the wavefront sensors required or available for use with each instrument. It is assumed that the OIWFS and/or PWFS are used in addition to the second PWFS that is assigned to primary mirror figure (aO) correction.

The values in each table are the 50% EED near to the 'knee' in the wavefront sensor performance curve. The location of the knee, corresponding to the point where there are insufficient guide star photons to overcome the noise sources, is critically dependent on the readout noise of the WFS detector, a parameter which is yet to be well characterised.

• OIWFS - a conservative assumption yields guide star magnitude limits in the R-band (for optical OIWFS e.g. GMOS) of 15-16 mag and in a composite J+H band (for near-IR OIWFS e.g. NIRI, GNIRS) of 17-18 mag. The guide star is assumed to be 1 arcmin off-axis.

Notes to table:
(1) Performance versus guide star off-axis angle not relevant for OIWFS.
(2) Image quality approximately 20% worse at ZD=40 deg under all conditions (from detailed modeling).

Last update October 13, 1999; Mark Chun and Phil Puxley