Summary of Tip-Tilt Corrected Image Quality Models at V (0.5痠)

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. Two sets of tables are presented to indicate (a) the dependence on zenith distance and (b) the effect of employing different wavefront sensors (on-instrument, OIWFS, and peripheral, PWFS). 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 detectors, parameters which are yet to be well characterised.

PWFS - a conservative assumption about detector performance corresponds to a guide star R-band magnitude limit of 15-16 mag, depending on wind speed. The guide star is assumed to be 4 arcmin off-axis i.e. near the inner edge of the unvignetted telescope field.
OIWFS - a similarly 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.

A summary of the dependence on guide star off-axis angle is given below each pair of tables.

PWFS, zenith

PWFS, 40 deg ZD

	wind
seeing	low	med	high
good	0.30	0.30	0.35
med	0.50	0.50	0.50
poor	0.65	0.70	0.70

	wind
seeing	low	med	high
good	0.35	0.40	0.40
med	0.60	0.60	0.60
poor	0.80	0.80	0.80

Notes to table:
(1) Image quality is worse (or better) by 3% per arcmin of guide star off-axis angle (from detailed modeling).
(2) Image quality is approximately 25% worse at ZD=40 deg under all conditions (from detailed modeling).

OIWFS, zenith

OIWFS, 40 deg ZD

	wind
seeing	low	med	high
good	0.30	0.30	0.30
med	0.50	0.50	0.50
poor	0.65	0.65	0.65

	wind
seeing	low	med	high
good	0.35	0.35	0.35
med	0.55	0.55	0.60
poor	0.75	0.75	0.75

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).
(3) PWFS worse than OIWFS by approx. 10% (from detailed modeling).

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