Hokupa'a Performance Calulations: Field Angle

Hokupa'a is able to use off-axis guide stars up to 30 arcseconds off-axis however a progressive reduction in performance will be incurred as the off-axis distance increases. The magnitude of this effect depends strongly on the intrinsic seeing (r0) and the effective altitude of the turbulence. (An atmospheric cell of fixed size subtends a smaller angle at higher altitudes).

As a guideline, the expected Strehl and FWHM variation versus field angle (at 10 and 20 arcsec) are given in the following table. The Strehl degradation is defined as the ratio of the off-axis and on-axis Strehls. The increase in the FWHM is the ratio of the FWHM in the off-axis PSF and on-axis PSF.

(0.5'' Seeing conditions)
Hokupa'a Performance Field dependence
Wavelength (microns) Strehl Degradation FWHM/FWHM0 50%EED/50%EED0
10" 20" 10" 20" 10" 20"
1.2 0.6 0.3 1.5 2.4 1.03 1.1
1.6 0.7 0.4 1.2 1.6 1.1 1.2
2.2 0.8 0.5 1.1 1.2 1.3 1.4






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Last update 24 August 2000; Mark Chun
In original form; Mark Chun