Results with the laser: 23% Strehl
June = 1999A=20 series of excellent nights during a technical run in June allowed ALFA = to make=20 huge progress in correcting the wavefront using a sodium laser guide = star in=20 order to reach high
For these results,
A series of 40 = consecutive=20 images were taken of the star in the K-band (2.2 microns) on the = Omega-Cass=20 camera. Each integration was 5 sec, and the only processing performed = was=20 sky-subtraction and bad pixel removal. In particular, no shift-and-add = or=20 deconvolution techniques were applied. Frames were added together = directly to=20 produce long exposure images.=20
![3D"open](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-open.gif") =20
| ![3D"open](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-open.profile.gif"=20) =20
| Open loop 50 sec K-band image of SAO 68075. The seeing indicated = a=20 FWHM resolution of 0.65'', an extremely good value. |
![3D"tiptilt](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-tiptilt.gif") =20
| ![3D"tiptilt](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-tiptilt.profile.gif"=) =20
| 50 sec image with tip-tilt corrected on the star. The FWHM has=20 decreased to 0.49''. |
![3D"lgs](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-lgs_all.gif") =20
| ![3D"lgs](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-lgs_all.profile.gif"=) =20
| All 40 images added together to give an effective 200 sec = exposure,=20 with tip-tilt corrected on the natural star and high order = correction on=20 the laser star. The FWHM is now only 0.19'' and the Strehl ratio = is 12.8%.=20 These values show what might be expected for long integration = times. |
![3D"lgs](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-lgs_best.gif") =20
| ![3D"lgs](3D"http://www.mpe.mpg.de/ir/ALFA/images/strehl23-lgs_best.profile.gif=) =20
| During the very best 10 sec, a Strehl ratio of 23.0% was = reached, and=20 the PSF is diffraction limited (0.14''). The first diffraction = ring is=20 clearly visible. |
What is a Strehl ratio ?
A Strehl ratio, or = simply Strehl,=20 is the ratio of the peak intensity of the measured point-spread function = (PSF)=20 to the theoretical maximum. In practice, it is only possible to measure = the peak=20 intensity integrated over one pixel, but with small pixels (in this case = 0.04'')=20 the derived Strehls are very similar to the absolute ones. The = theoretical PSF=20 depends on both the diameter of the telescope's main mirror (and its = central=20 obscuration) as well as the wavelength at which it is measured.=20Because the AO corrected PSFs have a `core+halo' shape, the FWHM very = quickly=20 approaches the diffraction limit even when the Strehl ratio is = relatively small.=20 Thus, if only high resolution is required, a Strehl of 10-15% might be=20 satisfactory. But if you are trying to detect faint objects then you = want the=20 Strehl as high as possible: even with a Strehl of 40% only 1/4 of the = energy is=20 in a diffraction limited core, the rest is in a broad halo making = detection of=20 nearby objects harder. However, either of these cases are far better = than a=20 seeing limited PSF: for K-band observations on the 3.5 m telescope at = Calar=20 Alto, in 1'' seeing the Strehl ratio is only 2.5% and only 1/20 of the = total=20 energy is in the same core region.=20
------=_NextPart_000_001C_01CA8710.B149C9F0--