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AO performance

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Adaptive optics

For ING staff

Contact
Ian Skillen
NAOMI Instrument Specialist
wji@ing.iac.es

NAOMI

Note that as part of ongoing rationalisation of the WHT's instrument suite, and following consultation with the ING's observing communities, the adaptive-optics instruments (NAOMI/INGRID, NAOMI/OASIS) are not available after 31st January 2015, i.e. from 2015A.

AO imaging and spectroscopy at the WHT

Above: Open-loop and closed-loop I-band images of a 0.36-arcsec star pair, click here for further images. For spectacular images from the commissioning of the laser guide star (GLAS), click here.

NAOMI, mounted at the WHT Nasmyth focus, delivers near-diffraction-limited images in J, H and K bands (FWHM ~ 0.15 arcsec), and significant correction at shorter wavelengths. There are currently two science instruments: the optical integral-field spectrograph OASIS, and the IR imager INGRID. OASIS offers a choice of 3 different spatial samplings, field of view up to 10 arcsec, and a choice of 15 spectroscopic modes. INGRID has a 40-arcsec field of view (0.04 arcsec/pixel), and is equipped with a variety of broad-band and narrow-band filters. A coronagraph, OSCA, can be placed in the light path to INGRID.

NAOMI requires a natural guide star (V < 14, within a few 10s arcsec). The science target itself can serve as a natural guide star, if it is sufficiently bright and compact. NAOMI has been used to study a wide variety of objects, including comets, binary asteroids, cicumstellar disks, nuclei of normal galaxies, AGN, QSO hosts and gravitational lenses.

The AO correction achieved depends on the magnitude of the guide star, separation of the science target from the guide star, the natural seeing, and the wavelength of observation. In median seeing (0.7 arcsec in the optical), a guide star with V = 11.5 typically yields FWHM ~ 0.2 arcsec, Strehl = 0.2, out to a radius ~ 20 arcsec in H band (1.6 microns). The correction is usually marginal for V > 14, or for radius of more than a few tens of arcsec from the guide star, or in natural seeing worse than 1.5 arcsec. In the optical, NAOMI typically improves the FWHM by a factor of two (e.g. from 0.6 to 0.3 arcsec). See the NAOMI performance page for details. (For guiding only, i.e. no AO correction, a star V < 19.5 within 0.9-1.5 arcmin of the target depending on its position angle and the telescope sky position angle, is required.)

The Rayleigh laser guide star, GLAS is not currently offered due to a technical problem - reduced power, which significantly compromises the level of correction achieved. The performance of the AO system with GLAS is still being characterised, but it is expected that it will be similar to that with a bright natural guide star (mag ~ 9.5). When observing with the laser guide star, a natural guide star is still required, to effect the tip-tilt correction, but this star can be much fainter, V < 17, and up to 60 arcsec from the science target.

Signal-to-noise predictions can be made using SIGNAL, the ING signal-to-noise calculator. The throughput to INGRID with NAOMI is about 0.5 times that to INGRID mounted at the Cassegrain focus (due to losses in the GHRIL derotator, and in the NAOMI optics). The throughput to OASIS is similar to that of OASIS at CFHT.

In the IR, the J and H background levels are similar to those measured with INGRID mounted at the Cassegrain focus. However, the K thermal background is ~ 2 - 3 mag brighter than that at Cassegrain, corresponding to ~ 100% emissivity from the telescope.

Observing overheads with adaptive-optics systems (e.g. for PSF calibration) are higher than for normal observing, and these overheads should be included in the request for time, see the page on planning observations.

All AO observations are carried out in service mode, by ING staff astronomers.

NAOMI was built by a team from Durham/ATC, led by Richard Myers.

ING welcomes enquiries about the technical feasibility of proposed observations.



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Contact:  (NAOMI Instrument Specialist)
Last modified: 07 October 2014