Subject: MCAO Date: Mon, 12 May 2003 18:14:13 -0200 From: "Laerte Sodre Jr" To: geminiaspen@lna.br + Gemini Multi-Conjugate Adaptive Optics system (MCAO) + Science case template Colegas, No caso do Gemini Sul, o dispositivo para otica adaptativa MCAO deve entrar em operacao em 2005 e, certamente, sera util para uma serie de projetos. Assim, fiz um resumo das caracteristicas do MCAO a partir do que encontrei no site do Gemini: http://www.gemini.edu/sciops/instruments/adaptiveOptics/AOIndex.html O resumo esta abaixo. Os "science cases" para justificar o MCAO estao num documento em pdf com 127 paginas que contem coisas que podem ser de muito interesse para os colegas e que pode ser acessado no site acima. E' interessante pensar em nossos projetos tambem em termos de "science cases" e, para ajudar nisso, coloquei tambem abaixo, um "template" para isso, adotado num workshop sobre o MCAO, mas que tambem pode ser util para muita gente. E', claro, apenas uma sugestao. Dimitri, talvez voce possa repensar sua proposta nos termos desse template. Abracos, Laerte ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Gemini Multi-Conjugate Adaptive Optics system (MCAO) The Gemini Multi-Conjugate Adaptive Optics system (MCAO) is a proposed instrument for Cerro Pachon that will deliver diffraction-limited images, with a uniform image quality over a one arc minute field of view. It does so by using several deformable mirrors opticaly conjugated at different altitudes and five wavefront sensors using laser beacons as their guide sources, to measure and compensate for the turbulence-induced phase aberrations in three dimensions. This new technique not only increases the compensated field of view by an order of magnitude or more, provides a uniform point spread function over this field, but also solves for the cone effect, a consequence of the use of lasers as guide stars, which in classical AO laser system reduces the performance at short wavelengths on large telescopes. In addition to the 10 fold gain in angular resolution, MCAO also pushes the detection limit by 1.7 magnitudes on unresolved objects with respect to seeing limited images. This instrument on Gemini provides a unique way to explore NGST-class science as soon as its commissionning in 2005. MCAO by itself can not produce data. It stabilizes the beam and restore the diffraction limit of the telescope. Therefore it has to come with a suite of instrument, designed along requirements which are derived from the study of the science case (cf MCAO Workshop). Efficient utilization of the MCAO system will depend upon the availibility of a large field IRMOS and a near-IR imager with complimentary design characteristics. Both instruments should be matched to the F/32 output beam of the MCAO system, and provide diffraction-limited image quality over the field of view. Such instruments would yield unique scientific capabilities before the launch of NGST sometimes around 2010. The IRMOS would remain competitive with NGST given sufficient spectral resolution. Imager Wavelength 1 - 2.5 microns Field 80'' x 80'' (4k x 4k) Sampling 0.02'' In addition, the following is desired: Low dark current (small pixels / narrow band imaging) Low and well characterized distortion (astrometric applications) Low and well characterized image aberrations Simple Spectral mode (cost issues) ? This instrument will most probably use bootable 2k x 2k arrays Note that this imager is slightly undersampled in J band, but this is proposed in a spirit of compromise to get the largest field exploitable at the output of MCAO. Spectrograph In the Strawman suite, the current idea is to have multiple Integral Field Units (15-25), each of them with the following characteristics: Wavelength 1 - 2.5 microns Field per IFU 3 - 5'' Spatial sampling 0.1'' Spectral resolution (2 pixels) 5000-10000 Patrol Field circular, 2' diameter Comments: Although the current baseline is an IFU, MOS type are not excluded. The rationale for which kind of spectrograph to be used will be at the heart of the up-coming MCAO workshop. The slit width has been tentatively set to 0.1'' because it is felt that smaller "pixels" will drastically reduce the number of observable objects (SNR). This is also under more detailled investigations and will be reviewed at the workshop. A possibility could be to dedicate a couple of IFUs heads to this finer spatial sampling for the brightests objects. The spectral resolution is set to a minimum of 5000 to be able to work between the OH lines. ......................................................................... Summary of the performance of the planned Gemini MCAO system: Most of these performance metrics are compared to what would be achieved with Classical Laser Guide Star AO (CAO in the following, with a single deformable mirror, wave front sensor and guide star). Some sensitivities comparisons with HST/NICMOS and NGST are also listed. -MCAO performance is very uniform over a 1 square arc minute field, both in terms of Strehl ratios and more general PSF characteristics; -Images are basically diffraction-limited, in term of FWHM, over the full 1 square arcmin field of view, even for the faintest NGS considered here; -Strehl ratio under median seeing conditions vary from 45% to 80% in the 1-2.5 micron range and 0-30 degrees zenith angle, with relative uniformity (relative Strehl ratio standard deviation) from 1.5 to 6% ; -The Strehl ratio degrades gracefully out of the 1 square arc minute central field. The useable field with Strehl ratio above 50% of the peak value is the full 2 arcmin field in H and K band, and approximately 1.5 arc minute at J band; -Three Natural Guide stars are needed to get the best compensation from the MCAO system but the magnitude limits (R=19) correspond to very useful values of sky coverage (~15% at the galactic pole and over 70% at 30o galatic latitude), even when sky background noise and windshake jitter are taken into account; -The overall performance is a weak function of the exact match between the deformable mirror conjugation altitude and the location of the turbulent layers; -Under median seeing conditions, MCAO brings a 1.5 to 1.7 magnitude sensitivity gain over the 1-2.5 micron range on point sources with respect to seeing limited imaging. Gains with respect to HST/NICMOS, in the same conditions, are 0.3 (J) and 1.2 (K) magnitudes; -For high spectral resolution spectroscopy, MCAO plus a spectrograph will have the same sensitivity as the NGST, insuring a competitive regime for this instrument after the NGST launch; -Multiplex gains with respect to CAO are factors of 10 to 20 in area; ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Science case template (para o workshop do MCAO): To insure uniformity and consistence, please use the following form for your proposed science case. Title PI + collaborators and institution Abstract Background Science Proposed observations Instrument requirements (field, spatial and spectral resolution, wavelength coverage) Sensitivity requirements Target density Time requirements Guide star availability Advantages over AO (try to quantify) Could this program be done with classical AO field of view (10-30";) ? Is this program enabled by MCAO ? In what sense ? Does this program need a very uniform image quality ? Is this program benefiting from the gain in angular resolution or from the gain in sensitivity (or both) ? Complementarity with other future facilities (ALMA, NGST, ...) Laerte Sodre' Jr. Departamento de Astronomia Instituto Astronomico e Geofisico Universidade de Sao Paulo Rua do Matao 1226 Cidade Universitaria 05508-900 Sao Paulo SP Brazil phone: 55 11 30912704 fax: 55 11 30912860 email: laerte@astro.iag.usp.br