Salvador Cuevas

Turbulence Profiles with Generalized Scidar at San Pedro Mártir Observatory and Isoplanatism Studies

The results obtained from 3398 vertical profiles of atmospheric turbulence measured during 11 nights at the Observatorio Astronomico Nacional in San Pedro Martir (Baja California, Mexico) are presented. The observations were carried out with the generalized scidar (GS) installed at the 1.5 m and the 2.1 m telescopes of that site, in 1997 March and April. The open-air seeing was measured with a differential image motion monitor (DIMM). The GS can detect turbulence profiles along the whole optical path, unlike the classical scidar, which is insensitive to low-altitude turbulence. For the first time, to our knowledge, profiles including turbulence near the ground are monitored and statistically analyzed. Isoplanatic angles for speckle interferometry and adaptive optics (AO) in either full or partial compensation are deduced, as well as the focus anisoplanatism parameter for sodium laser guide stars. The advantage of minimizing the distance between the turbulent layers and the conjugated plane of the deformable mirror of an AO system is studied. The comparison of GS profiles obtained at both telescopes, together with DIMM measurements, show that the turbulence near the ground is more strongly dominant at the 1.5 m telescope than at the 2.1 m telescope, where the median values of the seeing near the ground, in the free atmosphere and in the whole optical path are , and , respectively. These values are comparable to or better than those of the major astronomical observatories, although a larger data sample is needed for a definitive comparison.

Improved achromatization of phase mask coronagraphs using colored apodization

Context. For direct imaging of exoplanets, a stellar coronagraph helps to remove the image of an observed bright star by attenuating the diffraction effects caused by the telescope aperture of diameter D. The dual zone phase mask (DZPM) coronagraph constitutes a promising concept since it theoretically offers a small inner working angle (IWA similar to lambda(0)/D where lambda(0) denotes the central wavelength of the spectral range Delta lambda), good achromaticity, and high starlight rejection, typically reaching a 10(6) contrast at 5 lambda(0)/D from the star over a spectral bandwidth Delta lambda/lambda(0) of 25% (similar to H-band). This last value proves to be encouraging for broadband imaging of young and warm Jupiter-like planets. Aims. Contrast levels higher than 10(6) are, however, required for observing older and/or less massive companions over a finite spectral bandwidth. An achromatization improvement of the DZPM coronagraph is therefore mandatory to reach such good performance. Methods. In its design, the DZPM coronagraph uses a gray (or achromatic) apodization. We replaced it by a colored apodization to increase the performance of this coronagraphic system over a wide spectral range. This innovative concept, called colored apodizer phase mask (CAPM) coronagraph, is defined to reach the highest contrast in the exoplanet search area. Once this has been done, we study the performance of the CAPM coronagraph in the presence of different errors to evaluate the sensitivity of our concept. Results. A 2.5 mag contrast gain is estimated from the performance provided by the CAPM coronagraph with respect to that of the DZPM coronagraph. A 2.2 x 10(-8) intensity level at 5 lambda(0)/D separation is then theoretically achieved with the CAPM coronagraph in the presence of a clear circular aperture and a 25% bandwidth. In addition, our studies show that our concept is less sensitive to low than to high-order aberrations for a given value of rms wavefront errors.

On the normalization of scintillation autocovariance for generalized SCIDAR.

The Generalized SCIDAR (Scintillation Detection and Ranging) technique consists in the computation of the mean autocorrelation of double-star scintillation images taken on a virtual plane located a few kilo-meters below the telescope pupil. This autocorrelation is normalized by the autocorrelation of the mean image. Johnston et al. in 2002 pointed out that this normalization leads to an inexact estimate of the optical-turbulence strength C(2)(N). Those authors restricted their analysis to turbulence at ground level. Here we generalize that study by calculating analytically the error induced by that normalization, for a turbulent layer at any altitude. An exact expression is given for any telescope-pupil shape and an approximate simple formula is provided for a full circular pupil. We show that the effect of the inexact normalization is to overestimate the C(2)(N) values. The error is larger for higher turbulent layers, smaller telescopes, longer distances of the analysis plane from the pupil, wider double-star separations, and larger differences of stellar magnitudes. Depending on the observational parameters and the turbulence altitude, the relative error can take values from zero up to a factor of 4, in which case the real C(2)(N) value is only 0.2 times the erroneous one. Our results can be applied to correct the C(2)(N) profiles that have been measured using the Generalized SCIDAR technique.

Co-phasing of segmented mirror telescopes with curvature sensing

The applicability of the curvature method for co-phasing of segmented mirrors is investigated by means of simulations for the case of strongly defocused images. The simulations are performed for both the monochromatic and the white light as well. A simple wavefront reconstruction from curvature signal was made. The reconstruction quality of the piston modes and the aberrations up to the fourth order is analyzed. The dependence of the Central Intensity Ratio for a segmented mirror as a function of the rms segments aberrations is presented. The effect of turbulence-induced distortions on the quality of mirror co-phasing is analyzed. It is shown that the local pistons and the local tip-tilts can be measured directly from the curvature signal without any phase recovering procedure. The results obtained show that, even in the presence of the atmospheric turbulence, the curvature method is sensitive enough to detect the errors of segmented mirrors.

IRMOS: The near-infrared multi-object spectrograph for the TMT

We present an overview of the near-InfraRed Multi-Object Spectrograph (IRMOS) for the Thirty Meter Telescope, as developed under a Feasibility Study at the University of Florida and Herzberg Institute of Astrophysics. IRMOS incorporates a multi-object adaptive optics correction capability over a 5-arcminute field of regard on TMT. Up to 20 independently-selectable target fields-of-view with ~2-arcsec diameter can be accessed within this field simultaneously. IRMOS provides near-diffraction-limited integral field spectroscopy over the 0.8-2.5 μm bandpass at R~1,000-20,000 for each target field. We give a brief summary of the Design Reference science cases for IRMOS. We then present an overview of the IRMOS baseline instrument design.

Curvature equation for a segmented telescope

We demonstrate that the curvature equation can be modified using some properties of Distributions theory for segmented mirror techniques. It is shown that, additionally to the individual segment aberrations, the modified equation contains the information about the relative pistons and tip- tilts among the segments. The validity of the equation is verified by numerical simulations and by a laboratory experiment as well.

Efficiency of the Hartmann test with different subpupil forms for the measurement of turbulence-induced phase distortions.

The reconstruction quality of turbulence-induced phase distortions from Hartmann data is calculated for masks with different subpupil forms by means of computer simulations. Four subpupil forms are considered: the circle, the square, the hexagon, and the polar segment. We show that, for the case of a circular aperture, the mask with polar segment subpupils provides the best reconstruction quality.

Lab results of the circular phase mask concepts for high-contrast imaging of exoplanets

Circular phase mask concepts represent promising options for high contrast imaging and spectroscopy of exo-planets. Depending on their design, they can either work as a diffraction suppression system or as a focal plane wavefront sensor. While the apodized Roddier coronagraph uses a π-phase mask to obtain complete suppression of the star image in monochromatic light, the Zernike sensor uses a π/2-phase mask to measure the residual aberrations in the focal plane by encoding them into intensity variations in the relayed pupil. Implementations of the Zernike sensor can be considered in exoplanet imagers such as VLT-SPHERE, Gemini planet imager, Palomar-P1640 or Subaru-SCExAO to enlarge their capabilities. However, such concepts have not been validated experimentally up to now. Our goal is to perform lab demonstration of this concept on our visible coronagraph testbed at LAM and to propose an upgrade design for SPHERE. In this communication, we report on results of lab measurements of the Zernike sensor and determine its sensitivity to small wavefront errors.

FRIDA: integral-field spectrograph and imager for the adaptive optics system of the Gran Telescopio Canarias

FRIDA (inFRared Imager and Dissector for the Adaptive optics system of the Gran Telescopio Canarias) has been designed as a diffraction limited instrument that will offer broad and narrow band imaging and integral field spectroscopy (IFS) capabilities with low, intermediate and high spectral resolutions to operate in the wavelength range 0.9 - 2.5 μm. The integral field unit is based on a monolithic image slicer and the imaging and IFS observing modes will use the same Rockwell 2Kx2K detector. FRIDA will be based at a Nasmyth focus of GTC, behind the AO system. The main design characteristics of FRIDA are described in this contribution. FRIDA is a collaborative project between the main GTC partners, namely, Spain, Mexico and Florida, lead by UNAM.

San Pedro Mártir: astronomical site evaluation

The Observatorio Astronomico Nacional at San Pedro Martir is situated on the summit of the San Pedro Martir Sierra in the Baja California peninsula of Mexico, at 2800m above sea level. For as long as three decades, a number of groups and individuals have gathered extremely valuable data leading to the site characterization for astronomical observations. Here we present a summary of the most important results obtained so far. The aspects covered are: weather, cloud coverage, local meteorology, atmospheric optical extinction, millimetric opacity, geotechnical studies, seeing, optical turbulence profiles, wind profiles and 3D simulations of atmospheric turbulence. The results place San Pedro Martir among the most favorable sites in the world for astronomical observations. It seems to be particularly well-suited for extremely large telescopes because of the excellent turbulence and local wind conditions, to mention but two characteristics. Long-term monitoring of some parameters still have to be undertaken. The National University of Mexico (UNAM) and other international institutions are putting a considerable effort in that sense.