ABSTRACT The resolution of astronomical images taken from the ground is severely degraded by the atmosphere. One of the simplest methods for overcoming this problem is tip-tilt (TT) correction. This method is especially valuable for small telescopes, as it enables them to reach their diffraction limit. Unfortunately, the method requires a bright guide star to asses the blurring of astronomical images. In this study we show how various factors influence the feasibility of tip-tilt correction in very small telescopes (≤ 1 m). We evaluate the effect of: (1) the intensity of the guide star, (2) seeing conditions, (3) telescope size and (4) sky background. The results are obtained from data simulated using the Random Wave Vector method. The dependencies presented in this paper can be used to predict the sky coverage of small telescopes with TT correction for a wide range of observational conditions.

RESUMEN La resolución de las imágenes astronómicas obtenidas desde la superficie terrestre se ve fuertemente degradada por la atmósfera. Uno de los métodos más sencillos para superar este problema es la corrección ladeo-inclinación (TT por sus siglas en inglés: Tip-Tilt). Este método es valioso especialmente para telescopios pequeños, pues permite que alcancen su límite de difracción. Sin embargo, el método requiere de una estrella guía brillante para evaluar la degradación de las imágenes. Estudiamos la manera en que varios factores afectan la viabilidad de la corrección TT para telescopios pequeños (≤ 1 m). Evaluamos los efectos de: (1) la magnitud de la estrella guía; (2) la turbulencia atmosférica (el “seeing”); (3) el tamaño del telescopio; y (4) el fondo celeste. Los resultados se obtienen con datos simulados usando el m´etodo del vector de onda al azar. Las dependencias que encontramos pueden ser usadas para predecir la cobertura celeste de telescopios pequeños con corrección TT para una gran variedad de condiciones observacionales.

En este trabajo se presentan los resultados de una campaña observacional de tres años durante la cual se realizaron medidas de la calidad del cielo in-situ en seis sitios de interés astronómico en los departamentos de Santander, Boyacá, Tolima, Huila, Cesár y La Guajira. Se realizaron excursiones esporádicas y de corta duración a estos seis lugares con el fin de realizar mediciones de temperatura y humedad a lo largo de la noche, del nivel de turbulencia en la vecindad del telescopio ó Seeing, del brillo del cielo y de la extinción atmosférica. La metodología utilizada en las medidas del Seeing se validó a través del uso de imágenes satelitales simultáneas a nuestras visitas, obtenidas de la base de datos del satelite geoestacionario GOES 13. El Seeing es altamente variable a lo largo de la noche en todos los sitios, con valores entre 1" y 4". Las difíciles condiciones climatológicas en especial la elevada humedad registrada durante la campaña, impidieron obtener una estadística mínima de datos fotométricos por lo que el análisis comparativo de los lugares visitados está basado en las medidas de Seeing. Una prueba de Kolmogorov-Smirnov confirma que los valores más bajos se registraron en los lugares de mayor altura en el Oriente colombiano, en particular en el sitio La Bricha, una vereda del municipio de Macaravita en Santander en donde la variabilidad registrada fué la menor con un rms = 0,59''. Este hecho sumado a la humedad inferior a 85% incluso durante la madrugada y el bajo nivel observado en el brillo de cielo sugieren que de los lugares visitados durante la campaña, La Bricha puede considerarse como un sitio apropiado para la ubicación de instrumentación astronómica en el rango óptico. No obstante, se recomienda realizar un monitoreo a largo plazo con el fin de caracterizar completamente el lugar.

In this work we present the results obtained from an observational campaign aimed to conduct measurements in-situ of the quality of the sky at six sites with astronomical interest in Santander, Boyacá, Tolima, Huila, Cesár and La Guajira. Short and sporadic expeditions were done to those places with the main goal to conduct measurements of temperature, humidity during the night, the level of turbulence in the vicinity of the telescope, the Seeing, the sky brightness and the atmospheric extinction. The adopted methodology for the Seeing measurements was validated with satellite data simultaneous to our visits, obtained from the data archive of the geostationary satellite GOES 13. All sites present Seeing highly variable along the night with values between 1'' and 4''. The bad weather and specially, the high humidity registered during the campaign limited the quality of photometric data and subsequently, the comparative analysis of the visited sites is based on the Seeing measurements. An statistical Kolmogorov-Smirnov test confirm that the lowest values are observed in high altitude sites over the Eastern Andes in particular at La Bricha in Macaravita-Santander where we get the lowest variability with rms = 0,59''. This fact together with the low humidity, less than 85% even during the dawn and the low level of the sky brightness suggest that La Bricha is a suitable place for location of astronomical instrumentation in the visible range. However, it is highly recommended to conduct a long-term monitoring of the place with the aim to characterize the site properly.

We present a new star image shape transformer for astronomical slit spectroscopy that consists of a plano-convex lens attached to a plane-parallel rectangular glass plate on one of the small sides, with the aim of improving the performance of spectrometric systems. The system transforms the shape of the objects into a rectangular form with the purpose of filling with light the entrance slit aperture in spectroscopic systems. The plate is set sidewise with the lens as the entrance aperture and the exit aperture of the plate is the illuminated slit seen by the spectrometric system. The aim of this transformer is also to maintain the beam characteristics, the intrinsic resolution of the spectrometers and to improve the throughput of the spectrometer and detector systems.

Presentamos un nuevo transformador de la forma de las imágenes estelares para espectroscopia astronómica de rendija que consiste de una lente plano convexa pegada a una placa rectangular plana paralela de vidrio en uno de sus lados menores con el fin de mejorar el desempeño de los sistemas espectrométricos. El sistema cambia la forma de los objetos a la forma rectangular con el propósito de llenar con luz la abertura de entrada de la rendija en los sistemas espectroscópicos. La placa se pone de lado con la lente como la abertura de entrada y la abertura de salida es la rendija iluminada vista por el sistema espectrométrico. El objetivo de este transformador es también el de mantener las características del haz de luz, la resolución intrínseca del espectrómetro y de mejorar la eficiencia de los sistemas espectrométricos y detectores.

DRAGON equipment is a new astronomical instrument designed to carry out many types of experimental investigations related to adaptive optics, active optics, observational astronomy and astronomical instrumentation. In this paper we present the results of the first instrument testing at the OAN-Tonantzintla Mexican Observatory.

El equipo DRAGON constituye un nuevo instrumento astronómico diseñado para llevar a cabo diversas investigaciones relacionadas con óptica adaptativa, óptica activa, astronomía observacional e instrumentación astronómica. En este artículo presentamos los resultados de las primeras pruebas de este instrumento llevadas a cabo en el OAN-Tonantzintla (México).

The human eye, as our biological vision instrument, contains intrinsic optical defects, referred to as optical aberrations or ametropia. The immediate consequence of such aberrations is poor quality of images formed at the retina. With the advent of more precise pulsed lasers for eye surgery, the development of instrumentation to determine precisely the higher order aberrations of the eye became a crucial chalange. Current instruments available commercially (refractometers) measure only the lower optical aberrations of the eye, i. e., myopia, hyperopia and astigmatism. In the present work we have developed a high resolution refractometer based on the Hartmann-Shack (HS) wave-front sensor. The HS sensor was originally developed for aberration measurements in general optical systems, and is of wide-spread usage in adaptive optics applications such as astronomical telescopes. Preliminary results for a mechanical eye are presented here and the RMSE in dioptric power (D) and cylinder axis (in degrees) were as follows: 0.04D for sphere and cylinder and 4(0) for axis. It is known that refractometers have typical errors of 0.12D diopters for sphere and cylinder and 5(0) for axis. These preliminary results indicate that the HS sensor may be applied successfully on in vivo eyes and that, in the near future, this technology may be available in most eye hospitals and clinics throughout the world, therefore bringing benefits to the general population.

In the present paper we discuss the development of "wave-front", an instrument for determining the lower and higher optical aberrations of the human eye. We also discuss the advantages that such instrumentation and techniques might bring to the ophthalmology professional of the 21st century. By shining a small light spot on the retina of subjects and observing the light that is reflected back from within the eye, we are able to quantitatively determine the amount of lower order aberrations (astigmatism, myopia, hyperopia) and higher order aberrations (coma, spherical aberration, etc.). We have measured artificial eyes with calibrated ametropia ranging from +5 to -5 D, with and without 2 D astigmatism with axis at 45º and 90º. We used a device known as the Hartmann-Shack (HS) sensor, originally developed for measuring the optical aberrations of optical instruments and general refracting surfaces in astronomical telescopes. The HS sensor sends information to a computer software for decomposition of wave-front aberrations into a set of Zernike polynomials. These polynomials have special mathematical properties and are more suitable in this case than the traditional Seidel polynomials. We have demonstrated that this technique is more precise than conventional autorefraction, with a root mean square error (RMSE) of less than 0.1 µm for a 4-mm diameter pupil. In terms of dioptric power this represents an RMSE error of less than 0.04 D and 5º for the axis. This precision is sufficient for customized corneal ablations, among other applications.