Nuria Tomas

Design of an interferometric system for the measurement of phasing errors in segmented mirrors

One of the new problems that has to be solved for segmented mirrors is related to periodic phasing, because for such mirrors to exhibit diffraction-limited performance the segments have to be positioned with an accuracy of a fraction of a wavelength. We describe the optical design of an instrument that measures the phasing errors (i.e., tip, tilt, and piston) between two segments under daylight conditions. Its design is based on a high-aperture white-light Michelson interferometer. It was developed at the Center for Sensors, Instruments and Systems Development (CD6) of the Technical University of Catalunya, Spain, and its final testing was carried out on the Gran Telescopio Canarias test workbench.

Simple method for improving the sampling in profile measurements by use of the Ronchi test

We present a simple method for increasing the number of data points obtained during performance of profilometric measurements with the Ronchi test. The method is based on multiple ronchigram acquisitions that are superimposed after a few very simple data-processing operations. The measurement method, experimental setup, and data processing are described in detail from the ronchigram to the measured profile, and experimental results for a concave surface of an spherical ophthalmic lens are provided. The radius of curvature values measured for that surface are compared with the ones obtained with a high-precision radioscope, showing very good agreement and demonstrating the capability of the technique to measure topographic profiles of reflective samples.

Sources of variability of the van Herick technique for anterior angle estimation

The purpose of the present study was to investigate two potential sources of variability of the traditional van Herick technique for temporal anterior chamber angle estimation, namely the need to compare the depth of the peripheral anterior chamber (PACD) with the thickness of the peripheral cornea (PCT), and the possible loss of information resulting from restricting the assessment of the anterior chamber angle (ACA) to the temporal limbus.

Contribution of ghost and narcissus effects in MTF calculation

The MTF is a parameter used for quality analysis of a system in optical design. In optical design, this parameter is usually obtained without taking into account energy losses and ghost and narcissus effects. The proposed method to calculate MTF with this contribution has the same structure than traditional method. This one is based on ray tracing to obtain the PSF of the optical system, and then apply the Fourier transform to obtain MTF. The method we propose has three modifications from the traditional method: it uses a non-sequential ray tracing, it includes samples structures in the process to obtain the PSF and it also includes energy looses in the PSF function. In this work, we present the influence of and ghost and narcissus effects in MTF for a triplet system proposed by M. Laikin. We have obtained the traditional MTF with commercial available software Beam4, and our method without inclusion of ghost and narcissus effects. Also we have compared all results obtained with those that M. Laikin gives. Finally we have obtained the MTF with the contribution of ghost and narcissus effects using our proposed method.

Construction Method of Tailored Facets for Use in Freeform Reflectors Design

ABSTRACT A construction method to obtain sets of tailored facets to be used as the initial configuration in the design of freeform reflector surfaces is presented. The construction strategy starts from a collection of flat elementary facets and achieves the initial configuration of the reflector surface by tailoring the facets according to their Bézier surfaces description. The aim of this research is to describe a procedure capable of generating the Bézier surfaces that define elementary tailored facets; these facets could then be used as initial configurations in the design of freeform reflectors. Though beyond the scope of this article, the facets could be used on a subsequent optimization process; that is, the geometry of the elementary facets could be modified through a global optimization process taking into account all facets, in order to further improve the entire illumination system. Ray tracing is used to calculate the facet’s geometry; it applies elementary merit functions to find the parameters defining the Bézier curve that best meets specifications in each elementary facet. This method is based on a sequential 2D projection strategy that employs different strategic planes for calculations. A method to build up the facet’s geometry using a Bézier surface from 2D curves is provided. This approach will successfully generate a set of control points to describe a Bézier surface compatible with any standard optical optimization tool and suitable for use with computer-aided design (CAD) and other tools that represent solids.

Design of an interferometric system for piston measurements in segmented primary mirrors

Recently, telescopes with segmented primary mirrors are becoming increasingly popular due to their ability of achieving large apertures without the inconveniences caused by the fabrication and handling of monolithic surfaces with 8m (or over) in diameter. The difference in position of each pair of adjacent segments along the local normal of their interface (called piston hereafter), however, needs to be precisely measured in order to provide a diffraction- limited image. If a system yielding the nanometric accuracy required in piston measurements worked in daylight hours, the resultant saving in observation time would be an important advance on a majority of the state-of-the-art piston measurement systems. An interferometric piston measurement instrument accomplishing such objectives has been designed starting from the usual Michelson configuration at the CD6 (Terrassa, Spain), and its final test has been carried out in the test workbench of the Instituto de Astrofisica de Canarias (IAC, Canary Islands, Spain). Its optical layout relies on projecting the reference arm of the interferometer onto one of the segments of the pair considered, along the direction of the local normal to the surface while the measurement arm is projected onto the interface which divides the pair of segments considered. The field of view and its illumination are calculated to be equivalent in both segments. The lateral shift of the fringes in both interferograms determines the piston error present. A combination of monochromatic and white light is used, in order to remove the (lambda) /2 phase ambiguities present in piston measurements without losing the required resolution in the measurement. In this paper, the optical design of this interferometric piston measurement instrument will be presented. The particular configuration used in the interferometer, the implementation of an imaging system allowing to see both the interface of the segments and the interference fringes, the effect of the extension of the source and the use of both monochromatic and whit light will be discussed. A detailed study of the wavefront errors embedded in the wavefront at different stages of the interferometer is performed. The analysis shows the validity of the design of the instrument, and some experimental results obtained at the test workbench are provided to demonstrate the ability of the instrument to perform nanometric piston measurements under daylight conditions.

SERVICE-LEARNING IN OPTOMETRY HIGHER EDUCATION

Since 1993, the UVC has attended more than 25,000 patients, half of them being citizens with low incomes, referred from social services. These patients are attended within the framework of clinical practices, supervised by academic staff. If patients are in need of glasses, they are supplied thanks to partnership with the optical industry, which provides frames, ophthalmic and contact lenses, when needed. . SERVICE-LEARNING IN OPTOMETRY HIGHER EDUCATION

A New Slit Lamp-Based Technique for Anterior Chamber Angle Estimation

Purpose To design and test a new noninvasive method for anterior chamber angle (ACA) estimation based on the slit lamp that is accessible to all eye-care professionals. Methods A new technique (slit lamp anterior chamber estimation [SLACE]) that aims to overcome some of the limitations of the van Herick procedure was designed. The technique, which only requires a slit lamp, was applied to estimate the ACA of 50 participants (100 eyes) using two different slit lamp models, and results were compared with gonioscopy as the clinical standard. Results The Spearman nonparametric correlation between ACA values as determined by gonioscopy and SLACE were 0.81 (p < 0.001) and 0.79 (p < 0.001) for each slit lamp. Sensitivity values of 100 and 87.5% and specificity values of 75 and 81.2%, depending on the slit lamp used, were obtained for the SLACE technique as compared with gonioscopy (Spaeth classification). Conclusions The SLACE technique, when compared with gonioscopy, displayed good accuracy in the detection of narrow angles, and it may be useful for eye-care clinicians without access to expensive alternative equipment or those who cannot perform gonioscopy because of legal constraints regarding the use of diagnostic drugs.

New optimization process based on the contrast function for triplet systems design

In this work, we present a new image-quality-based optimization process using a USAAF test to estimate the contrast function in the image plane. This strategy permits us to estimate the contrast values with only one image for the analyzed field, so the computation time of the method is appropriated for an optimization process. This image-quality-based procedure does not present the typical problems in estimating the contrast value in optical systems with odd wave-aberrations because the method uses an image of one extended object. The optimization process is applied to design triplet systems. For a triplet lens system working with an f-number of eight, we obtain a design with a Strehl ratio of 0.91 at full field of view using our optimization process. Finally, we present the optimization process’ dependence on the starting systems which is not in correlation with the starting systems’ quality.

Self-evaluation in a Geometrical Optics laboratory with a large number of students

After each laboratory session, students must answer individually three random multiple choice questions. The corresponding software has been developed by us. This self-evaluation test motivates students before and during sessions performance, and provides objective information to the teacher. The estimated students’ mean satisfaction with this system is 8.1/10.