Ronchi test can detect piston by means of the defocusing term (original) (raw)

Experimental results on piston detection by using the classical Ronchi test

Applied Optics, 2006

We present some experimental results of piston detection by using a classical Ronchi test in a segmented spherical surface. We compare the Ronchi fringes frequency of both segments (reference and under test). When the piston term is present at the segment under test, it is moved iteratively until we obtain a minimum piston error. When the surface is almost cophased, both segments have the same fringe frequency. The fringes of each segment are compared by correlating the intensity versus position in each segment. The Ronchi test is used at the same time as Shack interferometry; this second method is used as a reference to avoid segments tilts. With the Ronchi test we have detected experimentally a piston of the order of 45 nm ͑͞14 with ϭ 632.8 nm). This result was compared with images of simulated Ronchigrams.

First approach to characterize tilts through multiple pistons in the classical Ronchi test

In this work, it is shown how a tilt can be introduced into a segmented surface through several piston terms, by using the classical Ronchi test. We have developed tilt error simulations by adding multiple constant terms to each point on the sagitta surface of a segment using ray tracing. Thereby a comparison between simulated Ronchigrams for piston and tilt has been performed for two adjacent segments, so that it is possible to appreciate the tilt and piston effects on the shape of the patterns. As a result, we show the behavior of the central maximum of the fringes in the presence of tilt and/or piston. Additionally we present evidence of introducing tilting without changing the surface shape by adding multiple pistons, and a description of how to characterize both piston and tilt using the Ronchi test.

Modified matching Ronchi test to visualize lens aberrations

We introduce a modification to the matching Ronchi test to visualize lens aberrations with simple and inexpensive equipment available in educational optics labs. This method can help instructors and students to observe and estimate lens aberrations in real time. It is also a semi-quantitative tool for primary tests in research labs. In this work by comparing a single lens with a doublet, we can clearly demonstrate the superior quality of the doublet over the single lens, and estimate their conic constants.

Rapid measurement of low-order aberrations using Fourier transforms of crystalline Ronchigrams

Ultramicroscopy, 2017

The aberrations of the objective lens should be measured and adjusted to realize high spatial resolution in scanning transmission electron microscopy (STEM). Here we report a method of measuring low-order aberrations using the Fourier transforms of Ronchigrams of an arbitrary crystal such as a specimen of interest. We have applied this technique to measure first-and second-order geometrical aberrations using typical standard specimens. Focus and twofold astigmatism are measured using two Ronchigrams obtained under different foci. Axial coma and threefold astigmatism are evaluated using the Fourier transforms of small subareas of a Ronchigram. The time dependences of focus and twofold astigmatism are examined using this technique for an aberration-corrected microscope.

Relationship between the f -number and the ruling frequency for a better discrimination of optical surfaces with the classical Ronchi test

Journal of Astronomical Telescopes, Instruments, and Systems, 2015

We present a criterion to properly choose the ruling frequency during the testing process of concave mirrors using the classical Ronchi test. It is known that when the number of lines per inch (ruling frequency) is low, the Ronchi test loses sensitivity; this fact implies that it is not qualitatively possible to determine the real surface shape; only an approximation would be obtained. In addition, if a higher ruling frequency is used, the ronchigram would be exposed to diffractive effects, making it even more difficult to identify the patterns for the real surface shape. We have found that by mathematically relating the f-number of the surface and the ruling spacing, the detection range of the Ronchi test can be improved. This allows us to know the shape of the patterns with the best certainty corresponding to a given optical surface. We have analyzed the behavior of real ronchigrams produced for two different parabolic mirrors to demonstrate this fact. In addition, real ronchigrams obtained from primary mirrors of telescopes that support the use of this criterion are shown.

Co-phasing of segmented telescopes: A new approach to piston measurements

Astronomy and Astrophysics, 2002

One of the main problems arising in the co-phasing of segmented telescopes is the problem of measurements of the relative pistons among the segments. These measurements become especially complicated when the pistons are in order of wavelength fractions. In this paper we present a new approach relative piston measurements which allows for a high-accuracy reconstruction of small pistons. Our approach is based on the shearing interferometry and compared to existed methods it has the following advantages: a high accuracy of piston reconstruction in presence of high levels of noise is combined with the ability to retrieve the pistons taking into account the segment's aberrations.

Sparse aperture differential piston measurements using the pyramid wave-front sensor

Adaptive Optics Systems V, 2016

In this paper we report on the laboratory experiment we settled in the Shanghai Astronomical Observatory (SHAO) to investigate the pyramid wave-front sensor (WFS) ability to measure the differential piston on a sparse aperture. The ultimate goal is to verify the ability of the pyramid WFS work in closed loop to perform the phasing of the primary mirrors of a sparse Fizeau imaging telescope. In the experiment we installed on the optical bench we performed various test checking the ability to flat the wave-front using a deformable mirror and to measure the signal of the differential piston on a two pupils setup. These steps represent the background from which we start to perform full closed loop operation on multiple apertures. These steps were also useful to characterize the achromatic double pyramids (double prisms) manufactured in the SHAO optical workshop.

Piston alignment for a segmented-aperture imaging system by using piston-sweep phasing

Optics Letters, 2017

This Letter presents a novel method for aligning the piston settings for a segmented-aperture imaging system. By sweeping the piston setting for a reference segment, a stack of images can be acquired that encodes information about the relative piston alignment for all segments in the system. We also demonstrate how a matched-filter processing method can be used to estimate the relative piston settings to align the imaging system at its full resolution.