We report the first optical and control performances of the Tor Vergata Fabry-P´erot interferometer prototype designed and realized in the framework of the ADvanced Astronomy for HELIophysics (ADAHELI) solar mission project. The characterization of the the coated surfaces of the two plates defining the optical cavity has been carried out with a Zygo interferometer able to measure the microroughness and global curvature of the cavity. The peak-to-valley errors are compliant with the manufacturer specifications and correspond to λ/70 and λ/80
@632.8 nm respectively. In addition, we present a first estimate of the interferometer spectral stability in stable open-air condition. A spectral uncertainty equal to 0.95 pm is found as the typical RMS over one hour of the passband central wavelength position.
In designing the optics of an imaging multi-etalon spectropolarimeter as a post-focus instrument for the Advanced Technology Solar Telescope (ATST), many constraints must be considered. Among these are the large entrance pupil diameter of the telescope (4 m), the demanded large field of view (≥90 arc sec ), high spectral resolving power (≥200,000 ), and limited field-dependent blue-shift of the instrumental profile [≤3 full width at half maximum (FWHM)], which require Fabry-Perot interferometers of large diameter (≥200 mm ), lighted by highly collimated beams. This implies large optical elements and long optical paths. Moreover, to use interference pre-filters with a relatively small diameter (≤70 mm ) and placed between the interferometers to reduce the inter-reflections in axial-mount, a “pupil adapter” must be included with a further increase of the optical path length. Although a multi-etalon spectropolarimeter works in quasi-monochromatic light, the Fraunhofer lines of interest cover a wide range of wavelengths (850 to 1650 nm), which demands a good chromatic aberration control. A low instrumental polarization (≤0.5% ) is also required to allow a high polarimetric precision. Finally, some secondary optical paths are required to perform the initial instrumental setup and to secure the best instrumental performances. A diffraction-limited optical solution for ATST is described that fulfills all the above requirements in a relative small volume.
ADAHELI ADvanced Astronomy for HELIophysics is a solar satellite designed to investigate the fast dynamics of
the solar photosphere and chromosphere performing visible and NIR broad-band and monochromatic observations
of selected atomic lines. ADAHELI is an Italian Space Agency (ASI) project, approved for a feasibility study
within the ASI Small Missions call. ISODY Interferometer for SOlar DYnamics is a Gregorian telescope and
its focal plane suite (FPS). The FPS is composed of a high-resolution fast acquisition system, based upon a
tandem of Fabry-Pérot interferometers operating in the visible and NIR regions on selected solar atmospheric
lines, a broad band channel, and a correlation tracker used as image stabilization system. In this contribution we
describe the Fabry-Pérot étalon prototype, based on the capacitance-stabilised concept, realized in our laboratory
to perform preliminary mechanical and optical tests with a view to a future Fabry-Pérot étalon prototype for
In designing the optics of an imaging classic mount multi-étalon spectro-polarimeter as a post-focus instrument for the
next generation of ground-based solar telescopes (Advanced Technology Solar Telescope, European Solar Telescope),
many constraints must be considered. The large entrance pupil diameter of the telescope (4 m), the demanded large field
of view (≥ 90 arcsec), high spectral resolving power (≥ 200000), and limited field-dependent blue-shift of the
instrumental profile (≤ 3 FWHM) require Fabry-Pérot interferometers of large diameter (≥ 200 mm), lighted by highly
collimated beams. This implies large optical elements and long optical paths. Moreover, to use interference pre-filters,
placed between the interferometers to reduce the inter-reflections in axial-mount, with a relatively small diameter (≤ 70
mm), a "pupil adapter" should be included, with a further increase of the optical path length. Although a multi-étalon
spectro-polarimeter works in quasi-monochromatic light, the Fraunhofer lines of interest cover a wide range of
wavelengths (850 nm - 1650 nm), demanding a good chromatic aberration control. Finally, a low instrumental
polarization (≤ 0.5 %) is required to allow a high polarimetric precision. In this paper a diffraction limited optical
solution is described, fulfilling all the above requirements in a relative small volume.
ADvanced Astronomy for HELIophysics (ADAHELI) is a Small Mission to study the structure and fast dynamics of
the low solar atmosphere, performing Visible-NIR monochromatic and broad-band observations. The mission will
achieve millimeter full disk observations as well. The ADAHELI Team has succesfully completed, in December
2008, the Phase A study awarded by the Italian Space Agency (ASI).
The Interferometer for SOlar Dynamics (ISODY), on board the ADAHELI satellite, comprises a Gregorian
telescope and its focal plane suite. The advanced design focal plane suite uses fast CMOS cameras for investigating
photospheric and chromospheric fast dynamics and structure. ISODY is equipped with a pioneering focal plane suite
composed of a spectral channel, based upon a tandem of Fabry-Perot interferometers operating in the visible-NIR
spectral region, a broad band channel for high resolution imaging, and a correlation tracker used as an image
stabilization system. ADAHELI's mission profile has been tailored to limit the spacecraft's radial velocity in the
Sunward direction, to not exceed ±4 km/s, during 95% of the yearly orbit, to allow a continuous use of the on-board
In this paper we describe the optical design of a catadioptric endoscope for the Joint European Torus (JET). The JET is the flagship experiment in the European nuclear fusion research programme. It is a large tokamak (Russian acronym for “toroidal magnetic chamber”) system located at Culham (UK). At the centre of this machine there is a toroidal (ring - shaped) vacuum vessel where the plasma is confined by magnetic fields. The endoscope explores in two wave bands (4.2 μm - 4.4 μm and 0.6 μm - 0.7 μm) an entire cross section of the vacuum vessel. It then creates for each wave band an image onto a separate area image sensor, located 5500 mm away from the plasma behind a concrete shield. The endoscope performs two different functions namely: infrared thermography on plasma facing components and in vessel inspection.
Relating two different methods of data analysis for assessing the absolute planarity of reference flats is reported. Considered methods are based on Zernike representation and pixel handling, respectively. Operations to be implemented on interferometric systems to use the same data set for comparative processing are described.
The Fritz's method using Zernike polynomials to assess the absolute planarity of test plates is revisited. Such method is based on four interferometric measurements, which are assumed perfectly correlated. In experiments, due to several instability sources, the data set is missing perfect correlation. Modifications of the Fritz's method are here presented, taking into account the residual uncorrelation of the data; such modified approach is demonstrated on a data set from experiments, achieving nanometer uncertainty level.
Observations of wave-optics effects in sunlight are reported. In particular, conditions are described that allow for visual detection of diffraction phenomena from line edges. Typical fringe patterns are demonstrated, also showing color features that account for the wavelength dependence of the diffraction process. Hints to optimize the observation are given, outlining the aspects of simplicity and naturalness of the occurrence.
The present work demonstrates the use of a general purpose interferometer to measure the deformations of a scattering surface by processing speckle patterns instead of smooth wavefronts. The operation of the interferometer are re- programmed to include algorithms adapted to handling speckle patterns. Details of the measuring operation are described, limits of applicability are discussed, and experimental results are presented.
The design, manufacture and testing of a high aperture and wide field of view optical system is reported. Under specified constraints, the design is discussed in terms of aperture, studying its effects on the performance, the barrel length, the weight and the costs of the system. Some design criteria are detailed, and mounting techniques are outlined in order to obtain optimum position and alignment of the lenses. A f/1.8 objective is described in particular, and system tolerances are given. Interferometric tests on prototype systems are required, comparing the results with the design data.
A homodyne interferometer is described, using a He-Ne laser source and working in Twyman- Green configuration. By means of proper retardation plates and polarizing beam splitters, three signals at 90 degree(s) phase lag are made available. Such signals are digitized and fed into a desktop computer. Fitting procedures and computing algorithms are then implemented working out a phase angle; monitoring such a phase provides information on the optical path difference between the two arms of the interferometer. The general features of the approach are reviewed; examples of application are given, monitoring the fine displacement of mechanical parts and measuring the optical thickness variation of a soap film in air.
This work relates to the glass polishing process in optical workshops, classically based on pitch lapping of ground surfaces. The process progressively removes the asperities and lowers the surface toward the bottom of the remaining pits. According to Preston's hypothesis, the polishing rate is proportional to the velocity of the lap and to its pressure on the area of contact. A computerized imaging technique is here reported to monitor the progress of the polishing action. A Nomarski microscope with a 16X objective has been equipped with a TV camera connected to a personal computer. A frame grabber provides image data that are elaborated to work out the surface features. Statistics are obtained on the fractional area covered by the residual pits. Referring to the Preston's hypothesis, the measured parameter is related to the actual finish grade of the surface inspected.
The application of interferometer as a spectral transducer in fiber optic sen- sors (FOS) provides the combination of high sensitivity of interferometric devises with absolute accuracy and reliability of spectral encoding. Different techniques may be used to analyze sensor's output spectrum. In1 an additional tunable inter- ferometer was applied to produce fringes of superposition. In this case, however, the severe problem of accurate measurement of the reference interferometer's opti- cal path difference (OPD) appears. It's overcoming results in dramatic sophistica- tion of FOS's optical and electronic processing schemes. An alternative approach deals with the straightforward and accurate detection of the output spectrum by means of micro-optic spectrometer with CCD photodetector. Recently, 2 3pressure and pe temrature FOS based on this principles have been reported' , but their measurement ranges werelimited by the single free spectral range of interferometer. Such performance produces an error when the change of OPD exceeds A, (where 2k, is the optical carrier wavelength). In the present paper we propose to detect simulta- neously several resonances of interferometer's spectral output and determine the number N of interference order for the exploited minimum (or maximum). This proce- dure provides the reliable fixed internal zero and the tremendous extension of the device's dynamic range.
After being processed by an optical system, either in reflection or in transmission, a probe wavefront contains information on the induced aberrations that are conveniently reported by interferometry. Yet, the probe beam also includes diffuse light, mostly produced by scattering at the optical surfaces. The actual disturbance that is studied via interferometry is a partially developed speckle field, made of a strong bias phaser plus the weak random contribution due to scattering. The standard deviation of the random contribution, normalized to the modulus of the bias phaser, is assumed as a characteristic parameter of the speckle field. Such a parameter has been measured with digital phase-shift interferometric techniques on a series of selected samples, corresponding to different optical finish. In excess of 120 samples have been studied, referred to a traceable polishing process. The results are interpreted on the basis of available models of polishing mechanisms. Data fitting to the equation of the theory is reported; the results are in fair agreement with the theory.