Eduard R. Muslimov

Combining freeform optics and curved detectors for wide field imaging: a polynomial approach over squared aperture

In the recent years a significant progress was achieved in the field of design and fabrication of optical systems based on freeform optical surfaces. They provide a possibility to build fast, wide-angle and high-resolution systems, which are very compact and free of obscuration. However, the field of freeform surfaces design techniques still remains underexplored. In the present paper we use the mathematical apparatus of orthogonal polynomials defined over a square aperture, which was developed before for the tasks of wavefront reconstruction, to describe shape of a mirror surface. Two cases, namely Legendre polynomials and generalization of the Zernike polynomials on a square, are considered. The potential advantages of these polynomials sets are demonstrated on example of a three-mirror unobscured telescope with F/# = 2.5 and FoV = 7.2x7.2°. In addition, we discuss possibility of use of curved detectors in such a design.

Design and modeling of a moderate-resolution astronomic spectrograph with volume-phase holographic gratings

We present an optical design of astronomic spectrograph based on a cascade of volume-phase holographic gratings. The cascade consists of three gratings. Each of them provides moderately high spectral resolution in a narrow range of 83nm. Thus the spectrum image represents three lines covering region 430-680nm. Two versions of the scheme are described: a full-scale one with estimated resolving power of 5300-7900 and a small-sized one intended for creation of a lab prototype, which provides the resolving power of 1500-3000. Diffraction efficiency modeling confirms that the system throughput can reach 75%, while stray light caused by the gratings crosstalk is negligible. We also propose a design of image slicer and focal reducer allowing to couple the instrument with a 6m-telescope. Finally, we present concept of the instrument’s optomechanical design.

Experimental Study of an Advanced Concept of Moderate-resolution Holographic Spectrographs

We present the results of an experimental study of an advanced moderate-resolution spectrograph based on a cascade of narrow-band holographic gratings. The main goal of the project is to achieve a moderately high spectral resolutionwith R up to 5000 simultaneously in the 4300-6800 A visible spectral range on a single standard CCD, together with an increased throughput. The experimental study consisted of (1) resolution and image quality tests performed using the solar spectrum; and (2) a total throughput test performed for a number of wavelengths using a calibrated lab monochromator. The measured spectral resolving power reaches values over R>4000 while the experimental throughput is as high as 55%, which is in good agreement with the modeling results. Comparing the obtained characteristics of the spectrograph under consideration with the best existing spectrographs, we conclude that the used concept can be considered a very competitive and cheap alternative to the existing spectrographs of the given class. We propose several astrophysical applications for the instrument and discuss the prospect of creating its full-scale version.

Advanced modeling of a moderate‐resolution holographic spectrograph

In the present article we consider an accurate modeling of a spectrograph with a cascade of volume-phase holographic gratings. The proposed optical scheme allows us to detect spectra in an extended wavelength range without gaps, providing relatively high spectral resolution and high throughput. However, modeling and minimization of possible cross-talk between gratings and stray light in such a scheme represents a separate task. We use analytical equations of the coupled-wave theory together with rigorous coupled-wave analysis to optimize the gratings parameters and further apply the latter together with a non-sequential ray-tracing algorithm to model propagation of beams through the spectrograph. The results show relatively high throughput up to 53% and the absence of any significant cross-talk or ghost images, even for ordinary holograms recorded on dichromated gelatin.

Design of pre-optics for laser guide star wavefront sensor for the ELT

Abstract In the present paper, we consider the optical design of a zoom system for the active refocusing in laser guide star wavefront sensors. The system is designed according to the specifications coming from the Extremely Large Telescope (ELT)-HARMONI instrument, the first-light, integral field spectrograph for the European (E)-ELT. The system must provide a refocusing of the laser guide as a function of telescope pointing and large decentring of the incoming beam. The system considers four moving lens groups, each of them being a doublet with one aspherical surface. The advantages and shortcomings of such a solution in terms of the component displacements and complexity of the surfaces are described in detail. It is shown that the system can provide the median value of the residual wavefront error of 13.8–94.3 nm and the maximum value <206 nm, while the exit pupil distortion is 0.26–0.36% for each of the telescope pointing directions.

Compact dual-band spectrograph

Abstract A concept and design method of a dual-band flat-field spectrograph is presented. In the proposed optical scheme, the working range of wavelengths is divided into two subranges. A reflection holographic grating is used in the first subrange, and a transmission concave holographic grating is used in the second one. Both of them are aberration-corrected flat-field gratings. As an example, a scheme of spectrograph for spectral range from 278 to 560 nm is considered.

Bringing high-spectral resolution to VLT/SPHERE with a fiber coupling to VLT/CRIRES+

Atmospheric composition provides essential markers of the most fundamental properties of giant exoplanets, such as their formation mechanism or internal structure. New-generation exoplanet imagers, like VLT/SPHERE or Gemini/GPI, have been designed to achieve very high contrast (< 15 mag) at small angular separations (<0.500) for the detection of young giant planets in the near-infrared, but they only provide very low spectral resolutions (R < 100) for their characterization. High-dispersion spectroscopy at resolutions up to 105 is one of the most promising pathways for the detailed characterization of exoplanets, but it is currently out of reach for most directly imaged exoplanets because current high-dispersion spectrographs in the near-infrared lack coronagraphs to attenuate the stellar signal and the spatial resolution necessary to resolve the planet. Project HiRISE (High-Resolution Imaging and Spectroscopy of Exoplanets) ambitions to develop a demonstrator that will combine the capabilities of two flagship instruments installed on the ESO Very Large Telescope, the high-contrast exoplanet imager SPHERE and the high-resolution spectrograph CRIRES+, with the goal of answering fundamental questions on the formation, composition and evolution of young planets. In this work, we will present the project, the first set of realistic simulations and the preliminary design of the fiber injection unit that will be implemented in SPHERE.

Curved detectors developments and characterization: application to astronomical instruments

Many astronomical optical systems have the disadvantage of generating curved focal planes requiring flattening optical elements to project the corrected image on at detectors. The use of these designs in combination with a classical at sensor implies an overall degradation of throughput and system performances to obtain the proper corrected image. With the recent development of curved sensor this can be avoided. This new technology has been gathering more and more attention from a very broad community, as the potential applications are multiple: from low-cost commercial to high impact scientific systems, to mass-market and on board cameras, defense and security, and astronomical community. We describe here the first concave curved CMOS detector developed within a collaboration between CNRS- LAM and CEA-LETI. This fully-functional detector 20 Mpix (CMOSIS CMV20000) has been curved down to a radius of Rc =150mm over a size of 24x32mm2. We present here the methodology adopted for its characterization and describe in detail all the results obtained. We also discuss the main components of noise, such as the readout noise, the fixed pattern noise and the dark current. Finally we provide a comparison with the at version of the same sensor in order to establish the impact of the curving process on the main characteristics of the sensor.

Curved CMOS sensor: characterization of the first fully functional prototype

Many are the optical designs that generate curved focal planes for which field flattener must be implemented. This generally implies the use of more optical elements and a consequent loss of throughput and performances. With the recent development of curved sensor this can be avoided. This new technology has been gathering more and more attention from a very broad community, as the potential applications are multiple: from low-cost commercial to high impact scientific systems, to mass-market and on board cameras, defense and security, and astronomical community. We describe here the first concave curved CMOS detector developed within a collaboration between CNRS-LAM and CEA-LETI. This fully-functional detector 20Mpix (CMOSIS CMV20000) has been curved down to a radius of Rc =150mm over a size of 24x32mm2. We present here the methodology adopted for its characterization and describe in detail all the results obtained. We also discuss the main components of noise, such as the readout noise, the fixed pattern noise and the dark current. Finally we provide a comparison with the at version of the same sensor in order to establish the impact of the curving process on the main characteristics of the sensor.

Freeform optics complexity estimation: comparison of methods

In the present paper we compare different approaches for estimation of freeform and aspherical surfaces complexity. We consider two unobscured all-reflective telescope designs: a narrow-field Korsch-type system with a slow freeform secaondary and a wide-field Schwarzschild-type system with an extreme freeform secondary. The performance improvement obtained due to the freeforms use is demonstrated. The Korsch telescope provides a diffraction-limited image quality for a small field 0.8x0.1° at F/3. The Schwarzschild design covers a large field of 20x8° and allows to increase the aperture from F/6.7 to F/3. Also, we analyze the freeforms shapes using different techniques. It is shown that the usual measures like root-mean square deviation of the sag are ineffective. One of the recommended way to estimate the surface complexity is computation of the residual slope and its conversion into fringes frequency. A simpler alternative is computation of the sag deviation integral.