G. G. Valyavin

Automation of the OAN/SPM 1.5-meter Johnson telescope for operations with RATIR

The Reionization And Transients Infra-Red (RATIR) camera is intended for robotic operation on the 1.5-meter Harold Johnson telescope of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. This paper describes the work we have carried out to successfully automate the telescope and prepare it for RATIR. One novelty is our use of real-time absolute astrometry from the finder telescopes to point and guide the main telescope.

Monitoring and modelling of white dwarfs with extremely weak magnetic fields - WD 2047+372 and WD 2359-434

Magnetic fields are detected in a few percent of white dwarfs. The number of such magnetic white dwarfs known is now some hundreds. Fields range in strength from a few kG to several hundred MG. Almost all the known magnetic white dwarfs have a mean field modulus ≥1 MG. We are trying to fill a major gap in observational knowledge at the low field limit (≤200 kG) using circular spectro-polarimetry. In this paper we report the discovery and monitoring of strong, periodic magnetic variability in two previously discovered “super-weak field” magnetic white dwarfs, WD 2047+372 and WD 2359-434. WD 2047+372 has a mean longitudinal field that reverses between about −12 and + 15 kG, with a period of 0.243 d, while its mean field modulus appears nearly constant at 60 kG. The observations can be interpreted in terms of a dipolar field tilted with respect to the stellar rotation axis. WD 2359-434 always shows a weak positive longitudinal field with values between about 0 and + 12 kG, varying only weakly with stellar rotation, while the mean field modulus varies between about 50 and 100 kG. The rotation period is found to be 0.112 d using the variable shape of the H α line core, consistent with available photometry. The field of this star appears to be much more complex than a dipole, and is probably not axisymmetric. Available photometry shows that WD 2359-434 is a light variable with an amplitude of only 0.005 mag; our own photometry shows that if WD 2047+372 is photometrically variable, the amplitude is below about 0.01 mag. These are the first models for magnetic white dwarfs with fields below about 100 kG based on magnetic measurements through the full stellar rotation. They reveal two very different magnetic surface configurations, and that, contrary to simple ohmic decay theory, WD 2359-434 has a much more complex surface field than the much younger WD 2047+372.

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.

Spectral multiplexed VPHG based on photopolymers: the first application on a spectrograph

Many of the current spectrographs available at state-of-the-art telescopes facilities, possess specifications that are strongly limited by the dispersing elements that are used. Therefore, a refurbishment of these devices would potentially increase the performances if innovative designs are considered. We propose a solution for designing stacked VPHG that is able to secure efficiently different spectra in a single shot. This could be possible considering parameters that are specific for a particular class of holographic material, the photopolymers, that are well known for bringing reliability and precise throughput. We demonstrate the applicability of our solution, through the example of the new spectrograph designed for the 1m telescope at SAO RAS. The spectrograph will cover a spectral range 444-706 nm with the spectral resolving power of R=4273-5176 and the throughput maximum of 64%. The working ranges of the gratings are selected to provide more diffraction efficiency around the main important lines used in astrophysics.

Combined narrowband imager-spectrograph with volume-phase holographic gratings

In the present work we discuss a possibility to build an instrument with two operation modes - spectral and imaging ones. The key element of such instrument is a dispersive and filtering unit consisting of two narrowband volume-phase holographic gratings. Each of them provides high diffraction efficiency in a relatively narrow spectral range of a few tens of nanometers. Besides, the position of this working band is highly dependent on the angle of incidence. So we propose to use a couple of such gratings to implement the two operational modes. The gratings are mounted in a collimated beam one after another. In the spectroscopic mode the gratings are turned on such angle that the diffraction efficiency curves coincide, thus the beams diffracted on the first grating are diffracted twice on the second one and a high-dispersion spectrum in a narrow range is formed. If the collimating and camera lenses are corrected for a wide field it is possible to use a long slit and register the spectra from its different points separately. In the imaging mode the gratings are turned to such angle that the efficiency curves intersect in a very narrow wavelength range. So the beams diffracted on the first grating are filtered out by the second one except of the spectral component, which forms the image. In this case the instrument works without slit diaphragm on the entrance. We provide an example design to illustrate the proposed concept. This optical scheme works in the region around 656 nm with F/# of 6.3. In the spectroscopic mode it provides a spectrum for the region from 641 to 671 nm with reciprocal linear dispersion of 1.4 nm/mm and the spectral resolving power higher than 14000. In the imaging mode it covers linear 12mm x 12mm field of view with spatial resolution of 15- 30 lines/mm.

Development of illumination optics in optical scheme of high-resolution fiber-fed echelle-spectrograph for the Big Telescope Alt-azimuth (BTA)

The report describes the development and optimization of optical scheme of the illumination optics of the entrance slit for the high-resolution fiber-fed echelle-spectrograph. The optical system of the illuminator provides the necessary agreement of the numerical apertures of the fiber and spectrograph, as well as it allows to install the necessary equipment to obtain the required structure of the image. As a result of the designing two components illumination system was obtained, which has a good transmission in a specified spectral range and low cost. This research provides a good instrument for performing modern researches for the astronomy.

Achieving the resolution of the spectrograph of the 6m large Azimuthal telescope

Special Astrophysical Observatory of Russian Academy of Sciences (SAO RAS) creates a spectrograph with high spectral resolution for the 6-meter telescope. The spectrograph consists of a mobile unit located at the focus of the telescope’s main mirror, a stationary part located under the telescope and optical fibers which transmit light from the mobile part to the stationary one. The spectral resolution of the stationary part should be R=100000. To achieve such a value, the scheme has two spectral elements, with cross-dispersion. The main spectral element is an echelle grating. The second spectral element is a prism with a diffraction grating on one facet.

Polarimetry as a tool to find and characterise habitable planets orbiting white dwarfs

There are several ways planets can survive the giant phase of the host star, hence one can consider the case of Earth-like planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU from the star would remain in the continuous habitable zone (CHZ) for about 8 Gyr. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf (CWD) is 102 (104) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a powerful tool to detect close-in planets around white dwarfs. Multi-band polarimetry would also allow one to reveal the presence of a planet atmosphere, even providing a first characterisation. With current facilities a super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known CWD. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue. Preliminary habitability study show also that photosynthetic processes can be sustained on Earth-like planets orbiting CWDs and that the DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence white dwarfs are compatible with the persistence of complex life from the perspective of UV irradiation.