3. Further steps in designing and using additional instruments at Terskol
During the past few decades, the situation in ground-based and space astronomy has changed dramatically, to a certain degree thanks to new observational facilities. However, not only new telescopes and new satellites or planet missions give the possibility to answer some of the fundamental questions. Many advances in astronomy come from the development and use of specific instruments and techniques. Thus, ground-based telescopes (including small- and medium-aperture telescopes, equipped with CCD cameras and additional instruments) still provide good enough opportunities for long-term astrometric, photometric, polarimetric, and other observations.
3.1. Exploration of Solar System bodies
3.1.1 The focal reducer for high-quality imaging
Observations of Solar System bodies started at Terskol in 1996 with the aid of the two-channel focal reducer attached to the Cassegrain focus of the 2-m telescope. This instrument was built by the Max-Planck Institute for Solar System Research (formerly Max-Planck Institute for Aeronomy - MPAe) and brought to the Terskol Observatory according to the agreement on cooperation between MPAe and ICAMER.
The focal reducer is a lens system consisting of collimator, parallel beam and camera lens with high light gathering power. It shrinks the telescope image to a size that matches the seeing disk and the required field of view with the detector (CCD) resolution. This property makes the reducer suitable for taking atmospheric turbulence into account (Jockers 1997).
Atmospheric turbulence is one of the factors limiting the magnification of optical ground-based telescopes. With the 2-m telescope at Terskol it is impossible to get a resolution much better than 1/20000 of a degree, i. e. 0.18 arcsec. However, the focal length of the telescope can be adjusted to the task of the observations.
Two-color image of comet C/1995 O1 (Hale-Bopp) obtained at Terskol with the Two-Channel Focal Reducer on April 13, 1997. The orange color represents the dust grains of the cometary atmosphere. Note the dust spirals observed in this huge comet. The blue color represents the distribution of cometary ions (here the ion OH+) (Jockers 2003)
In its Cassegrain focus, the 2-m telescope has a focal length optimized to the best conditions of atmospheric turbulence. The two-channel focal reducer reduces this focal length by a factor of 2.86 to a resolution of about 1 arcsec. This allows one to obtain sharp images even under non-optimum conditions of atmospheric turbulence. At the same time the light-gathering power increases by the square of the same factor (2.862 = 8.2), i. e. less time is needed to get a well-exposed image (Jockers 2003).
As an additional feature, the focal reducer has two channels to conduct simultaneous observations in two wavelength bands between 350 nm and 1000 nm. Moreover, it is equipped with Fabry-Perot etalons for narrow-band observations.
The Io torus, imaged in the forbidden lines of S+ ([SII]) and S++ ([SIII]) (Jockers 2003)
Using this electronic imaging device, in 1996-2002 the MPAe researchers (Prof. Klaus Jockers and his group), in collaboration with Russian, Ukrainian and Bulgarian astronomers, successfully studied gas and dust in comets, conducted polarimetry of cometary dust and asteroids, and astrometry and photometry of the inner Jovian satellites (Jockers et al. 1998; Kulyk & Jockers 2004). Figure 7 shows a two-color image of comet C/1995 O1 (Hale-Bopp) obtained with the two-channel focal reducer at Terskol. Moreover, the morphology and brightness of the plasma torus of the Jovian satellite Io were investigated in order to derive its physical properties.
In recent years, a new focal reducer based on the original Meinel camera, was developed and constructed at the Terskol Observatory.
3.1.2 Multi-Mode Cassegrain Spectrograph and other instruments to support research activities
The Multi-Mode Cassegrain Spectrograph (MMCS) was designed in 2003 for observations of faint objects at the Cassegrain focus (F/8) of the 2-m telescope. It has two modes of functioning: (i) grating / echelle spectrometer mode (300 nm -1200 nm, R =1000…15000, the limiting magnitude is ~15.5 mag); (ii) CCD photometer mode.
The main components of MMCS are as follows:
- collimator is a parabolic mirror with diameter of 75 mm and focal length of 600 mm;
- special (with a central hole) echelle grating with 75 gr mm-1 and 63. 5° blaze angle;
- diffraction grating with 600 gr mm-1 and 8° blaze angle;
- diffraction grating with 300 gr mm-1 and 4° blaze angle;
- diffraction grating with 300 gr mm-1 and 6° blaze angle;
- diffraction grating with 200 gr mm-1 and 28° blaze angle;
- a 45° crown prism;
- Schmidt-Cassegrain camera with F=150 mm;
- lens camera with F=180 mm;
- detectors: Wright Instruments CCD, 1242x1152 pixels of 22.5 µm; Wright Instruments CCD (back-illuminated), 1252x1152 pixels of 22.5 µm.
Some of the observational capabilities of MMCS are presented in Table 1.
Table 1.
| Mode: | echelle (64°+4°) | quasi-echelle (28°+prism) | classic 8°-grating | classic 4°-grating |
| Maximum resolution | 13500 | 3200 | 1200 | 600 |
| Limiting magnitude, S/N~10, 1h exposure | ~12.5 | ~14.5 | ~15 | ~16 |
MMCS and some specific automatic photometers, which were developed and constructed at the Terskol Observatory, are in productive scientific use on the telescopes. These devices contribute significantly to achieving important results in the following fields of research:
- detection and monitoring of potentially hazardous objects (Earth-approaching asteroids, comets) and space debris,
- precise astrometry and photometry of comets and asteroids,
- photometric observations of trans-neptunian objects (Rousselot et al. 2005),
- high-resolution mapping of planetary surfaces by the short exposure method.
Moreover, the abovementioned instruments are heavily used within other observational programmes, which have been run at Terskol: photometry of variable stars and cataclysmic variables (also within the framework of the Whole Earth Telescope), search for optical afterglow of gamma ray bursts, etc.
The program on NEOs observations is carried out in collaboration with the Institute of Astronomy (Russia) and includes an astrometric position determination and taxonomic investigation of the objects. The technique applied allows us to detect potentially hazardous bodies of decameter size at a distance of some million kilometers. Using photometric and spectroscopic observations, a larger number of asteroids were classified with regard to size and taxonomic type.
As for observations of satellites in or near geosynchronous orbit, several important results were also obtained. For instance, a critical approach of the geosynchronous satellite Arabsat 1C to a Russian satellite was revealed in February 1997.
3.2 Up-to-date science with a high-resolution spectrometer
Important advances in observational techniques, data acquisition and processing have been made at the Terskol Observatory when in the late 1990s an echelle spectrometer was developed and put into operation. This work was performed in collaboration with Prof. Jacek Krelowski and his colleagues from the Toruń Center for Astronomy of the Nicolaus Copernicus University (Poland).
The schematic layout of the 3-camera cross-dispersed echelle spectrometer
The spectrometer called MAESTRO (MAtrix Echelle SpecTROmeter) was completed and installed in an isolated and temperature-stable coude-room in the tower of the 2-m telescope. It offers a range of resolutions: R=λ/δλ = 45,000; 120,000; 210,000 and 500,000. These resolutions can be achieved by means of three Schmidt cameras with focal lengths of 450 mm, 875 mm and 1960 mm, respectively. The limiting magnitude of the spectrometer is about 11m (S/N ~ 10, 1h exposure).
The main components of the cross-dispersed echelle spectrometer MAESTRO are as follows:
• collimator is an off-axis parabolic mirror with diameter of 200 mm and focal length of 7100 mm;
• mosaic R2: built from two 200x250 mm echelle gratings with 37.5 gr mm-1 and 63.5° blaze angle;
• mosaic R6: built from three 220x320 mm echelle gratings with 37.5 gr mm-1 and 80.5° blaze angle;
• cross-disperser is a 45° crown prism;
• cameras: 1 - Schmidt (folded), ƒ = 450 mm; 2 - Schmidt with outer focus ƒ= 875 mm; 3 - Schmidt with outer focus ƒ = 1900 mm;
• detectors: Wright Instruments CCD, 1242x1152 pixels of 22.5 µm; Wright Instruments CCD (back-illuminated), 1252x1152 pixels of 22.5 µm.
The spectrometer attached to the 2-m telescope opens up entirely new fields of research. For instance, absorption spectra of dark interstellar clouds reveal the complexity of physicochemical processes inside them. Some observed interstellar absorption features, especially diffuse interstellar bands (DIBs), still remain unidentified. An analysis of their high-resolution profiles seems to be the most prospective way to identify their carriers. Since the Terskol Observatory is located at a remote and high-altitude mountain site there are a very dark sky background and the seeing better than anywhere in Europe. The very high resolution (up to R=500000) achieved allows analysis of profile shapes of interstellar spectral features. The first spectra acquired at Terskol with the aid of the MAESTRO spectrometer clearly showed that the substructure of DIBs can be easily traced in these data. The profile details matched perfectly with those observed at other observatories.
Profiles of the 5797 DIB acquired for the target HD179406 with the aid of the HARPS spectrograph at ESO and for HD24398 using the MAESTRO at Terskol (R=500000). The shapes of the two profiles are clearly identical; all substructure details are seen in both.
It should also be mentioned that the Terskol echelle spectra allow a very precise (up to 0.0003 nm) wavelength determination of any of the detectable features due to the application of a global dispersion curve.
With the aid of the echelle spectrometer installed at the coude focus of the 2-m telescope, various studies of interstellar spectral features have been conducted at Terskol. The spectrometer is capable of recording the infrared spectra of homonuclear molecules such as C2 (Phillips 2-0 band) or C3; the first images of the Phillips 2-0 band with the resolution of R=120000 were those from MAESTRO (Krelowski et al. 2003).
Another important observational result was evidence for the existence of the neutral (independent of wavelength) interstellar absorption.
In 2007, the UV branch of the MAESTRO spectrograph was developed. At present, the spectral region to be covered is from 300 nm to 1000 nm. This allows us to observe OH and NH molecular bands, as well as TiII lines in order to extend our constraints to the chemistry of oxygen and nitrogen in the interstellar medium.
