Transient Double-beam Spectrograph for the 2.5-m Telescope of the Caucasus Mountain Observatory of SAI MSU

Abstract: The Transient Double-beam Spectrograph (TDS) is designed for optical low-resolution observations of non-stationary and extragalactic sources with the 2.5-m telescope of Caucasus Mountain Observatory of the Sternberg Astronomical Institute. It operates simultaneously in a short-wavelength (360--577 nm, reciprocal dispersion 1.21 A/pixel, resolving power R=1300 with a 1 arcsec slit) and long-wavelength (567--746 nm, 0.87 A/pixel, R=2500) channels. The light is split by a dichroic mirror with a 50% transmission at 574 nm. In the "blue" channel, the automatic replacement of the grating by a grism with a double resolving power is possible. Two CCD-cameras use E2V 42-10 detectors cooled down to $-70^\circ$C with a readout noise of 3 $e-$ at a readout rate of 50 kHz. The spectrograph is equipped with a back slit viewer camera and a calibration unit allowing to record a comparison spectrum from a hollow cathode lamp for wavelength calibration or from an LED source with a continuous spectrum (the "flat field") to take into account the vignetting and uneven slit illumination. The throughput of the entire optical path without slit loss is 20% at the zenith in the "blue" and 35% in the "red" channel. Excluding the atmosphere and the telescope, the efficiency of the TDS itself reaches a maximum of 47% and 65% respectively. The spectrograph is permanently mounted in the Cassegrain focus of the 2.5-m telescope of CMO SAI MSU sharing the port with a wide-field photometric CCD-camera. The spectrograph is fed by the light from a folding mirror introduced into the optical path. Since November 2019, TDS has been used for regular observations of non-stationary stars and extragalactic sources up to 20-th mag in a 2-h exposure with a signal-to-noise ratio >5 per pixel.

• The paper demonstrates how a transient double-beam spectrograph refines redshift estimates of distant galaxy clusters and doubles the known high-z sample.
• The methodology employs deep photometric imaging and precision spectroscopy with δz≈0.001–0.003 accuracy, leveraging multi-telescope observations.
• The paper significantly expands the catalog by identifying seven clusters, including one with strong gravitational lensing, enhancing cosmological sampling.

Overview

The paper "Transient Double-beam Spectrograph for the 2.5-m Telescope of the Caucasus Mountain Observatory of SAI MSU" presents systematic observations focused on distant galaxy clusters using optical identifications and spectroscopic measurements. It particularly concentrates on clusters from the second catalog of Sunyaev-Zeldovich signal sources from the Planck Observatory survey.

Objective and Methodology

The primary objective of the study is to enhance the current understanding of distant galaxy clusters with redshifts approximately in the range $z\approx0.7$–$0.9$. The approach involves comprehensive optical observations and spectroscopic measurements to refine redshift estimates and confirm the presence of galaxy clusters corresponding to Sunyaev-Zeldovich sources. The data was collected using various telescopes, including the 1.5-m Russian-Turkish Telescope (RTT-150), the 1.6-m telescope at the Sayan Observatory, the 3.5-m CAHA Observatory telescope, and the 6-m SAO RAS telescope.

Key Findings

The research successfully identified seven distant galaxy clusters, including one significant cluster, PSZ2 G126.57+51.61, which contributes to the cosmological sampling. It was observed that the central areas of two clusters exhibited strong gravitational lensing arcs, with one galaxy showing a redshift of $z=4.262$. These observations effectively double the known number of galaxy clusters from the Planck survey at high redshifts. The completion degree of optical identifications varies with redshift, with high-redshift, low-luminosity clusters being harder to identify optically due to reduced completeness.

Observational Techniques

Key observational methods involved using deep imaging in the photometric SDSS system, alongside spectroscopic data from instruments like the SCORPIO and SCORPIO-2 on the BTA telescope. The spectra allowed for detailed redshift measurements, achieving precision levels around $\delta z=0.001$–$0.003$. The research utilized data processing techniques to correct for shifts and calibrate spectral data. Moreover, significant computational resources were devoted to analyzing redshifts systematically, utilizing software like IRAF for data analysis.

Implications

The study significantly expands the catalog of high-redshift galaxy clusters and provides a more precise determination of their spectroscopic properties. The success of such surveys demonstrates the efficacy of using comprehensive multi-telescope instruments to track cosmic structures and contribute essential data for cosmological models.

Conclusion

This endeavor to map and analyze distant galaxy clusters through precise observations using a variety of telescopic resources has proven to be a substantial contribution to the field of astrophysics. The expansion of known galaxy clusters at high redshifts by approximately double signifies an improvement in catalog completeness. Such findings provide valuable insights into the mass distribution and evolution of cosmic structures, bearing implications for our understanding of the universe's expansion and mass-energy content. Further observations and continued refinement of these methods are anticipated to yield even more extensive datasets, enriching our understanding of high-redshift galaxy clusters.