The Shelyak eShel III is a high-performance, third-generation Echelle spectrograph, specially developed for demanding astronomical observation programmes. It combines high spectral resolution with a broad spectral range. This makes the instrument suitable for scientifically-oriented amateur astronomers, universities, observatories and research groups wishing to quantitatively analyse stellar spectra and investigate temporal changes in astrophysical objects.
With a resolution of R = 10,000 in the spectral range from approximately 370 to 910 nanometres, the eShel III covers large parts of the near-ultraviolet, the entire visible spectrum and part of the near-infrared. The simultaneous detection of numerous spectral lines in a single exposure is particularly useful for analysing stellar atmospheres, measuring radial velocities, conducting observation of variable stars and studying objects with emission lines.
The Echelle principle: high resolution over a wide spectral range
Conventional grating spectrographs often face a fundamental design trade-off: with a given sensor size, high spectral resolution comes at the expense of the wavelength range that can be covered simultaneously. The eShel III resolves this trade-off through the Echelle method, in which a diffraction grating operates at high diffraction orders.
Light from the observed object is coupled into the spectrograph via an optical fibre and directed onto the Echelle grating. The high diffraction orders produce a fine spectral resolution. As the individual orders overlap in their wavelength ranges, a cross-dispersion system further separates the orders spatially from one another. In the eShel III, a prism performs this task.
As a result, the camera sensor does not capture a single, continuous spectral band, but rather a two-dimensional Echelle spectrum comprising numerous spectral orders arranged side by side. The analysis software extracts the individual orders, calibrates them and combines them into a scientifically usable spectrum.
The eShel III utilises, in particular, spectral orders 25 to 60. This allows a wavelength range of approximately 370 to 910 nm to be covered.
Fibre-coupled design for stable measurement conditions
A key design feature of the eShel III is the separation between the light-coupling system located on the telescope and the spectrograph itself. The starlight is transmitted to the spectrograph via an optical fibre with a core diameter of 50 µm.
This design offers several advantages:
- Thermal stability: The spectrograph can be housed in a temperature-controlled room. This reduces thermally induced changes in the optical geometry and facilitates reproducible measurements
- Improved measurement conditions for radial velocities: A more stable instrument environment facilitates the precise comparability of spectra taken in sequence over time
- Flexible installation: The telescope and the spectrograph do not need to be mounted immediately next to one another
- Convenient operation: The sensitive spectroscopic optics can be operated independently of the mechanical and thermal conditions at the telescope.
The concept is based on the manufacturer’s experience with professional fibre-coupled spectrographs such as MUSICOS and Narval. However, fibre transmission alone does not guarantee a specific radial velocity accuracy: this also depends on the calibration, the stability of the wavelength scale, the instrument configuration and the data reduction.
Optical system and detector
The eShel III spectrograph module uses a specially developed Petzval optical system with a focal length of 122 millimetres and an aperture ratio of f/2.2. This camera optics system images the two-dimensional spectrum generated by the Echelle system onto the scientific camera sensor. The design is optimised to ensure the most uniform possible focusing of the spectral orders used.
For spectral imaging, Shelyak recommends a monochrome camera with a Sony IMX571 sensor, such as the ZWO ASI2600MM Pro. The camera is not included with the spectrograph and must be purchased separately.
The combination of Echelle optics and a large-area CMOS sensor enables the simultaneous capture of numerous spectral orders.
Two versions for focusing
The spectrograph is fitted with a manual helical focuser for focusing the spectral image. However, a version with a motorised focuser from PrimaLuceLab is also available, which allows for remote-controlled focusing.
Calibration and light input
A scientifically usable spectrum requires reliable calibration of the spectral lines. Shelyak therefore offers a calibration unit featuring a thorium-argon hollow-cathode lamp (ThAr). This can be purchased as accessories or as part of a complete system together with the spectrograph.
The numerous known emission lines of these lamps serve as a reference for mapping detector positions to wavelengths.
The calibration system also includes a halogen lamp, which can be used for Flatfield images. Such images aid in the correction of sensitivity distribution and instrumental inhomogeneities.
Light is transmitted via two optical fibres, each 20 m long:
- Object fibre with a core diameter of 50 µm: Transmits the starlight collected by the telescope to the spectrograph.
- Calibration fibre with a core diameter of 200 µm: Conveys light from the calibration unit.
At the telescope, the fibre is connected via the FIGU F/6 fibre injection and guiding unit. This unit facilitates the precise coupling of the target object into the object fibre, which is only 50 µm thick, and enables tracking of the object. The complete system configuration also includes the associated high-voltage power supply for the calibration lamp, connection cables and adaptors.
Scientific applications
- Be stars and other emission-line objects: The study of emission lines, in particular the Balmer series of hydrogen, allows conclusions to be drawn about circumstellar matter, gas movements and changes in stellar environments. The eShel III is suitable for repeated observations of Be stars, as well as Wolf-Rayet stars, novae and young stellar objects.
- Variable stars and time-resolved spectroscopy: By repeatedly acquiring calibrated spectra, changes in line profiles and line positions can be detected. Pulsating stars such as RR Lyrae are of particular interest, as their spectra provide information about atmospheric motions and the phase of pulsation.
- Radial velocities and exoplanet research: Shifts in spectral lines relative to a reference can be used to determine a star’s radial velocity. Under suitable measurement conditions, this enables the study of spectroscopic binary stars and selected radial velocity programmes. The actual measurement accuracy depends largely on instrument stability, calibration, the signal-to-noise ratio and the analysis method.
- Solar and stellar spectra: The large number of spectral lines recorded simultaneously allows for the study of numerous absorption lines. Under suitable analysis conditions, this provides information on temperature, chemical composition and physical conditions in stellar atmospheres.
- Planetary nebulae, asteroids and planetary bodies: In the case of planetary nebulae, the focus is on emission lines and their intensity ratios. Spectroscopy of asteroids and other bodies in the solar system, on the other hand, can provide insights into surface properties, reflected sunlight and spectral classification characteristics.
Software and scientific data reduction
For operation and analysis, Shelyak provides the Demetra software in a version tailored to the eShel. It supports the workflow from calibration through to the processing of spectral data.
With Echelle spectrographs, data reduction is an essential part of the measurement process: the individual spectral orders must be identified, extracted, wavelength-calibrated and, where necessary, corrected for instrumental sensitivity. For further analysis, the spectra obtained can then be used, for example, to investigate line profiles, equivalent widths and Doppler shifts.
As an additional reference, Shelyak mentions an analysis tutorial for the ISIS software. This provides a further approach to processing astronomical spectra alongside the manufacturer’s own software.
The Shelyak eShel III is a powerful solution for users who wish to go beyond simple spectral classification and acquire reproducible, quantitatively analysable spectra. Its combination of Echelle technology, a wide wavelength range, fibre-optic cables and calibration makes it particularly suitable for demanding observational programmes in stellar astrophysics.
