Shelyak eShel III Complete System

The eShel System: Professional Spectroscopy for the Advanced Amateur

The eShel is a professional-level, multi-order, fiber-fed instrument for when you cannot choose between resolution and spectral range, as in exoplanet studies.

Developed from Shelyak's experience with professional instruments like the Musicos and Narval spectroscopes, Shelyak developed the eShel. The complete system includes the spectrograph, the fiber injection and guiding unit, the calibration module and the optical fibers; the Demetra software is a free download.

Echelle Optical Design: Capturing Orders 32 to 53 Simultaneously

The power of the eShel lies in its echelle optical design. An echelle grating is used at high diffraction orders, from 32 to 53. This spreads the starlight into a highly dispersed spectrum where many orders overlap. A second optical element, a cross-disperser prism, then separates these orders vertically, stacking them across the entire surface of the acquisition camera's sensor.

The resulting 2D echellogram can be read "like a book," with the end of one spectral order found at the beginning of the next. This way the whole visible range is obtained over some twenty orders in a single exposure.

R > 10,000 Resolution Across a 390 nm to 700 nm Spectral Range

The eShel delivers a resolving power of R > 10,000, which is the ability to distinguish fine details within the spectrum. It reaches more than 10,000 with an ATIK 460EX camera in 2x2 binning (camera not included), and the fiber feed makes radial velocity measurements more accurate.

The range covers about 390 nm to 700 nm, so lines such as H-beta, the sodium doublet and H-alpha are recorded in the same exposure.

The Fiber Feed: A 20m Link for Stability

A key feature of the eShel is its fiber-optic feed. The system uses a 20-meter optical fiber to transport light from the telescope to the spectrograph. This allows the spectrograph to be physically separated from the moving telescope and placed in a quiet, temperature-controlled room. Shelyak chose fibers to increase the accuracy of radial velocity measurements and for user comfort; the manual says the fiber link gives the best mechanical and thermal stability.

F/6 Fiber Injection & Guiding Unit (FIGU): Precision Light Collection

The F/6 Fiber Injection & Guiding Unit (FIGU) is the critical interface between the telescope and the fiber. It contains a small mirror with a 50µm hole at its center. A guiding camera views this mirror, allowing you to see the star field and precisely center your target on the hole. When the star is centered, its light passes through the hole and is injected into the 50µm object fiber.

The FIGU also takes the 200µm calibration fiber. A remote-controlled mirror switches from one source to the other, so the light from the calibration fiber feeds the 50µm object fiber and thus the spectrograph.

Telescope Integration: Matching Your Optics to the eShel System

The eShel is designed to be adaptable to a variety of telescopes. The FIGU has a standard M42x0.75 T-mount thread and a backfocus of approximately 45mm. The included T-mount variable adapter simplifies connection, especially for Schmidt-Cassegrain telescopes. The supplied FIGU is the F/6 version.

For telescopes with a different native focal ratio, such as a common f/10 SCT, a focal reducer (like a 0.63x reducer) with the variable T-adapter (using its 15mm extension tube) is what the manual recommends. An F/9 FIGU and other F-ratios are available from Shelyak.

Camera Requirements: Sensor and Connection Specifications

The eShel system does not include an acquisition camera. The spectrograph's objective is a Canon EOS lens, so a Canon DSLR can be fitted quickly, and astronomical cameras attach with the proper Canon EOS adapter. Camera mounting is critical: the backfocus range is limited (less than 9mm).

The manual describes a KAF1600 sensor (about 13x9mm, 9µm pixels) as well adapted: the size gives good overlap between orders and the pixel size suits the dispersion. It recommends QSI516ws or QSI532ws cameras with the BA0023 adapter, and Shelyak's example spectra were taken with an ATIK 460EX in 2x2 binning, a QSI516 and a Canon 40D.

Calibration Module: A Thorium-Argon Reference for Precision Work

Accurate wavelength calibration is fundamental to spectroscopy. The eShel complete system includes a dedicated calibration module that provides the necessary light sources, controllable via the Demetra software. The primary source is a Thorium-Argon (ThAr) lamp, which produces a dense forest of narrow, well-known emission lines across the entire visible spectrum.

This ThAr spectrum is used as a precise "ruler" to determine the wavelength solution for every pixel on the sensor. The module also has a tungsten lamp, whose smooth spectrum is used to correct the blaze function, and LEDs used with the tungsten light for the flat that traces the geometry of the orders.

Demetra Software: From Automated Acquisition to Calibrated Spectrum

Demetra for eShel is free, runs under MS Windows and accompanies you from acquisition to data reduction. Demetra communicates with the calibration module to automate the acquisition of reference frames (bias, darks, flats, and ThAr spectra).

It also processes the complex echellograms automatically. The software identifies the orders, extracts the 1D spectrum from each one, applies the wavelength calibration using the ThAr reference, corrects for instrumental response, and merges the orders to produce a final, continuous, science-ready spectrum.

Science with the eShel: From Exoplanets to Pulsating Stars

Shelyak's examples show what the eShel records.

• Exoplanet Studies: Exoplanet studies are a case where you need both resolution and range, and the fiber feed is designed for radial velocity accuracy.
• Stellar Astrophysics: Study the atmospheres of Be stars like Delta Scorpii by monitoring emission lines such as H-alpha, or novae (example raw spectra include WR 140 and a nova in Delphinus).
• Pulsating Stars: A time series of BW Vul (5-minute exposures over about 5 hours, around the sodium doublet) was taken with a 60cm telescope at Pic du Midi.
• Emission-Line Stars: Shelyak's raw spectra include Beta Lyrae and P Cygni.

System Components and Getting Started on Your First Night

The eShel is a complete system that includes all the core spectroscopic components. In the box, you will find the main spectrograph module, the F/6 FIGU, the calibration module with ThAr lamp, a 20-meter 50µm object fiber, a 20-meter 200µm calibration fiber, and a T-mount variable adapter. You will provide the telescope, mount, acquisition camera, and guiding camera.

Installation should be performed during the daytime. The process involves assembling the components, connecting the fibers, and configuring the Demetra software with the specific parameters of your setup. The goal is to align the system and perform initial calibrations using the ThAr lamp so that you are ready to begin observations as soon as it gets dark. The installation and maintenance manual covers focusing the objective, mounting the camera and the processing parameters.