Shelyak eShel III Spectroscope

The eShel Spectroscope: A Professional-Level Instrument

Shelyak developed the eShel from its experience with the Musicos and Narval professional spectroscopes. It is not a standalone device: this spectroscope module is the centre of a complete system.

To become operational, the eShel spectroscope must be paired with a Fiber Injection and Guiding Unit (FIGU), a calibration module, and optical fibers. All of them are included in the complete eShel system (ES0006). The fiber lets the instrument sit away from the telescope, if possible in a temperature-controlled room.

Echelle and Cross-Dispersion: Capturing Orders 32 to 53

The eShel achieves its combination of high resolution and broad wavelength coverage using an echelle optical design. An echelle grating disperses the light at high orders (32 to 53), and these orders overlap.

A second optical element, a cross-disperser prism, then separates these orders vertically. This "slices" the spectrum into successive orders that occupy the surface of the camera sensor. An echelle spectrum reads "like a book": the end of one order is found at the beginning of the next.

R=10,000 Resolving Power Across a 390nm to 750nm Range

The eShel delivers a resolving power of R=10,000; it reaches more than 10,000 with an ATIK 460EX camera in 2x2 binning (camera not included). The fiber feed increases the accuracy of radial velocity measurements.

Shelyak's specification gives 390 to 750nm (its descriptive text says about 390 to 700nm). The whole range comes in one exposure, so lines such as H-alpha and the sodium doublet are recorded together.

The 50µm Fiber and the FIGU

The eShel is a fiber-fed instrument. The starlight collected by the telescope is focused into a 50µm optical fiber by the Fiber Injection and Guiding Unit (FIGU). This fiber transports the light to the spectrograph, which can be placed in a quiet, temperature-controlled room away from the telescope.

The installation manual says the fiber link gives the best mechanical and thermal stability for accurate radial velocity measurements.

Matching the eShel to Your Telescope

The Fiber Injection and Guiding Unit (FIGU) is designed to be mounted on the telescope and has an M42x0.75 T-mount interface with about 45mm backfocus. For an f/10 Schmidt-Cassegrain, the manual recommends a standard SCT 0.63x focal reducer and a variable T-adapter. The FIGU F/6 is the standard version; an F/9 FIGU is also offered.

The FIGU contains a small mirror with a 50µm hole at its center. A guiding camera views this mirror, allowing you to precisely center the target star on the hole. When the star is centered, its light goes into the object fiber and the star center disappears from the guiding image, so your autoguiding software must be able to guide through an optical fiber.

FC/PC, T-Mount, and Canon EOS Connections

The spectroscope has an FC/PC connector for the 50µm object fiber.

• Telescope Side: The FIGU (sold separately) attaches to the telescope through its M42x0.75 thread, with the 2-inch adapter or a T-adapter.
• Spectrograph Input: The 50µm object fiber connects to the main spectrograph module via an FC/PC input connector.
• Camera Side: The spectrograph outputs its focused spectrum through the included 85mm F/1.8 lens, a Canon EOS lens. A Canon DSLR can be fitted directly, and astronomical cameras attach with the proper Canon EOS adapter; the backfocus range is limited (less than 9mm).

Matching a Camera Sensor

The installation manual describes a KAF1600 sensor (about 13x9mm) as well adapted to the eShel: its size allows good overlap between orders, needed to get a continuous spectrum.

A pixel size of around 9µm provides appropriate sampling of the spectral lines for the instrument's dispersion. The manual recommends QSI516ws or QSI532ws cameras with the BA0023 adapter, and says any camera meeting these requirements can be used, even a DSLR.

Tungsten, LED, and Thorium-Argon Calibration Sources

Accurate data reduction requires a series of calibration frames, which come from the Calibration Module (sold separately). Its light reaches the FIGU through the 200µm calibration fiber, and a mirror in the FIGU sends it into the object fiber.

• Thorium-Argon (ThAr) Lamp: Used for wavelength calibration.
• Tungsten Lamp: Gives a smooth spectrum (a 2800 K Planck function) that the software uses to correct the blaze function, the grating's spectral vignetting on each order.
• LEDs: Used with the tungsten light for the flat image that lights all orders and gives the geometry of the spectrum; the tungsten adds more red signal.

Data Acquisition and Reduction with Demetra

Demetra for eShel is free, runs under MS Windows and accompanies you from acquisition to data reduction, with remote control of the FIGU and calibration module.

The workflow includes:

• Instrument Control: Demetra connects to your camera through ASCOM and controls the calibration module, for acquisition sequences of object, flat, dark and ThAr frames.
• Session Management: It organizes your night's work, tracking which calibration files are needed for each target and using color-coding to show the status of each observation set.
• Automated Processing: Once all raw frames are acquired, Demetra performs the complex data reduction. This includes dark correction, order tracing, wavelength calibration, blaze correction and merging the orders into a calibrated 1D spectrum.

From Exoplanets to Pulsating Stars like BW Vulpeculae

Shelyak's example spectra show what the eShel records.

• Exoplanet Studies: Shelyak's example of a case that needs both resolution and spectral range.
• Be Stars: Studying the emission lines, such as H-alpha in stars like Delta Scorpii, which originate from circumstellar disks of gas.
• Pulsating Variables: A time series of BW Vul (5-minute exposures over about 5 hours around the sodium doublet, 60cm f/3.5 telescope at Pic du Midi).
• Novae and Eruptive Stars: Raw spectra of WR 140 and a nova in Delphinus, and of Beta Lyrae and P Cygni.

Unboxing the 85mm F/1.8 Lens and Base for Your First Night

This package contains the core eShel spectroscope module, the heart of your system. Included are the spectroscope, the base (for stability) and the 85mm F/1.8 lens that focuses the spectrum onto your camera.

Getting started involves a clear, methodical assembly process. You will mount the spectrograph to its base, attach the Canon EOS lens, and then mount your camera to the lens. Then connect the 50µm object fiber from the FIGU and take Thorium-Argon spectra to focus the lens, as the installation manual describes.