A fibre-optic spectroscope

The eShel III spectroscope enables work across a wide spectral range (370 to 910 nm) at high resolution (R = 10,000). This is made possible by the use of multiple orders (in this case, between 25 and 60); an Echelle spectrum reads ‘like a book’: the end of one order is found at the start of the next.

The decision was taken to offer a fibre-optic solution in order to improve the accuracy of radial velocity measurements (the instrument is separate from the telescope, ideally in a temperature-controlled room) and to enhance user comfort. The optical fibre is 50 µm in diameter.

We have developed this instrument based on our experience with the Musicos and Narval professional spectroscopes.

Echelle spectroscopy

This technique enables high spectral resolution to be achieved as it use the higher diffraction orders of a diffraction grating (orders greater than 20). The resulting spectrum passes through a cross-disperser – a prism in this case – which separates the successive orders across the entire surface of the camera’s sensor. This provides the entire spectral range within the visible spectrum, spanning several tens of orders.

Schematic diagram of an Echelle spectrograph (© François Cochard)

Example of an Echelle spectrum: spectrum of a Be star (the Balmer emission lines are visible). The image has been coloured in afterwards

Spectrum of Sunlight

Orders #25 to #60 are uniformly focused… even slightly beyond these limits.

Spectrum of Chi Oph Be star

This allows us to process orders #25 through #60 and extend the usable spectral range from approximately 370 to 910 nm. The manual version of this lens comes with a helical focuser, enabling precise focusing of the spectrum. There is also a motorised version fitted with a PrimaLuce Lab motorised focuser (ideal for remote operation or when the spectrograph is not located next to the control PC).

Applications

• 122mm focal length Petzval lens @ f/2.2
• Manual helical focuser
• Requires the use of a SONY IMX571 sensor (we recommend the ASI 2600 MM Pro), which is not supplied with the eShel III
• Resolution power R = 10,000
• Spectral range covered: approximately 370 to 910 nm
• FC connector for 50µm object fiber
• Weight (excluding the camera): 3.8 kg

• eShel III spectroscope with its 122mm focal length Petzval optics and manual focuser
• 1 interface board for an ASI 2600 MM Pro camera
• the tools required to adjust the spectroscope

• Installation manual
• Tutorial ISIS: http://www.astrosurf.com/buil/isis/eshel/tuto1.htm

• Demetra software (eShel version)