Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany

The optical Components of the Echelle Spectrometer

F = focus of the main mirror and entrance diaphragm of the spectrometers

1 = collimator mirror: the collimator mirror produces a parallel beam

2 = Echelle grating: the Echelle grating has 316 lines per millimeter and is used in orders 40 to 61. The Echelle grating is a blazed grating, i.e. the grooves are shaped as steps and the front sides of these steps have a reflecting coating, so that the reflected intensity of the grating is optimized for the desired reflection direction.

3 = cross disperser grating: The cross disperser grating has 1200 lines per millimeter, it is also a spherical mirror which focusses the parallel beam onto the detector. The grooves are perpendicular to those of the Echelle grating. Thus the individual orders of the Echelle grating are separated and are mapped one below each other.

4 = Echelle detector: The detector is a high sensitivity, photon counting and imaging detector with a sensitive area of 40mm x 40mm. It is capable of detecting up to 30.000 photons per second.

The spectral resolution of the Echelle spectrometer is 10⁴, i.e. at a wavelength of 100 nm one resolution element has a width of 0.01 nm. The Echelle spectrum covers a wavelength range from 90 nm to 140 nm in the Echelle orders 40 to 61.

The Principle of blazed Gratings

The grooves of a blazed grating are shaped as steps. One side of these steps is mirroring, so that the incident beam will be reflected optimally back in the desired direction. The further the outfalling beam is away from this optimized direction, the weaker the beam will be.

In a given diffraction direction only certain wavelengths are reflected: those wavelengths for which the path difference from adjacent steps is an integer multiple m of the wavelength. The number m is called diffraction order. The grating equation applies to a given diffration direction (lambda = wavelength):

For the ORFEUS Echelle grating the orders used range from 40 to 65 (but usually only the orders up to 61 contain useful information, as wavelengths below the Lyman limit are absorbed by interstellar hydrogen). Thus a certain diffraction direction may contain different wavelengths from different orders.

A second grating is necessary to separat these orders lying one upon the other: a cross disperser grating whose grooves are perpendicular to those of the Echelle grating. This configuration maps the individual Echelle orders one below the other onto the detector.

How the Echelle Detector registers the Echelle Spectrum

The sensitive area of the Echelle detector is 40mm x 40mm. The Echelle spectrum is mapped as bright stripes onto the detector. Each stripe is one order of the Echelle spectrum. Orders 40 and 60 are marked in the image, also some wavelegth positions are marked within these orders. Additionally the hydrogen Ly-beta absorption line is marked. Absorption lines are visible as dark spots within the bright stripes of the individual orders.

The image shows a simulated Echelle spectrum. It is composed from actual spectra recorded with the Berkeley spectrometer during the ORFEUS I mission and with the IUE satellite.
A laboratory measurement of an Echelle spectrum is found here and 2 examples for ORFEUS II Echelle images are found here.

For extraction of the spectrum the brightness along each stripe is measured and each position on the detector is associated a certain wavelength. Thus a diagram is produced which shows the intensity against the wavelength: the extracted spectrum.

Efficiencies

This graphics shows the efficiencies of the individual optical components of the Echelle spectrometer and the resulting total efficiency of the whole instrument. The efficiencies of the individual components are given in percent (left axis), while the efficiency of the whole instrument is given as effective area in cm² (right axis).

Components:

• Mirror: main mirror of the telescope (reflectivity)
• Collimator: moveable collimator mirror (reflectivity)
• Echelle: Echelle grating (reflektivity and grating efficiency within the respective order)
• Cross-Disperser: cross disperser grating (reflectivity and grating efficieny)
• Detector: Echelle detector (quantum efficiency). The values given are the pure quantum effiency values without electronical losses due to selection or dead time effects. The losses due to electronic thresholds are about 5% to 10% and depend on the actual gain of the detector. Dead time losses are count rate dependent, the dead time is about 13 microseconds per photon pulse. The maximum count rate that can be registered is about 30000 counts per second.
• Effective Area: the effective area of the instrument (solid line, right axis) is calculated from the total light collecting area multiplied by the efficiencies of all individual optical components. This value helps to evaluate the intensity that will be registered by the instrument for certain stellar objects: the photon fluxes (photons per cm²)for most objects are known or may be evaluated from the brightness in other wavelength regions. By simply multiplying these photon fluxes by the effective area the expected count rate for this object may be calculated.

Technical Data

Photo of the Echelle spectrometer

This photo shows the Echelle spectrometer during mounting into the ORFEUS telescope.
The left side of the spectrometer is located inside the telescope. This is the location of the cross disperser grating.
Through the opening in the center the Echelle detector is visible. The yellow protective covers of the high voltage connectors are visible, and also the round longish motor of the detector shutter.
The slant at the right side carries the Echelle grating.
The collimator mirror is not mounted within the Echelle spectrometer, but is mounted within the telescope itself.

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Jürgen Barnstedt | Impressum Last modified 07 Feb 2005