Echelle spectrograph

The 2.3m Echelle is a high-resolution echelle spectrograph that operates at mid-optical wavelengths, previously mounted on the ANU 2.3m telescope.

meeting_room Facility



Echelle spectrograph


Basic specifications

The 2.3m Echelle is a high-resolution echelle spectrograph that operates at mid-optical wavelengths. When in use, it is mounted at the Nasmyth B focus of the ANU 2.3m telescope at Siding Spring Observatory. With typical settings, the spectrograph provides wavelength resolutions of R = 24,000 (FWHM), or 0.02nm at 550nm. 

The spectrograph camera contains a 2k x 2k pixel E2V CCD detector with 13.5 μm square pixels.

The spectrograph has a 79 grooves/mm echelle (R2) disperser, and a choice of three cross-dispersers:  316/750nm, 600/750nm, and  300/300nm. The 316/750nm grating provides coverage between 500 and 1000nm; 300/300nm between 300 and 500nm.

Thorium-argon (Th-Ar) and neon-argon (Ne-Ar) arc lamps are available for wavelength calibration.

There is a choice of two cutoff filters, GG13 and GG495, to remove light below 3900A and 5000A respectively.

Information for observers

The spectrograph has a 79 g/mm echelle (R2) and a choice of three cross-dispersers. It is mounted in a fixed position at Nasmyth_B. It is not possible to rotate the spectrograph because the CCD dewar extends too far from the axis to allow rotation. The dewar has an auto-filler for LN2. All adjustments to the spectrograph are done manually.

Suggested settings

The spectrograph is often used with settings close to the following: cross-disperser 316/7500Å angle near 6 degrees to put orders 33 to 53 (6845 to 4262Å centers) on the CCD, echelle angle near zero, dekker setting 13, slit width 0.17 mm. This gives an almost complete coverage of the wavelength range, a PSF FWHM of 3.0 pixels, resolution 24,000, GG395 filter.

With these settings in good seeing, a star of V = 11.0 gives about 3300 counts per pixel or SN ~ 100 per resolution element in order 35 (6454Å) in one hour.

Slit unit

The slit unit is near the front of the spectrograph on the near side (dome shutter side). It has a dekker to limit the length of the slit, and allows slit rotation. The dekker is moved via the central knob on the slit unit: 0 is the maximum length and 20 the minimum. The slit rotation is adjusted with the outer knob. A dekker setting of 13 is OK with the 316/7500Å cross-disperser set to observe orders 33 to 53.

To adjust the slit width, the slit unit must be removed from the spectrograph. Please remove and replace it very carefully: it should be kept square to its mounting face to avoid damaging optical components. A slit with of 0.17 mm matches the typical seeing at SSO, projects to about 3 CCD pixels and gives a FWHM resolution of about 24,000.

Collimator focus

The collimator focus adjustment is near the back of the spectrograph. Release the three clamping screws and clamp them up again gently after setting the focus. The current focus setting is near 3.0 on the clock gauge.

Echelle orders

The center of the orders is given by n.wavelength(Å) = 225875. The echelle angle is set by a push-pull screw adjustment on the far side of the spectrograph: undo the clamps on the near and far sides of the spectrograph before adjusting, then reclamp. A setting near zero puts the center of the orders near the center of the CCD rows. The angle can be read from the vernier scales.

Cross dispersers

The 316/7500Å cross disperser is used redward of about 5000Å. The angle is set by releasing the clamp, taking hold of the two handles, lifting the unit very slightly and rotating the whole unit. Then reclamp in position again. The angle can be read from the vernier scales. A cross-disperser setting of 5.9 degrees puts orders 33 (6845Å central wavelength) to 53 (4262Å) on the E2V 2K x 2K CCD.

The two other cross dispersers are 600/7500Å and 300/3000Å.

Order-separating filter

When observing in the red, an order-separating filter may be needed. The filter slide is near the echelle angle adjustment on the far side of the spectrograph.

Comparison and flat-field lamps

The lamp unit in the telescope control room has a Th/Ar lamp, a Ne/Ar lamp and a QI lamp. Please be sure to turn the lamps off after use. Typical exposure times for orders 33 to 53 are about 20 sec for the Th/Ar lamp and about 150 sec for the QI lamp.


The spectrograph currently has an E2V 2K x 2K detector. For orders with N > 35, the whole FSR falls on the detector. The PSF width a bit spatially over the detector. To reduce readout time, one can bin 1 x 2 (across the dispersion). The dewar can be rotated relative to the spectrograph, in order to have the orders parallel to the CCD rows: get local advice before doing this.