With the imaging mirror out of the beam and the grating and both prisms in place, orders 6 through 15 of the cross-dispersed spectrum fall on the CCD. The dispersion is a constant 11.5 km/sec/pixel in velocity. The dispersion in Å/pixel varies from order to order, as does the spatial scale along the slit. The bluest order (15) has a linear dispersion of ~0.15 Å/pixel and the dispersion increases moving towards the red (lower order number) inversely by the order number. The scale in the dispersion direction is 0.154 arcsec/pixel; 1 arcsec projects to 6.5 pixels. Due to variable anamorphic magnification the scale along the slit varies with order (Table 1). Figure 1 shows a schematic of the light path forthe echelle mode. Table 1 lists the orders andpectral ranges of each order and Figure 2 shows the format of the spectrum in this mode. Note that the free spectral range of every order listed falls completely on the detector. The bluer orders contain significant amounts of data outside the free spectral range as well.

Figure 1: The optical layout for the echelle configuration.

The echelle mode is selected from the GUI top-level panel (the box labeled "Observing Mode"). Selecting echelle mode automatically reconfigures the spectrograph to send cross-dispersed light to the camera, rotates the middle wheel to center a small aperture under the echelle slits and rotates the lower wheel to the open position ("Clear_S", which stands for the open position for spectroscopy). The slit width is user selected as is the CCD binning and window. To get the full wavelength coverage for all orders, you want to read out the full CCD in echelle mode! Full frame can easily and quickly be chosen by means of the QuickFrame menu button on the main GUI display.

The maximum slit length (set by the interorder spacing in the red) is 20". The slit orientation in this mode is along the radius of the wheels, vertical as seen by the guide camera. Six slit widths are available, two pinholes, and a line of pinholes (called "MultiHoles"). These are listed in Table 2, along with the projected slit widths in pixels and the corresponding resolutions.

Figure 2: Stellar spectrum in echelle configuration [pixel (0,0) is at upper left].

Table 1 below shows the free spectral range of each of the orders. In the blue, there is considerable overlap in the total wavelength coverage of adjacent orders. The redder orders are almost completely independent. Figure 3 below gives a good indication of the overlaps.

Figure 3 shows the extracted spectra from all orders for Wolf 1348, a DA white dwarf. There is considerable wavelength overlap in the blue orders (order 15 is the bluest). The strong absorption in orders 8 (6900 Å) and 9 (7600 Å) are the atmospheric B and A bands. Order 7 shows significant fringing and the CCD loss of sensitivity beyond 9200 Å reduces throughput in order 6 significantly.

Figure 3: Extracted orders from the DA white dwarf star Wolf 1348. Order 6 is at right, order 15 at left.

The IRAF task scombine in noao.imred.echelle will average over common wavelengths for multiple orders to create a single spectrum. Here is the combined Wolf 1348 spectrum.

Fig. 4. The Wolf 1348 orders averaged together. (Note that it would be better to show the orders summed.)

In this mode, the CuAr hollow-cathode tube and HgNe and Xe lamps are used for wavelength calibration. A set of Web pages contain sample spectra with some of the stronger lines in each order identified.