The Coude Echelle Spectrographs (Expert Information)

Appendix C3: The UCLES SETUP Routine

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The command CENTRE causes the software to calculate the echelle grating angles to place the specified wavelength in the centre of the field. However, since the centre of the detector may not coincide precisely with the optical axis of the camera, it is usually necessary to determine small offsets to be applied to the calculated angles. Similarly, since the detector is rotated to align the orders with the detector, it is necessary to offset the calculated slit angle so that the slit image is perpendicular to the echelle orders. These offsets are determined by running the SETUP routine.

When UCLES is run, it uses offsets determined the last time it was used. REMEMBERS TEK v MITLL?? If you suspect that the offsets applied are wrong, i.e. if you can't recognize the ThAr spectrum, you may zero the offsets by using the command OFFSETS. Note also that the ThAr lamp polarity must be correct, or an unrecognizable spectrum will result.

Figure 1 (left): UCLES, 10 sec ThAr lamp, SLAUTO, TEK1K_SETUP, 79 lines/mm grating at 5009.334 Å, XMEM orientation.
Figure 2 (right): Same setup as Figure 1, with the TEK1K_SX2 window. Note overscan strip on right hand edge.

Ray Stathakis (
Last Update: 22 March 2000