The UCLES SETUP Routine

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 the same type of detector (CCD or IPCS) was used. If you suspect that the offsets applied are wrong, e.g. 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.

• You must be running from a terminal with imaging capabilities, probably an xterminal in which case SETUP will want to create its own image display (FIGDISP) window; there must be no FIGDISP windows already active on this terminal.

• UCLES: SETUP

• SETUP prompts for a central wavelength, which should correspond to a moderately strong, easily recognised spectral line near the wavelength region of interest. SETUP configures the spectrograph to the requested central wavelength using the 79 lines mm grating (since this gives the greatest order spacings), and turns on the ThAr lamp.

• Take an exposure with this setup. If using a CCD, use a narrow window, e.g. THOMSON_CENTRE. The image should resemble Fig .

    
  Figure: UCLES, 10 sec ThAr lamp, SLAUTO, TEK1K_SETUP, 79 lines mm grating at 5009.334 Å, XMEM orientation.
  

• The program initiates a FIGARO IMAGE command. Note: This version of IMAGE will not recognize MAX, MIN or `' as responses to these prompts.

• SETUP calls ICUR, producing an image cursor, and asks the user to mark three points along the target spectral line, which should be obvious near the centre of the field. Place the cursor at each end and in the middle of the spectral line, hitting the space bar on each occasion. Type Q when this is done.

• SETUP calls ROTCEN to calculate the offsets. The slit angle is determined by cross--correlating the ends of the spectral line. The program prompts for the range of data values to extract; the defaults are usually suitable. The position of the central point is used to determine the echelle angle offsets.

• SETUP reconfigures the spectrograph with the new offsets and gives the option of repeating the process. This is advisable to ensure that the offsets were reasonable. The ech_theta and ech_gamma offsets should be of order 0.01.

• UCLES should now be ready for the observer. A spectrum taken with the full window ( e.g. TEK1K_SX2) should resemble that shown in Fig. , though it may be at a different wavelength or with the 31.6 lines mm grating in which case there will be many more orders, more closely spaced.

    
  Figure: UCLES, 10 sec ThAr lamp, SLAUTO, TEK1K_SX2, 79 lines mm grating at 5009.334 Å, XMEM orientation. Note overscan strip on right hand edge.