The Coude Echelle Spectrographs (Expert Information)
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Appendix C3: 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
When UCLES is run, it uses offsets determined the last time it was used.
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.
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.
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. Use a narrow window, e.g.
TEK1K_SETUP. The image should resemble Figure 1.
The program initiates a FIGARO IMAGE command.
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 the left end,
the middle and the right end 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 Figure 2, 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 2 (right): Same setup as Figure 1, with the TEK1K_SX2
window. Note overscan strip on right hand edge.
Ray Stathakis (email@example.com)
Last Update: 22 March 2000