The Coude Echelle Spectrographs
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The following sections give a description and list software commands for the components of UCLES, roughly in order along the light path from telescope to detector.
For a summary of commands see the Cookbook or Appendix C6 for a complete list.
Summary of the light path: Coude 5 or calibration lamps - beam rotator - slit assembly - focal modifiers - slit filters - slit shutter - collimator - cross-dispersion prisms - echelle - camera & shutter - detector.
Figure 1.2.1 The optical path of UCLES
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Since the beam rotator introduces an optical element upstream of the slit, the night assistant must be kept informed if it is being moved in and out while observing, as the telescope focus has to be offset each time.
Information concerning the RA and Dec of the telescope is fed from the telescope control computer via CAMAC to the UCLES program on the VAX, and commands are available which cause the slit orientation to be maintained at a given position angle on the sky (TRACKPA pa) or to be kept vertical to the horizon, i.e. at the parallactic angle (VERTICAL). Communication via CAMAC requires that the UCLES software be run on the AAT40A.
The most common use of the beam rotator is to track the parallactic angle, so that the UCLES slit is aligned with atmospherically dispersed images. The value of this, especially for bluer wavelengths, may be judged from Figure 2 which shows the difference in refraction for several wavelengths compared to 5500 Å, near the peak in sensitivity of the acquisition TV. For zenith distances exceeding 25o, a stellar image at 4000 Å will lie on the edge of a 1 arcsec wide slit unless it is orientated close to the parallactic angle.
Figure 1.2.2 Atmospheric disperision
Main commands: BR IN/OUT; TRACKPA pa; VERTICAL; CANCELPA;
CANCELVERTICAL
The UCLES calibration lamp system has two 8 position filter wheels containing neutral density and colour filters. The current filter specifications are found by typing HELP FILTERS. (It is not usually necessary to have lamp filters in the beam with any of the CCDs.)
Table 1.2.1 Calibration Lamp Filters for UCLES as at April 2000
| LF1 | LF2 | ||
| Position | Filter | Position | Filter |
| 1 | CLEAR | 1 | CLEAR |
| 2 | 0.39ND | 2 | 0.19ND |
| 3 | 0.75ND | 3 | 0.56ND |
| 4 | 0.93ND | 4 | BG24 |
| 5 | 1.35ND | 5 | BG38 |
| 6 | 1.58ND | 6 | UG5 |
| 7 | 2.14ND | 7 | CLEAR |
| 8 | 2.62ND | 8 | UHRF QTZ LAMP |
Main commands: ARC QUARTZ/THAR/LASER; LF1 n, LF2 n
A TV autoguider (operated by the night assistant) is used for all but
the shortest exposures.
The slit is continuously adjustable from 0.1 to 10 mm in width and from 2.5 to 40 mm in length. A slit width of 1" projects to 54 um on the detector, or 39 um with the focal modifier. Along the slit length 1" will project to 83 um on the detector, or 63 um with the focal modifier.
It is necessary to limit the slit length to prevent orders overlapping. With the 31.6 lines/mm grating the maximum recommended slit lengths range from ~11" in the blue (3100 Å) to 3.5" in the red (7500 Å). The corresponding values with the 79 lines/mm grating are 2.5 times greater. When setting the spectrograph with CONFIG, the keyword SLAUTO will automatically set the slit length to the maximum value which avoids order overlap and leaves one clear interorder pixel.
Table 1.2.2 : Spatial scales for Tek and MITLL detectors (unbinned)
and slit projection factors (SPF)
| Setup | Scale (Tek) | Scale (MITLL) | SPF (Spatial) | SPF (Spectral) |
| UCLES | 0.29"/pixel | 0.18"/pixel | 8.63 | 13.25 |
| UCLES+FM | 0.38"/pixel | 0.24"/pixel | 11.25 | 18.15 |
Main commands: SW x MM/PIX/ARCSEC; SL x MM/PIX/ARCSEC;
CONFIG SLAUTO
Main commands: PERI IN/OUT
UCLES has two focal modifiers, optimized for light bluer and redder than 5500Å. The control program will automatically select the correct one to use, unless overridden by a specified colour.
Main commands: FM IN/OUT
Because these filters are positioned after the slit they will affect
the calibration light as well as the starlight.
(It is not usually necessary to have filters in the beam with
any of the CCDs.)
Table 1.2.3: Slit Filter Specifications.
| SF1 | SF2 | ||
| Position | Filter | Position | Filter |
| 1 | CLEAR | 1 | CLEAR |
| 2 | 0.35ND | 2 | UG11 |
| 3 | 0.80ND | 3 | RG630 |
| 4 | 0.96ND | 4 | GG400 |
| 5 | 1.35ND | 5 | GG435 |
| 6 | 660/11 | 6 | GG475 |
| 7 | BG12 | 7 | OG530 |
| 8 | BG13 | 8 | OG590 |
Main commands: SF1 n; SF2 n; HELP FILTERS
The slit shutter, which opens and closes for each exposure, is positioned in the wall separating the main coudé east room from the pre-slit area. It is controlled through OBSERVER or via a "panic button" in the control room. These controls are in series, so both must be open for the shutter to operate. The slit shutter will normally show SHUT in the spectrograph status display, unless the DISPLAY command is issued while an exposure is in progress, in which case it should show OPEN.
Camera shutters are located directly in front of the detectors and are used as dark slides. They are controlled by switches on a panel in the coudé east anteroom, and also by microswitches on the doors to the main coudé room and to the pre-slit area which close the shutters if either of these doors is opened. If this happens, the camera shutter must be re-opened manually from the panel in the coude east anteroom.
The "panic" indicators (a holdover from the days of the IPCS) are red
when shutters are open.
There is a choice of two collimators, coated with aluminium and silver respectively. The software selects the aluminium one for central wavelengths shorter than 4000Å and the silver one at longer central wavelengths, unless overridden. Both are off-axis paraboloids.
A half-circular mask which may be driven in front of the collimator and rotated into either of two positions enables focussing via the Hartmann test.
Main commands: CF x; HP UP/DOWN/OTHER
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