The Coude Echelle Spectrographs (Expert Information)

Appendix C11: Technical Calibrations for UHRF

The Hilger slit | Measuring the slicer output | Alignment of the slit angle | Recalibrating UFC and USC

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The Hilger slit

Although the UCLES slit closes down to 10 um, it is not possible to calibrate the resolution of the
spectrograph for very narrow slits due to the inaccuracy of the encoders. A calibrated Hilger slit is stored in the adjacent darkroom, and locates on the same kinematic mount as the slicer. The Hilger slit requires slit angle SA 2000 (ADU) to be horizontal.

Measuring the slicer output

You can use the diffraction pattern of the slicer to determine its output slit width. The distance h between the two first minima (either side of the central maximum) is inversely proportional to the slit width sw

      sw = (0.632815 x 2 x 1220) / h

 where h is measured in mm and sw in microns; 0.632815 is the wavelength of the laser in microns. The distance between the alignment plate on the slit-end of the UHRF cross-disperser assembly and the slit itself is 1220 mm.

Alignment of the slit angle

The slicer output must lie horizontal to maintain maximum resolution through the spectrograph optics. The required slit angle is currently SA 2160 (ADU), but can be checked as follows:

  •  Use a spirit level to mark the horizontal across the back of the slit/slicer carriage.
  •  Tape a white straight-edge ( e.g. the edge of a sheet of paper) level with this.
  •   Illuminate the slicer with the quartz lamp.
  •   View the output face of the slicer with the UKST 3 inch Questar.
  •   Change the slit angle SA to make the two parallel.
  • The alignment can also be checked using an eyepiece at the UHRF focus, provided the detector is not yet mounted:
  •  Use the eyepiece mounting plate with the eyepiece from the binocular in the chart room.
  •  Configure the spectrograph for the laser.
  •  You will have to remove the diffuser disk in front of the laser and possibly the diffuser from the pupil simulator to get enough light to the eyepiece.
  •  You should be able to determine the rotation required to make the output horizontal.
  • Recalibrating UFC and USC

    The programs UFC and USC interpolate in tables of ADU encoder values corresponding to increments of 1 mm in the mechanisms without any known start value. To convert this table of ADU's to something useful, one needs a start value, i.e. the ADU value at a known mm position. This position must be redetermined if certain mechanical changes are made to the spectrograph.

    Position USC and UFC at the 4246 Å positions indicated on the mechanisms, driving the appropriate mechanisms in manual mode. These correspond to 0 mm in USC and 0 mm in UFC. The distance between the echelle and cross-disperser gratings should be 3532 mm, and the air distance between the secondary collimator and cross-disperser should be 382.4 mm. Read the encoder values for USC and UFC LVDTs (linearly variable displacement transducers) in ADUs, and determine the mm offset by consulting the table of encoder values.

    For example, if at 4246 Å = 0 mm USC reads 24400 ADU, and USCLVDT.DAT has entries 49 = 24140 and 50 = 24470, 24400 corresponds to position 49.7879, which is then the offset.

    The programs UFC and USC must be edited to change the offset value they use.

    Ray Stathakis (
    Last Update: 17 April 2000