Spectra appear offset (shifted) from where the peak is expected. Generally, this can happen after re-attaching a camera or changing the grating turret. When this happens, the wavelength drive requires re- calibration using the following method.
The Offset Adjustment Control (Figure 1) allows the user to adjust the position of the wavelength motor drive to correct for any shift in the spectra. Detector or grating offset adjustment is selected via the tabs on the Offset Adjustment Control (this can be added to the Shamrock panel from Display >> Offset Adjustment).
Figure 1: Detector and grating offset adjustment control.
The Detector offset value can be adjusted to correct for any mechanical change that shifts (offsets) the displayed spectrum from the expected calibrated position. For example, if a camera is removed and refitted, or a grating turret is removed and refitted there will most likely be a small mechanical change in the exact re-positioning of the component. Any Detector offset introduced will be ‘global’, i.e., any offset will be independent of the grating being used and will apply to all gratings.
The Grating offset values are used to correct for any shift between the displayed spectra from the expected calibrated position and will only apply to the currently selected grating. The grating offsets are normally only adjusted when a new grating turret is installed or upon initial setup.
The following describes the procedure for the detector offset adjustment, but the grating offset adjustments are performed in the same manner.
If the offset adjustment control is not already displayed press the Display button and select the offset adjustment control. Click the Detector tab on the offset adjustment control.
Set a low-pressure mercury (Hg) pen-ray, or Neon (Ne) lamp (or similar calibrated light source) at the entrance slit of the system.
Set the entrance slit width to 10 um and select the centre wavelength as a known spectral E.g., the Hg 546.07 or 435.83 nm line or the Ne 703.24 nm line.
Acquire The target line should appear close to the centre of the display. Pressing the centre line button on the offset adjustment control will draw a red line on the centre CCD column of the display and serves as a useful aid to offset adjustment (see figure 2 below).
If the spectral line is positioned on the centre line, then an adjustment to the detector offset value is not If there is a discrepancy between the displayed centre wavelength and the actual wavelength position of the spectral line the offset should be adjusted.
Press the appropriate + or - offset adjustment buttons to bring the centre of the spectral line onto the red centre The adjustment step size may be selected from a drop-down list (right-click either of the keys).
Figure 2: Spectra acquired before (left) & after (right) Detector Offset Adjustment
When complete, the Shamrock EEPROM is automatically updated with the new Offset values and will be applied each time the spectrograph powers on.