Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 51)

4.2.4
Focus Adjustment
The optical system includes the long sample cell, with the lens holder at the dewar end, and the beamsplitter and
detectors at the other end. To focus the system, this entire assembly is moved closer or farther away from the dewar.
1) To adjust the focus, first note the sample detector signal at the current focus position. Then loosen the axial
clamping screw, slide the optical assembly either forward or back a short distance (~2 mm), and retighten the axial
clamping screw. Readjust the horizontal and vertical alignment to find the maximum sample detector signal at this
new focus position.
2) Compare the sample detector signal at this focus position to the signal at the previous focus position. Step the focus
again in the same direction if the signal improved, or move it the other direction if the signal decreased. Repeat this
process until the sample detector signal has a single narrow peak of maximum height. It may be helpful to record
the focus position and sample signal in a table, along with a qualitative assessment of the focus: whether there is
one or multiple peaks, and if the peak seems broad or narrow.
3) Generally the goal is set the focus for a single narrow peak, giving the maximum sample signal. In some cases it
may be desirable to intentionally defocus the system:
• Isotope ratio measurement accuracy may be improved by defocusing to reduce detector nonlinearity
• If the laser signal is large enough that the detector temperatures must be raised above ~0 °C to avoid detector
saturation, defocusing will reduce the signal and it will also reduce detector nonlinearity
• If it is difficult to coalign the reference and sample detector (see section 4.2.5) it may be helpful to defocus the
system
In cases where the optics are to be intentionally defocused, start at the position of best focus and move the optical
system away from the dewar by no more than 5 mm. It is generally best to defocus just enough to give a single
relatively broad, flat peak.
4.2.5 Reference Detector Coalignment
Once the optical alignment has been optimized for the sample detector, check the coalignment of the reference and
sample detectors. Ideally, the sample and reference detectors are optically coincident, and adjusting the horizontal and
vertical alignment gives a maximum response for both detectors at the same position. Evaluate this by watching both
detector signals while adjusting the horizontal and vertical alignment. If they are not coincident, the reference detector
alignment must now be adjusted. The process is different for older and newer systems:
• On older systems, the beamsplitter mount can be rotated to adjust the vertical coalignment of the reference
detector to the sample detector. Loosen the three beam splitter clamping screws, rotate the beam splitter mount
to maximize the reference detector signal, and retighten the beam splitter clamping screws. It is recommended
that the system be at normal operating pressure (vacuum pump on) for this step. If it is not possible to achieve
adequate signal on the reference detector signal by rotating the beamsplitter, it may be necessary to make a
small adjustment to the horizontal, vertical, and axial alignment to reach a compromise between the reference
detector signal and the sample detector signal.
• Newer systems have a combined reference cell / detector holder that includes horizontal and vertical alignment
cams. For these systems, the beamsplitter mount should be rotated to center the three clamping screws in their
slots. Align the reference detector to the sample detector by loosening the three screws that attach the
reference detector holder to the beamsplitter block, turning the alignment cams to maximize the reference
signal, and then retightening the mounting screws.
The optical alignment is now complete. However, if the laser is exceptionally bright, if it is difficult to coalign the
reference detector to the sample detector, or if detector nonlinearity is a problem, it may be desirable to defocus the
system intentionally. The optical assembly may be moved back (away from the dewar) by as much as 5 mm from the
position of best focus (see section 4.2.4).
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