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depression with a tetrahedral corner at the other end of the pushrods locates a 60-degree cone
mounted on the end of the grating lever arms. The pushrods are secured between the micrometer
shafts and lever arms by tension springs. The material of the pushrods has been selected to
minimize differential temperature effects.
The micrometers are rotated by stepper motors . The motors drive two 10-tooth gears which are
kinematically linked to 60-tooth gears on the micrometer shafts. The drive shafts are coupled to the
motor shafts with universal joints
One motor step represents 0.006 nm on the exit-slit plane. Backlash of the micrometers and cross-
spring bearing systems have been measured at 0.002 nm. The temperature range of operation for
the stepper motors and micrometer adjustment is -16° to +40°C.
Slit Mask Assembly
Refer to Table 2.4 and Figure 2.6
Located between the two spectrometers is a slotted cylindrical mask which may be located in one
of eight positions Six of the eight positions allow light to enter one of the six exit slits. The second
mask position blocks light from all slits so that a dark count measurement can be made. The eighth
mask position exposes two of the exit slits so that the dead time of the photon counting system
may be determined. The correspondence between slit mask position and selected wavelength is
summarized in Table 2-4.
The axis of the cylinder is attached to the shaft of a stepper motor. The mask is aligned with the
exit slits by adjusting the vertical position of the motor and adjusting the horizontal position of the
cylinder on the motor shaft.
By rotating the slit mask, each of the mask slots aligns with a corresponding slit in the exit plane,
effectively selecting a single wavelength. During an HG test the first slot in the slit mask aligns with
the Hg calibration slit in the exit plane.
Table 2.4:
slit mask position vs nominal wavelength
Slit Mask Position
Figure 2.6: Slit Mask Assembly.
18
UV Wavelength Setting (nm)
0
Hg Slit:
1
Dark Count:
2
Wavelength 1:
3
Wavelength 2:
4
Wavelength 3:
5
Wavelength 4:
6
Wavelength 5:
7
Wavelength 2 & 4 for Deadtime Test
MKIII OPERATOR'S MANUAL
303.2nm (Cal=302.1)
--------
306.3
310.1
313.5
316.8
320.1
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