Kipp&Zonen Brewer MkIII - Manual (Page 20)

MKIII OPERATOR’S MANUAL 17
Spectrometers Detailed Description
Light enters the entrance slit and passes through a tilted lens [LE 5] which corrects for the coma
and astigmatic aberrations inherent in an Ebert system. In the first spectrometer the light is
collimated by a spherical mirror onto a diffraction grating where it is dispersed. A second mirror
reflection focuses the spectrum onto the focal plane of a slotted cylindrical slit mask positioned at
the entrance of the second spectrometer. Following wavelength selection by the slit mask, the light
passes through the second spectrometer where it is recombined and directed onto the exit slit
plane. Six exit slits are located along the focal plane at the appropriate wavelength positions.
Entrance and Exit Slit Plates
The entrance slit and six exit slits [ES 1, EX 1] are laser-etched into 0.1-mm-thick disks of hard
shim steel. One of the six exit slits (slit #0) is used for wavelength calibration against the 302-nm
group of mercury lines; the other five are for intensity measurements and are nominally set at
306.3, 310.1, 313.5, 316.8, and 320.1 nm. The dimensions for the entrance and exit slits are listed
in the Selected Parts List.
Both slit plates are positioned on their respective housings by locating pins which orient the slit axis
to within 0.1°. Both plates are blackened to minimize light reflections.
Correction Lens
The correction lens [LE 5] has a convex-cylindrical surface (radius 170.0 mm) and a concave-
spherical surface (radius 230.0 mm).
Both surfaces are coated with a layer of magnesium fluoride to minimize reflectance at 315.0 nm
for an incidence angle of 29°. The lens is mounted in the entrance-slit housing at an angle of 29°
to
the optical axis with the concave-spherical surface facing the entrance slit. The axis of the
cylindrical surface is positioned in the horizontal plane to within 1°.
Spherical Mirrors
The spherical mirrors [SM 1 & SM 2] each have a 324 mm radius-of-curvature. The spherical
surfaces are ground from rectangular pyrex blanks. The surfaces are polished, coated with
aluminum, and then coated with magnesium fluoride to maximize reflection at 315.0 nm.
Spring-loaded mounts secure the spherical surfaces of the mirrors against three adjustment screws
which are normal to the spherical surfaces in the horizontal plane of the spectrometers. The mirrors
are allowed to move on a spherical surface defined by the three adjustment screws, up to a limit of
0.25 mm in the horizontal and vertical. Nylon screws prevent the mirrors from moving beyond this
limit.
Diffraction Gratings
The diffraction gratings [GR 1 & GR 2] are 3600 line / mm holographic plane-reflectance types,
operated in the first order. The gratings have optimum efficiency over the range 225 to 450 nm in
the first order.
The gratings are secured with high-quality adhesive to three small blocks which provide kinematic
mounts, as well as fine adjustment for rotation of the gratings about the two axes perpendicular to
the grating grooves. The three blocks are thus part of the grating and are the basis of point, slot,
and plane mounts which allows adjustment by three screws fixed in the grating-mount plates.
These plates are suspended on a set of cross-springs which constrain the gratings to rotate in the
vertical axis (the axis parallel to the grating grooves). The cross-spring suspension acts as a
frictionless bearing. Rotation of the gratings is controlled by two micrometers acting at the end of
lever arms such that a 0.03 mm adjustment of the micrometers represents approximately a 0.1 nm
wavelength change at the exit-slit plane.
Micrometers
Metric micrometer heads clamped to the spectrometer frame are used to adjust the grating rotation
for each half of the spectrometer. Micrometer #1 adjusts the grating in the dispersing half (top) and
Micrometer #2 adjusts the grating in the recombining half (bottom). The micrometer shafts are
ground to 60 degree cones which insert into bearings at one end of floating pushrods. A conical
2. SYSTEM DESCRIPTION