270M
Rapid
Scanning Imaging Spectrograph
8.3 Exit Slit Considerations:
Bandpass
in
optical spectrometers
is
determined by multiplying the dispersion by the
exit slit width,
or
the width
of
the image
of
the entrance slit that is formed at the exit,
whichever is the greater
of
the two widths. The difference is significant in instruments
such as the 270M which have a magnification factor due to the exit arm focal length
being greater than the entrance arm focal length. The 270M has a nominal 220 mm
entrance arm, and 270 mm exit arm. The magnification factor is the ratio
of
the
entrance arm
to
the exit arm lengths, or 1.23. Hence, the image
of
the entrance slit at
the exit is 23% larger than the entrance slit width.
If you set the entrance and exit slits
to
the same width, the resolution will be consistent
with the entrance slit width, but throughput will be limited by the exit slit width. With the
270m, setting the exit
up
to
23% wider than the entrance will give more throughput
without sacrificing resolution. Beyond that, the resolution degrades, as the increase
in
exit slit width will allow a larger bandpass to exit.
8.4 Spectrograph Port Focusing:
Please refer to the instructions provided with your array detector, and set it up in a
realtime display mode for final adjustment. If reatime display is not available, the
alignment will take longer, as you must view the change in detected signal after each
trial adjustment.
If you have a linear photodiode array with raw video output, displayed the video on an
X-Y monitor
or
oscilloscope with video intensity on the Y axis versus horizontal pixel
position on the X axis. The raw video display updates in true realtime. With no lag
time, you will be iess likely
to
overshoot the optimum.
The 270 M spectrograph port provides for translation in and out
to
locate the array in
the focal plane, and rotation
to
align the array pixels with the vertical slit image.
The goal is
to
get the best focus and vertical alignment.
to
start, set up a spectral
lamp at the entrance (e.g. Mercury, neon,
or
fluorescent desk lamp). Acceptable
. results can often be attained without coupling optics, provided that the source
or
reflected beam is directly on the optical axis entering the slit. (For best results, the
lamp should be properly coupled
to
the entrance slit. See the Optical Interfacing
section for further information.)
Set the spectrometer drive
to
the wavelength
of
a known spectral line, such as the
mercury 546.1 nm emission line. Set the entrance slit
to
80%
of
the width of a pixel on
your array,
or
slightly smaller (this takes into account the magnification factor
of
the
Revieed
November
24.
1992
31
Leica Microsystems, Inc.'s Ex. 1033
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