M77781A
MS257
™ USB/RS232
MONOCHROMATOR AND SPECTROGRAPH
52
Slits and the Instrument's Bandpass
This is the band of wavelengths passed by the monochromator at any one wavelength setting. It
may be obtained by multiplying the slit width by the reciprocal linear dispersion, and is usually
specified in nanometers. At large slit widths the bandpass is synonymous with resolution. But at
small slit widths the affect of aberrations tend to limit the resolution even though the bandpass
may be very small. Refer to Appendix A, Bandpass and Resolution for more information.
Using the term
“resolution” instead of” bandpass” is more meaningful. However, the concept of
bandpass is particularly well suited to spectrographs. In this case, a wide wavelength range is
spread over an array or CCD. The reciprocal linear dispersion is meaningless in this scenario
because it can vary significantly over the focal plane. The bandpass does still vary with grating
angle but other than at extreme angles it is a good representation of the wavelength range lover
the focal plane.
The bandpass of the MS257 can be calculated using the data from Section 9.3. Assuming that
instrument has the same width input and output slits, multiply the reciprocal dispersion by the slit
width. The number obtained has to be greater than the resolution. If it is not, it means that the
instrument will operate below the resolution limit and its bandpass will be determined by the
resolution, rather than spectral bandpass of the slit.
Mounting Slit Assemblies
All slit assemblies are mounted using four 4-40 screws. The input slit assembly is oriented by an
alignment pin in the MS257 housing that fits into a precision hole in the slit assembly. The exit slit
assemblies do not have alignment pins. This allows one to slightly rotate the exit slits in order to
obtain the best throughput and resolution.
The MS257 comes with all slit assemblies installed and the instrument calibrated. Removing and
replacing or re-installing the exit slits should only be attempted by personnel who are experienced
in optical calibrations. It is strongly suggested to have slits replaced by an authorized service
person. Removal of the input slit is never recommended. Recalibration is required whenever a
slit is installed
– even if it is the same slit being re-attached. Never remove slit assemblies or slit
holders for storage or transportation.
When mounting an exit slit assembly, use the following process:
Use a light source to illuminate the input slit.
Set the wavelength to zero, or to a prominent spectral line if using a calibration lamp.
Adjust or change the input slit to a small slit width, or insert a narrow width fixed slit into the
slit holder.
Adjust or change the exit slit to be the same slit width as the input slit.
Adjust the rotation of the exit assembly until it appears to be parallel to the image. The
image height should be at a maximum.
Tighten the mounting screws.
