10
Student Spectrometer 012-02135F
¨
13. Illuminate the slit with an external light source. Ro-
tate the telescope directly opposite the collimator
and focus the collimator only (do not disturb the
telescope focus) until the illuminated slit is in sharp
focus. If the collimator slit is not vertical, loosen the
lock ring, align the slit vertically, and then retighten
the lock ring. Then align the fixed edge of the slit
with the vertical cross-hair.
14. Adjust the collimator leveling screws until the slit is
vertically centered in the field of view of the tele-
scope. (As with the telescope, you may need to ad-
just the collimator so that its optical axis is square to
the axis of rotation.) The telescope, collimator, and
spectrometer table are now properly aligned.
15. If you are going to use the grating, plug the Gaussian
eyepiece back in and rotate the spectrometer table
until the vertical cross-hair is again aligned with its
reflected image. This insures that the grating is per-
pendicular to the optical axis of the spectrometer.
16. If you wish, you may replace the Gaussian eyepiece
with the original eyepiece. The focus of the tele-
scope will be maintained if you slide in the original
eyepiece until the cross-hairs are in sharp focus.
Alignment Error
The multiple reflections from the glass slides of the grat-
ing introduce some error into the alignment procedure.
Normally, centering the cross-hairs between the reflected
images will reduce the error below the 1-minute resolu-
tion that is obtainable when reading the vernier scales.
To verify the alignment, use a light source with discrete
spectral lines such as a sodium or mercury vapor lamp.
If the alignment is correct, corresponding spectral lines
on opposite sides of the optical axis will have equal
angles of diffraction. If necessary, adjust the rotation of
the spectrometer table until the measurements are the
same.
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