Photos
and Drawings of Littrow Spectrograph
In one
direction, the grating in the Littrow spectrograph acts like a mirror,
reflecting a cone of light from a diagonal back through the collimation lens
and onto the camera chip. You can click on the image to enlarge it.
In a
perpindicular direction, the grating disperses the spectrum across the camera
chip.
This is a
photo of the tiltable grating housing in the Littrow spectrograph. Like the
grating holder in the classical spectrograph, there is a micrometer to adjust
the grating angle and the grating has two stops, a 1st order stop
(the micrometer) and a 0th order stop (an adjustable screw).
The
diagonal mirror has four knobs to adjust its angle / orientation / tilt. It is very
difficult (or it was for me, at least) to get the diagonal mirror in the
right orientation. The aluminum you see in the picture is a tube I machined on
our lathe. The black is an macro adapter for the Pentax lens I used in my
spectrograph.
This is an
image of the camera with the spectrograph body (but not the Pentax lens)
attached. You can see the diagonal mirror with the red adjustment knobs, and
the hole cut out of the spectrograph’s back where the spectrum hits the camera
chip.
An image of
the slit created for the Littrow assembly. Unlike the classical spectrograph,
both razor blades are adjustable. I machined the long parts that hold the
adjustment screws with a saw and a file. This slit is unique, compared to my
classical spectrograph, because you can adjust the slit without removing the
entire spectrograph. That is immensely useful when one is taking astronomical
spectra (and Raman spectra as well, for that matter) because you don’t end up
jarring the telescope out of alignment and don’t have to bother with taking the
slit out. It has a slot for taking astronomical spectra (to be covered up for
Raman spectra), on the bottom of the slit instead of the middle so that it
takes up less room. When I first made up the slit I made the mistake of having
the magnets such that the blades were attracted towards one another and
attracted a lot of charged particles in the gap between the slits. I was able
to fix this problem.
In this
image of the spectrograph body (in which the Pentax lens and grating cup are
not attached) you can see the SCT ring with the adapter that attaches it to the
body of the spectrograph, and the diagonal underneath.
Please send
questions or comments to marymast@gmail.com
last
updated June 15, 2006