Auxiliary Equipment
Spectrograph
I built a spectrograph as a class project for Stellar Astrophysics
while earning a MSc at Swinburne Astronomy Online.
I
was given a monochromater which was easily adapted for use as a
spectrograph. An eyepiece was also made to replace the CCD, converting
the the spectrograph into a spectroscope. A monochromater is a device
that is designed to provide a narrow wavelength band from a continuous
spectrum by rotating the grating around its long and looking at the
exit slit. The difference between the two devices is the entrance slit
is rotated 90o from a spectrograph and the monochromater has
an exit slit which is different form a spectrograph. The exit slit is
parallel to the entrance slit. When the Grating is rotated a particular
narrow, almost single wavelength, portion of the continuous spectra can
be selected, hence the name monochromator. The spectrograph on the
other hand makes the entire spectra available. The original
monochromater schematic is shown in Figure 1.
Figure 1
In order for the monochromater to be used as a spectrograph several
modifications needed to be done. The first thing was to change the
orientation of the entrance slit and remove the exit slit. A method of
attaching the spectrograph to the telescope had to be made and a
mounting needed to be made to place the CCD camera where the exit slit
was placed, which is the image plane. Figure 2 shows the parts that
were modified to adapt the monochromator for use as a
Figure 2
spectroscope. The entrance slit as I said was rotated 90o,
Figure 7. This was replaced with a wider slit with the proper
orientation for a spectrograph. An adapter was machined from PVC so the
original entrance assembly could be installed in a standard 1.25"
focuser. The exit assembly was shortened to allow the image of the
spectrum to be accessed. An old T adapter to 1.25" was perfect to
modify to allow focusing and adapt the spectrograph to the T ring on
the CCD camera. In this case an Apogee CCD. Two problems were
encountered when I tried to use the apparatus as a spectrograph. The
first was the Exit aperture on the aluminum body was to small to pass
the entire spectrum. The body of the device exit was enlarged to fix
this problem. The second was part of the body of the spectrograph kept
the CCD form getting close enough to the focal plane. Unfortunately the
area is part of the rotation mechanism that selected the portion of the
spectra to be examined. Excess material was removed without effecting
this function.
The grating in the of the spectrograph is 1200 lines/mm and is concave.
A concave grating simplifies the construction of the spectrograph since
collimating optics are replaced by the concave grating. An advantage of
having the rotating grating provided by the original monochromater is
particular portions of the spectra can be selected and placed on the
small area of the CCD sensor. The dispersion of the grating is large
enough that a full spectrum cannot fit on the small CCD image area.
This necessitates moving the grating to place different portions of the
spectra on the sensor allowing the full spectra to be imaged. I would
like to eventually build a long linear array for the spectrograph.
Figure 3 shows the top of the spectrograph removed.
Figure 3
The grating and the rotating grating mount can also be seen in figure 3.
The entire apparatus including the camera is very small about 4" x 7"
and weighs a few pounds, Figure 4. The initial experiments were done
with
Figure 4
a borrowed Apogee camera with a KAF 1600 chip. The CCD was only capable
of seeing 1/3 of the entire spectra generated by the spectrograph.
Figure 5 is the spectrograph attached to the newtonian focus of the
telescope.
Figure 5
The monochromater was made by Instruments SA. They are still sold new.
The version I got was part of some kind of device.
I also made an adapter so that an eyepiece could replace the CCD so the
spectra could be viewed visually. Figure 6
Figure 6
Using the instrument as a spectroscope was kind fun. I used the this
configuration to focus the star on the slit. I found the set up and use
to be very difficult. I think that a flip mirror would be very helpful
in setting up and using the spectrograph. The horizontal slit is seen
in Figure 6.
Unfortunately, I had only one night to collect images after I finished
the spectrograph before I had to return the Apogee CCD camera.
Consiquently I do not have any spectra fit to display. I hope to get to
using the spectrograph with the Genesis CCD or perhaps film when the
weather clears and I have some time to devote to debugging for the
Genesis CCD. Eventually I would like to put a 1" long linear CCD
so the entire spectrum could be obtained. It would also be nice to have
the CCD mounted permanently so focus could be set.
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