The CCDS gratings are located in the OSU instrument cabinet in the 2.4-m
telescope laboratory. They are stored in a long rectangular box in
which the interior is custom contoured to safely store up to 5 gratings
in their cells. Only the MDM Observatory staff or designated OSU
instrumention group personnel handle the CCDS diffraction gratings
because they are delicate and expensive. For the CCDS user, this
means that you are constrained to use only one grating during a night,
the choice of which is made during the afternoon, so that the MDM staff
can install the grating for you.
In spite of this policy, we give below the instructions for changing
CCDS gratings just in case special situations arise (brightest GRB ever,
a Galactic supernova, etc.). In general, these procedures should
first be followed under the direction of an MDM staff member or other
qualified OSU person during the afternoon. Users should note that
changing a grating during the day with the help of others is very
different from doing it by yourself in a dark dome at night up on the
platform. For this reason, users are strongly encouraged to let the MDM
staff perform these operations.
- First, locate the cover for the grating that is currently installed in the
CCDS. It will probably be in the OSU instrument cabinet. If not,
look in the control room or in the telescope chamber. Put it near the
CCDS so it is easily available.
- Start the CCDS software as described elsewhere in this
documentation. Verify that the grating information as presented in the
Prospero status window (grooves, order, and central wavelength)
matches the grating that is installed in the CCDS.
- Center the grating to zero-th order by issuing the
GRATZERO command in Prospero, then taking a short lamp
exposure to verify that you are indeed centered at or near zero-th order.
A single bright image of the slit should be visible near the center
of the CCD.
If you don't see this, do a full mechanism reset by typing
GRESET at the Prospero command line, then issue GRATZERO
- In the dome at the CCDS, locate the grating stepper motor
assembly. It has two black anodized handles protruding from it with a
large brass gear in the middle.
- First Turn off the AC power on the Instrument Electronics
box (the "IE" for short), and only then disconnect the power cable going
to the grating stepper motor. Second, remove the three (3) allen
screws that hold the grating assembly onto the body of the spectrograph.
Put them away for safe keeping. Third, grasp the two handles
and pull the grating assembly straight out of the CCDS making sure to
support the other end of the assembly since it is heavy and will have
a tendency to want to drop downward.
- Place the grating assembly on a table or other safe, clean, and dry
location. Promply put the plexiglass cover back over the grating to
protect its ruled surface. Remove the four (4) allen screws which
attach the grating holder to the grating assembly. Keep them
nearby. Put the unused grating back in the grating box.
- Install the selected grating in the grating holder. It can only go
in one way since there are 2 locater pins on the back of the grating along
one side but only 1 locator pin along the opposite side. There are
corresponding cutouts in the grating turret for these locater pins.
Attach the grating to the turret by replacing or tightening the four allen
screws. Do not overtighten. Remove the grating cover and
put it in a safe place.
- Grab the grating assembly handles and carefully insert it back into the
CCDS lining up the mounting holes in the assembly with the tapped holts in
the body of the CCDS. Attach the assembly to the body of the CCDS with
the three (3) allen bolts. Reattach the power cable to the grating
assembly, and power-up the IE box again.
- Back in the control room, using Prospero, type STARTUP at
the command prompt to resynchronize the data-taking system after
power-cycling the IE box.
- Reset the grating mechanism by typing GRESET at the Prospero
command prompt. When it is done, move the grating to the zero-order
position by typing GRATZERO.
- Define the new grating parameters with the GROOVES and ORDER
commands, respectively. You can also use the GRSETUP command
to step you through the process.
- Tilt the grating to the desired central wavelength using
the CENTER <lambda> command, where <lambda>
is the desired central wavelength.
- Take a comparison lamp spectrum to verify the central wavelength. If
necessary, update the center wavelength by using
the NEWCENTER command.
- If needed, re-position the grating to the desired central wavelength
with the CENTER <lambda> command.
Updated: 2006 November 10 [rwp/osu]