Changing Gratings

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.

1. 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.
2. 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.
3. 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 again.

4. 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.

5. 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.

6. 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.
7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. Tilt the grating to the desired central wavelength using the CENTER <lambda> command, where <lambda> is the desired central wavelength.

13. Take a comparison lamp spectrum to verify the central wavelength. If necessary, update the center wavelength by using the NEWCENTER command.

14. If needed, re-position the grating to the desired central wavelength with the CENTER <lambda> command.