Version 1.0, XX Yyy 2007
This guide provides instructions to the Magellan instrument
scientist for installing and setting up the MMTF. It outlines
detailed procedures for installation and takedown at the beginning and
end of an observing run, for changing the setup between nights, and
for troubleshooting.
1. Secure the MMTF etalon in the disperser wheel using the same procedure as for other gratings. The protective covers should remain in place until the etalon has been installed. In the disperser wheel service position, the MMTF cable sockets should be pointing out and down (in the "5 o'clock" position as viewed from the rear of the instrument, looking toward the telescope).
2. Plug into the MMTF the 5 cables connecting the MMTF to the CS-100 controller. Be sure that the X, Y, and Z cables are matched properly with their respective sockets. Pictures of the mounted etalon.
3. Install the blocking filters specified by the observer.
4. Find and install an IMACS slit mask that has 10 or more guide star holes (square apertures) placed near the center of the field-of-view. This mask will be used for determining the optical axis. If charge-shuffling has been specified by the observer, also insert MMTF slit mask "A".
5. The CS-100 is located in the IMACS electronics rack. Ensure that the CS-100 "Balance/Operate" knob is in the "Balance" setting. Turn on the CS-100. (NOTE: The cables between CS-100 and MMTF should never be plugged or unplugged when the CS-100 power is on.)
6. Ensure that the CS-100 "Offset/Quad" knob is in the "Offset" setting. Using the knobs on the left side of the CS-100, dial in the following numbers:
coarse fine quad
X -1 6.95 4.20
Y 0 6.72 3.75
Z 1 1.92 4.26
The X, Y, and Z settings control the spacing between the etalon plates in X tilt, Y tilt, and Z piston. In balance mode, you are balancing out the capacitance bridges in the feedback loop, but not actually moving the plates. In operate mode, changes to the front panel knobs change the position of the etalon plates.
Once the dials are set, turn the "Balance/Operate" knob to "Operate". After a few seconds, the green "?" light should illuminate, indicating the etalon is now in active operation and functioning properly. If instead the red "Out of Range" light appears, the etalon must be re-balanced; follow the Balancing instructions below.
7. Once the etalon is in proper operation, set the Z_coarse setting specified by the observer. To do this, turn to "Balance" mode. Turn the Z_coarse dial by one unit in the direction of the desired Z_coarse. Turn back to "Operate" mode. If the green light appears, repeat this procedure until the desired Z_coarse is reached.
Once the CS-100 in the rack is powered on, you can start the IMACS software. [PROCEDURE FOR LOADING THE ETALON SETTINGS . . .]
The IMACS software must be configured properly to allow execution of MMTF observing scripts. This requires a file in the observer's home directory entitled ".imacs_use_script". In the observer's home directory, type
% touch .imacs_use_script
Restart the CamGUI.
To enable use of the MMTF software, . . .
NOTE on etalon Xfine CS-100 values vs. software values.
There is a wavelength gradient in the MMTF due to the angle at which rays pass through the etalon. This gradient is circularly symmetric aboout the optical axis (the projection of MMTF normal axis onto the CCD). The optical axis is in the center of the field of view, but shifts slightly from run to run (due to the re-mounting of the IMACS CCD) and must be re-measured. To find the optical axis, we use the fact that there is a faint ghost reflection between MMTF and the CCD. Use the following procedure:
1. Insert into the beam the multislit slit mask [and an MMTF filter]. Take a flatfield lamp exposure. The exposure must be long enough that faint reflections (ghost images) of the guide star boxes (primary images) appear. These reflections are symmetric about the optical axis. For instance, a primary image centered at location [1024,2048] on chip 2 will have a ghost image at [1024,2048] on chip 8 if the optical axis is perfectly centered (and assuming constant chip gaps).
2. In IRAF, find the centers of each primary and ghost image using the ICBOX task in the IMACS package. (We set "sz=25" when using this task, but the result is probably not sensitive to this choice.)
3. Create a file that contains on each line the chip number and pixel coordinates of each primary and guide star image:
chip(primary) X(primary) Y(primary) chip(ghost) X(ghost) Y(ghost)
Because the optical axis is at the field center, it is simple to match up primary and ghost images. . . .
4. Run "findghosts" on this file. findghosts has a model of the CCD mosaic geometry and will find the midpoint between each box and its ghost, and make a plot. Record the optical axis location (note that it is possible for it to be between chips). If the RMS error . . .
Parallelism is the process by which the two etalon plates are made parallel to one another. If the plates are not optimally aligned, the transmission profile will be broad and/or asymmetric. This diminishes the light transmitted in the core of the profile, and thus the system throughput (for an emission-line source) and the instrumental resolution. It is thus crucial that proper parallelism is achieved and maintained throughout the run.
Parallelism is achieved and maintained through a capacitor and piezo-electric feedback system embedded in the etalon. See this page for a brief description.
There are several methods for achieving parallel plates. We have developed a rather simple procedure that takes advantage of the ability to synthesize a spectrum by scanning the transmitted image in the radial coordinate. The plate alignment is scanned along both axes of movement using a (N x N) grid of [Xfine, Yfine]. At each value of [Xfine, Yfine], an image is taken of an emission line. Each image is then azimuthally averaged to create an emission-line spectrum. The profiles of emission features are then compared by eye (and, where possible, with line fits) to find the narrowest and most symmetric profile.
Here is a detailed procedure:
- Choose a lamp appropriate for the filter being used. (See this table for options.)
- With the lamp on, adjust the fine etalon spacing Zfine and take snapshot images until you observe a bright emission line well separated from other lines in the field of view. Further adjust Zfine until the ring appears just outside the two outer vertical chip gaps in the image display. This will place the ring far enough out in wavelength space, and also prevent loss of profile coherence across the gaps. (The latter issue can be minimized using software.) If a single, well-separated emission line is not present, a doublet will serve.
- Click "Create" to bring up the script creation window. Choose "Parallelize" and enter the starting [Xfine, Yfine] (x, y), step size in X and Yfine (dx, dy), and number of steps in X and Yfine (nx, ny). Assuming the image does not saturate, use 4x4 or 8x8 on-chip binning to minimize the readout time. Click "Run" to bring up the script execution window, and run the loop.
- When the script has finished, create azimuthally-averaged spectra with the program makerings.
- Finally, run paraltest on the resulting spectra. Search for the value of [Xfine, Yfine] that optimizes the etalon transmission profile.
- If necessary, repeat steps 3-5 with different starting and ending values of [Xfine, Yfine] until the optimal parallelism settings are found.
- Enter the chosen [Xfine, Yfine] pair into the IMACS software.
- makerings (wavelength at optical axis in Å) (-s
first_exposure_#) (-e last_exposure_#) OR
makerings (wavelength at optical axis in Å) (--list=list_with_image_names) - Calls: skyring2autobin
- Description: This script performs azimuthal averages on multiple files at once. It is important to get the wavelength at the optical axis broadly correct (e.g., 6600 Å at H-alpha, not 8200 Å) for the output wavelength scale to be accurate.
- paraltest
- Calls:
- Description:
The etalon will sometimes fail to operate when the "Balance/Operate" knob
Turn on CS-100 in balance mode with the meter knob on offset, and adjust the X, Y, Z coarse and fine controls to null the meters. Then switch the meters to quadrature error and adjust the quad balance knobs to null the meters. Iterate: switch the meters back to offset and re-null with coarse and fine; then switch to quad error and re-null that. Finally switch the CS-100 to operate mode and make sure it operates (doesn't go out of range). Write down the knob settings.
Note that sometimes you may have difficulty getting the CS-100 to "operate" at the lower or higher ranges of Zcoarse - that is the etalon will operate briefly, or operate only until X,Y, or Z is changed slightly, then go out of range. If this happens, it is possible to approach the desired Zcoarse by retuning the quadrature balances. For example, suppose you have difficulty getting it to operate at Zcoarse=-2. You can set the CS-100 to Zcoarse=-1, put it into operate mode, and switch the meters to quadrature error. You may find that one of the channels is significantly off. Re-null this channel using its quadrature balance knob while in operate mode - you will probably only have to move it a small amount. Then switch to balance, switch Zcoarse to -2, and switch to operate. The etalon should operate without going out of range.