Wavelength calibration involves solving the equation relating wavelength and plate spacing for the slope B and zero-point A:

λ = A + B Zfine

Once A and B are known, this equation yields the value of fine plate spacing (Z_fine) necessary to observe the wavelength of interest.

A varies with time (possibly due to temperature changes) and with the parallelism setting. It must be calibrated at the beginning of the run and checked frequently using single, full-field images of rings. B (or dλ/dZ) depends on the coarse plate spacing Z_coarse and the interference order. However, it is stable and has been measured for 3 values of Z_coarse in each filter (see the table below).

The instrument scientist provides values of A and B to the astronomer at the beginning of a run. It is the astronomer's responsibility to track changes in A (due to, e.g., temperature drift).

The table below has links to MMTF arc lamp spectra. These were taken by scanning the etalon spacing through somewhat more than 1 free spectral range (FSR): a "sausage cube." The spectra are aquired using a small CCD subraster near the optical axis. This table also lists suggested lamps for doing wavelength calibration and parallelism tests, including whether it is internal or external ("Int/Ext") and good exposure times. The final column gives details about the etalon properties derived from these spectra, including values for B and FWHM and approximate Z_fine ranges used to achieve a particular order.

¹For the 6800 Å filter, we recommend the NeHg lamp for parallelism and for finding the λ - Z zero-point. For full wavelength calibration (including the λ - Z slope), near-simultaneous sausage cubes of Ne, NeHg, and He may be necessary.

²Calibration is ongoing; further filter data will be published as it is completed.

During the afternoon preceding your run, the instrument scientist will acquire sausage cubes and reference ring images to determine the wavelength zero point (A) for each filter you plan to use. He/she will also provide the value of B (see also the table above). It is the observer's responsbility to track changes in A with time. We recommend checking A at least every 30 minutes at the beginning of the night, when the dome temperature changes rapidly.

Changes in A, and hence the values of fine plate spacing necessary to observe the wavelength of interest, are tracked using the FPCALC online tool. To begin, enter the relevant information for your target into FPCALC: the value of B provided by the instrument scientist, the wavelength of interest, and the object position with respect to the optical axis (which is approximately at the field center). Next, insert the reference A value into the Aref box, and click the button next to it. This will assign A = Aref and compute the value of Z_fine when the reference data was taken.

To update A and Z_fine, use the following procedure:

1. Take a binned exposure at the same Z_fine setting and binning used for the reference image, using the same arc lamp. Let's call the new image ccd1159 and the reference image ccd1156, and assume that we are using the 6400 Å filter.
2. In a terminal window, type the following:
   

   % ring ccd1159 6400
   % ringdrift 1156 1159
   

   The resulting spectrum is shown at right.
3. Enter the change in A specified by ringdrift into the ΔA text window in FPCALC. Press the "set A = Aref + dA and compute new Z" button.
4. Enter the new value of Z_fine for the wavelength of interest into the IMACS observing computer.
5. Observe!

During the afternoon preceding an MMTF run, the instrument scientist provides A and B to the observer for each filter. B comes from the calibration table above, and A from a sausage cube (see the figure at right). A reference ring image must also be taken before each sausage cube. We describe these procedures below.

Procedure to aquire reference ring image.

1. IMPORTANT: This image should be taken immediately before or after a sausage cube. The etalon should also be parallelized before any wavelength calibration proceeds.
2. Turn on the lamp suggested for the filter of interest (see table above).
3. Vary the etalon Z_fine and take Snap exposures until a relatively strong emission-line ring appears in the field-of-view. The ring should be isolated from other lines, and its radius should be approximately halfway between the center and edge of the aperture.
4. Take a binned image of the emission-line ring, using the exposure time suggested in the table above. To minimize readout time, use the coarsest binning possible while maintaining an unsaturated image (preferably 8 x 8, but finer binning if necessary).
5. Give the number of this exposure, as well as the Z_fine used, to the observer.
6. In a terminal window, check the spectrum by typing the following (fill in the proper exposure no. and filter central wavelength):
   

   % ring ccd0001 6600

Procedure to aquire sausage cube.

1. Running a well-sampled sausage cube typically takes ~20-30 minutes; this is dominated by CCD readout time.
2. Turn on the arc lamp recommended in the table above for the filter of interest.
3. Before taking the sausage cube, take a reference ring image as described above.
4. Turn on CCD subrasters by changing the ExpMode from Full to Subraster. When the subraster menu pops up, load the file defining the MMTF sausage cube subrasters: mmtf_sausage.sub in the default path, /Users/imacs/subrasters/. Click the Load button to load this subraster. REMEMBER: Set the SaveMode to Minimal to minimize readout time. Click Apply. Exit this window by clicking Done.
5. Ensure that CCD binning is set to 1x1, and that the exposure time is appropriate for the lamp you are using (the same exposure time as for the reference ring is fine).
6. Under the Script menu, click Create. In the pop-up window, select the Scan(seq) Mode. Set Z0 equal to the the lowest Z_fine in the reference sausage cube spectrum given in the table above. Keep dZ at the default value of 20, and set nZ equal to 50.
7. Save the etalon script.
8. To begin the sausage cube, click Exec under the Script menu.
9. Once the sausage cube has finished, type the following in a terminal window:
   

   % fitsausage 1 51 2

   where the first two numbers are the beginning and ending frame values in the sausage cube (leading zeros ignored) and the last number is the IMACS CCD chip. Always use chip 2, which is closest to the optical axis. Compare the resulting spectrum to the one given in the table, and make sure it is identical (there may be a small horizontal shift).
10. Copy the IRAF commands listed by the fitsausage output into an IRAF terminal window. Using the splot fitting routines (e.g., 'k' and 'l' for a single-line fit, or 'd' and 'd' for a multi-line fit), fit a Lorentzian profile to one of the emission lines. Preferably, choose a strong line that is closest to the observer's wavelength of interest and free of blends.
11. Insert the wavelength and measured Z_fine value of this emission line into FPCALC. Enter the subraster radius output by fitsausage. Enter the appropriate value of B from the table above (for the given filter, Z_coarse value, and interference order corresponding to the emission line that was measured). Click the compute A button, and note the resulting value of A. Give this value to the observer, and copy it to the Aref box in FPCALC.