Fabry-Perot Interferometer

A Fabry-Perot Interferometer (FPI) was recently installed for the APOGEE instruments to help improve our wavelengths and radial velocity precision.

APOGEE Wavelength Calibration with the FPI

There are several processing steps to take advantage of the FPI to improve our wavelength calibration.

1. Redetermine precise FPI wavewlengths and wavelength solutions

  • Determine precise wavelength solutions using a week of arclamp exposures and full-frame FPI exposure at beginning of the night.

  • Fit wavelength solutions to many arclamp exposures (over a week) simultaneously

  • Determine median wavelength of each FPI line using wavelength solutions and Gaussian position over the 300 fibers

  • Refit wavelength solution of each fiber using the wavelengths of the FPI lines

  • This is created by the mkfpiwave which calls fpi.dailyfpiwave). This should be run every day with the daily calibration products. The code still needs to be tested.

2. Correct wavelength solution using 2-fiber FPI data

  • For each science exposure use 2 FPI fibers to measure pixel shift and correct the wavelength solution

  • Use 2 FPI fibers to fit linear surface and find pixel shift correction for each fiber

  • Correct the wavelength solution for each fiber by offseting the x-values of each pixel and refitting the wavelengths (maybe holding the higher-order terms fixed).

    • from original wavelength solution get wavelengths (x array and w array).

    • offset x array, x2=x+deltax

    • refit wavelength solution using x2 and w arrays. Maybe only refit constant and linear terms

  • The FPI lines allow us to make a precise shift measurement relative to the FPI lines at the beginning of the night.

  • Currently we use the sky lines to do this, so maybe add the FPI part to the same program, or write a similar one that is called if we have FPI data.

  • This is run by the ap1dwavecal script which calls fpi.fpi1dwavecal() which is called at the end of ap2dproc.pro. The code still needs to be tested.

3. Measure Dither shift

  • Measure dither shift between science frames like we have always done, by cross-correlating the two science spectra.

  • Fit linear or quadratic polynomial to the dither shifts vs. fiber.

  • These should be more accurate (on a fiber by fiber basis) than the FPI shifts which are interpolated/extrapolated to most of the fibers.

  • This should be very similar to what we are already doing.

  • This is done in apdithershift.pro. This uses existing code.

4. Dither combination/final wavelengths

  • Use the dither shifts from #3 to perform the dither combination.

  • Shift the wavelengths using the same dither shift as the spectra.

  • For each fiber, average the wavelengths of the multiple exposures.

  • The wavelength part of this is new code that can be written into the dither combination program.

  • This is done in apdithercomb.pro. This still needs to be tested.