Nighttime calibrations

 As with most spectroscopic observations, the reduction of Fabry-Perot data requires a number of calibration observations. Since some of these calibrations consist of entire data cubes, and are therefore time-consuming, it is best to take them during the daytime. A few, however, are more easily obtained at night:

• velocity calibration
  In most instances, the observations can be velocity-calibrated (wavelength-calibrated) using the images of the monochromatic calibration lamp. Alternatively, if the night sky lines are sufficiently bright in the data frames, they may be used for wavelength calibration.

• flux calibration
  If the Palomar ET-50 etalon and narrowband blocking filters are being used, the Fabry-Perot data cannot be flux-calibrated easily. This is due to the fact the the Palomar filters are approximately 20Å wide, which is a factor of two greater than the free spectral range of the etalon (~11Å). This means that almost two orders of interference are passed simultaneously through the system.

  However, if a different etalon or blocking filter is used, you may want to flux-calibrate your observations. In this case, a planetary nebula or spectrophotometric standard star should be observed at a few etalon settings, in each filter, preferably each night and preferably at a couple of different air masses. Several tables of planetary nebulae fluxes can be found in the literature, including Lang 1974 (Astrophysical Formulae, Springer-Verlag, New York). Tables of spectrophotometric standards are available at the observatory.