A typical observing sequence consists of taking a series of exposures of an astronomical target, changing the wavelength setting of the FP system for each exposure to cover the relevant spectral range about a spectral feature of interest. Atmospheric transparency is monitored during each exposure by the guider. Wavelength zero-point calibration exposures of a standard spectral lamp are taken before and afer the sequence. Flat-field and full wavelength calibration sequences are run during daylight hours.
The spectral resolution of an etalon is set by the size of the spacing between its plates and by their reflectivity; this resolution is fixed for a given etalon (although the lowest resolution etalons have small enough gaps that they can be tuned by their piezos through approximately a factor of two in resolving power). The system has four spectral resolution modes: tunable filter (TF), low (LR), medium (MR), and high (HR). Tunable filter and low-resolution modes use a single etalon, with an interference filter to select the desired interference order (corresponding to wavelength). The medium- and high-resolution modes use two etalons in series, with the low-resolution etalon and its filter selecting the desired order of the medium- or high-resolution etalon, respectively.
Be aware of the standard spectroscopist's rule of thumb: with reasonable signal-to-noise, you can measure a velocity accurate to about 1/20 the FWHM of the line profile. For example, in measuring galaxy rotation curves, you are probably interested in a precision of about 5 to 10 km/sec; that suggests a resolution of about 1500, or MR mode. If, on the other hand, you are investigating line profile shapes, you obviously need higher resolution. Don't over-resolve! The cost in total observing time varies as the square of the resolution
Approximately 30 interference filters (of resolving power R=50) will be required to isolate the FP orders over the entire spectral range. Only 15 will be installed in the magazine at any one time, so the operating queue will be structured to limit the number of filters needed on a given night.
The wavelength of a single FP image is not constant over the field, but varies quadratically with distance from the optical axis. The field of view at approximately constant wavelength (the so-called "bull's-eye") is 1.3' x (10450/R)^(1/2), set by the focal length of the PFIS collimator. The total wavelength variation from the optical axis to the edge of the field of view is 0.9969 x the central wavelength (2.1 nm at 656.3 nm).