GPI has five wide band filters. Because the spectral resolutions increases toward longer wavelengths, the K band is split into two filters K1 and K2. These filters overlap for the 2.12-2.19 micron range. There is also a small overlap between Y and J for 1.12-1.14 microns.
|Filter||Min Wavelength||Max Wavelength||Spectral Resolution|
The above table gives the 50% cut-on and cut-off wavelengths. The 10% cut-on and cut-off wavelengths are typically an additional 0.01 microns on either side.
Filter transmission profiles are distributed as part of the GPI data pipeline, in files in the $GPI_DRP_DIR/pipeline/config/filters subdirectory. These are vendor-provided cryogenic tests made on witness samples with identical substrate and coating produced along side the GPI IFS filters.
The wavelength dispersion axis is in the Y direction in GPI raw 2D data files. Longer wavelengths are toward the bottom.
(Physically the optical plane of dispersion within the IFS is horizontal when the instrument is level on the ground, but the X and Y axes are set by detector fast and slow readout axes rather than being referenced to any given notion of ‘horizontal’ or ‘vertical’).
For a given lenslet, the pixel positions as a function of wavelength are given as follows:
where, from a measured wavelength solution calibration as produced by the data pipeline, are the starting positions of that lenslet at the reference wavelength , the spectral dispersion is microns per pixel, and the spectral tilt with respect to the Y axis is radians.
The spectral dispersion is very roughly 0.017 microns/pixel.
TODO update here with better information of dispersion vs filter band. It’s not exactly cosntant.