Gemini Planet Imager Observational Calibration XIII: Wavelength Calibration Improvements, Stability, and Nonlinearity

TL;DR

This paper improves the wavelength calibration of the Gemini Planet Imager's integral field spectrograph by incorporating quadratic terms, significantly enhancing accuracy and stability across multiple spectral bands.

Contribution

The authors introduce a quadratic correction to the wavelength solution, reducing calibration errors by a factor of ten and enabling more precise spectral measurements.

Findings

Quadratic terms improve wavelength solution accuracy by a factor of 10.

Achieved wavelength uncertainties of ~0.2 nm across all filters.

Validated non-linearity corrections using on-sky and simulated data.

Abstract

We present improvements to the wavelength calibration for the lenslet-based Integral Field Spectrograph (IFS), that serves as the science instrument for the Gemini Planet Imager (GPI). The GPI IFS features a 2.7" $\times$ 2.7" field of view and a 190 $\times$ 190 lenslet array (14.1 mas/lenslet) with spectral resolving power ranging from R $\sim$ 35 to 78. A unique wavelength solution is determined for each lenslet characterized by a two-dimensional position, an n-dimensional polynomial describing the spectral dispersion, and the rotation of the spectrum with respect to the detector axis. We investigate the non-linearity of the spectral dispersion across all Y, J, H, and K bands through both on-sky arc lamp images and simulated IFS images using a model of the optical path. Additionally, the 10-hole non-redundant masking mode on GPI provides an alternative measure of wavelength dispersion within a datacube by cross-correlating reference PSFs with science images. This approach can be used to confirm deviations from linear dispersion in the reduced datacubes. We find that the inclusion of a quadratic term provides a factor of 10 improvement in wavelength solution accuracy over the linear solution and is necessary to achieve uncertainties of a few hundredths of a pixel in J band to a few thousands of a pixel in the K bands. This corresponds to a wavelength uncertainty of $\sim$0.2 nm across all filters.