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Wavelength accuracy

Wavelength Calibration Accuracy & Flexure

This page is focussed on the wavelength calibration accuracy and the IFS flexure compensation.

Wavelength Calibration Overview

Similar to other near-IR spectrographs, GPI derives its wavelength calibration based on the precisely known emission lines of an arc lamp, specifically using the Xenon and Argon lamps available in GCAL. The GPI data pipeline can use these data to provide wavelength calibrations with current accuracy of better than 0.5% for all bands. As discussed below, flexure within the IFS due to varying orientation with respect to gravity causes small shifts of the spectra on the detector, which must be properly compensated for in order to assemble accurately calibrated datacubes.

Creating Wavelength Solutions

Due to variations across the field of view, a unique wavelength solution is determined for each lenslet for each band (Y, J, H, K1 and K2). The wavelength solution is characterized by the starting positions (x0, y0) of a lenslet at the reference wavelength (λ0), the spectral dispersion (ω) in microns per pixel, and the spectral tilt (θ in radians) with respect to the Y axis. For a given lenslet, the pixel positions as a function of wavelength (x(λ),y(λ) are given as follows:

Wavelength solution

Wavelength Solution files are written with the extension “_wavecal.fits” in the form of a 3D datacube with 5 slices of size [nlens x nlens] giving information about each lenslet as described below. Note: Due to the field rotation, only a central rotated-square portion of these arrays will have valid values. The outer regions are flagged with the NaN value.

  • Slice 1: Y-positions (y0) of spectra (Y=spectral direction) at [λ0]
  • Slice 2: X-positions (x0) of spectra at [λ0]
  • Slice 3: λ0 [um]
  • Slice 4: Dispersion w [um/pixel]
  • Slice 5: Tilts of spectra, θ [radians]

Wavelength calibration files can be created from Xenon or Argon arc lamp data (obtained from GCAL) using the “2D Wavelength Solution” primitive. This primitive fits the image of each lenslet individually by simulating an arc lamp lenslet spectrum 2D image at the spectral resolution of the GPI IFS for a given band. Each spectral line is represented by a gaussian PSF at a location predicted by the x0, y0, w and θ values given in a prior wavelength calibration file. The model and detector images are compared using a nonlinear least squares optimization and the values for x0, y0, w and θ are updated for each lenslet.

Wavelength solution datacubes

The figure above shows the Xenon and Argon spectra for Y, J and H bands. Vertical lines show the individual emission lines and the plotted curve shows the spectral response at GPI’s spectral resolution, at the the 37 wavelength channels in each band (in interpolated output cubes which are oversampled relative to Nyquist). The xenon spectrum provides more cleanly separated peaks and is easier to fit. The argon spectrum has many more lines which are strongly blended at GPI’s low resolution; however the much greater brightness of the GCAL Ar lamp (3-10x brighter than Xe) means it is preferred for practical reasons in many cases. The GPI instrument team has worked to ensure that good wavelength calibrations can be derived from either lamp; comparisons of results from the Xe and Ar and further optimizations are ongoing.

Note: The “Quick Wavelength Solution” primitive can also be used to fit a subset of the lenslets and update the x0 and y0 positions only. This is significantly faster than the “2D Wavelength Solution” primitive and is useful to correct for offsets due to flexure (discussed below).

IFS Flexure

Gravitationally induced flexure within the IFS causes shifts of the lenslet spectra ranging from a small fraction of a pixel to several pixels in magnitude. The figure below shows flexure in the x and y direction as a function of elevation for several epochs of data. During a single epoch, the flexure behavior is relatively repeatable and can be accounted for using the “Update Spot Shifts for Flexure” primitive. Between GPI observing runs when GPI is moved to non-standard orientations, larger shifts of a few pixels can occur.

[MISSING FIGURE]

It is recommended that a snapshot arc lamp image be taken at the elevation of any interesting science targets for the best flexure correction. We recommend two 60 second Argon lamp images in H band. These data can then be reduced using the “Quick Wavelength Solution” primitive in the GPI pipeline. Such data are available for some but not all observations in the GPI early release data.

If a given observation does not have a contemporary arc lamp calibration image, the “Update Spot Shifts for Flexure” primitive will consult a look-up table that will return an estimation of the shifts due to flexure given the value of the ELEVATIO keyword in the image header, relative to whatever the closest-in-time available wavelength calibration in. This primitive also allows for user defined x and y pixel shifts; see below for how to use these.

Symptoms of Imperfect Flexure Compensation

Having an incorrect estimation of the shifts due to flexure will affect both the accuracy of the wavelength solution and the relative brightness of the final science data products. Each wavelength bin is extracted using a three pixel box in the x direction. If the x flexure shift values are incorrect, flux from the source will be lost. By eye, this will result in a strong moire (or checkerboard) pattern. An example of this is in the figure below.

[MISSING FIGURE]

The spectra are dispersed in the y-direction on the detector. Consequently, incorrectly estimating the y-direction flexure shifts will cause the wavelength solution to be off by a value in microns equal to the shift in pixels times the spectral dispersion. If the flux dramatically drops off near one end of the channels in the final datacube, this could indicate that the flexure correction is incorrect, however, this depends strongly on the spectrum of your source.

Adjusting Wavelength Solutions Shifts Manually

The quality of a given wavelength solution for a given science observation can be checked using gpitv’s Plot Wavelength Solution function to overplot the wavelength solution on the 2D detector image. Misalignments of the spectra with their wavelength solution by more than a few tenths of a pixel can be seen by eye. The Plot Wavelength Solution tool allows interactive adjustment of the X and Y shifts. This can be a useful manual approach to deriving an approximately-correct flexure-compensated wavelength solution for a set of science data. Once you have found a set of X and Y shifts that align the spectral traces with the spectra themselves, these can be entered as parameters to the “Update Spot Shifts for Flexure” primitive.

The GPI team is working on updated algorithms that will automatically measure the shifts for each individual science frame. These will be available in a future update of the GPI data pipeline.