MIRI Wavelength Solutions for MRS, LRS

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A number of measurements were performed during the MIRI test campaigns to determine the spectral dispersion properties of the LRS, required for wavelength calibration of the spectra. The as-designed dispersion of the instrument is strongly nonlinear (see Fig. 3), particularly at the shortest wavelengths (4:56 μm). While the turnover of the dispersion below 4:5 μm was miti- gated by a blocking filter, the steepening of the slope introduces a degeneracy in the pixel-wavelength relation (Fig. 3) near the turnover point. The nominal 510 μm range is dispersed over approximately 135 pixels, giving a mean scaling of 37 nm=px. Beyond 14:5 μm, the slit spectrum no longer falls onto the detector (or within the subarray in the slitless case); however, the instrument throughput effectively limits its performance to ?1213 μm. 

Using the available filters in the MTS, the dispersion relation was constrained at a number of points along the LRS range, and compared with the as-designed dispersion. These measurements used spectra of point sources, positioned at the center of the slit. The differences between the observed and predicted locations of the reference positions were used to fit the dispersion profile to the observations. The final best-fit pixel-wavelength table to- gether with the measurement points is shown in Figure 5. The wavelength calibration relation was found to be accurate to within <10 nm within the 5–10 μm LRS nominal range. Outside this range the uncertainty rises to 10 to 15 nm longward of 10 μm, and to ∼35 nm for λ < 5 μm. The accuracy is cur- rently limited by the lack of unresolved line sources or other strong reference features over the full LRS wavelength range. 

Of critical importance for the wavelength calibration is the precise knowledge of the source position in the slit. During test- ing this knowledge was limited by the calibration of the point source scanning mechanism coordinate system with respect to the detector, to approximately 0.2 px. 

During the mission, the LRS wavelength calibration will benefit from the excellent pointing accuracy of JWST and the availability of astronomical targets with unresolved emission lines in the LRS wavelength range. Based on current data, for the expected pointing accuracy of 4.6 mas, we estimate an accuracy of the in-flight wavelength calibration of 12 nm within the nominal wavelength range. Further measurements over the course of the test campaigns showed the dispersion to have excellent stability over time, varying by no more than 1.7 nm over any given 7-day period. There was no evidence of any changes in dispersion profile with source position along the slit. However, when the source is moved away from the slit cen- ter in the across-slit direction the PSF quickly becomes trun- cated, affecting the accuracy of the wavelength calibration. Spectra taken of a point source placed at `1:25 μm from the slit center showed a deviation of 30 nm in dispersion at a given reference position (2:5 μm corresponds to the expected pointing accuracy of 4.6 mas). Slit spectroscopy with MIRI therefore re- quires careful positioning of the source in the center of the slit, and detailed knowledge of the telescope pointing. 

The test campaign at CEA Saclay in 20092010 included a finely stepped monochromator scan to measure the as-built spectral resolving power Rð1⁄4 λ=ΔλÞ near 7:5 μm. After cor- recting for the intrinsic linewidth of the monochromator, the LRS resolving power was found to be 95:3 ` 0:6 at 7:5 μm. Measurements over the range 78 μm are shown in Figure 5. These data were not corrected for the increasing PSF size with wavelength, which may affect the measured value of R. While the resolving power was not measured over the full wavelength range, the data available suggest a linear trend close to the as- designed specification. 


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