Wavelength calibrations are built up primarily from observations of P Cyg, J900, PN M 1-42, NGC3242, NGC6720, NGC6781 for the high-resolution modules; and PN M 1-42, NGC7293, NGC6781, NGC7027, G333.9+00.6, and NGC6543 for the low-resolution modules. The lines used for each module are listed in Table 4.2.
Table 4.2 Lines used for Wavelength Calibration
SL
SH
LL
LH
[Mg V] 5.610
H2 S(5) 6.910
[Ar II] 6.985
[Na III] 7.318
H I (6-5) 7.460
[Ar III] 8.991
H2 S(3) 9.665
[S IV] 10.511
[Ne II] 12.814
[S IV] 10.510
[Ne II] 12.814
[Ar V] 13.102
[Ne V] 14.322
[Ne III] 15.555
[S III] 18.713
[Ne III] 15.555
H2 S(1) 17.035
[Fe II] 17.936
[S III] 18.713
[Fe III] 22.925
[Ne V] 24.322
[O IV] 25.890
[S III] 33.481
[Si II] 34.815
[Ne III] 36.014
[Fe II] 19.058
[Cl IV] 20.311
[Ar III] 21.830
[Fe III] 22.925
H I (10-13) 22.331
[Ne V] 24.318
[Fe II] 24.519
[O IV] 25.890
[Fe II] 25.988
H I (9-8) 27.803
H I (10-12) 29.840
[Fe III] 33.038
[S III] 33.481
[Si II] 34.815
[Fe II] 35.349
[Ne III] 36.014
Bright extragalactic line measurements have also proven essential to adjusting the calibration in spectral regions that do not include bright Galactic lines. The SH calibration has been further refined by using observations of Titan.
The IRS wavelength calibration is generally good to approximately 1/5 of a resolution element. Table 4.3 lists the root mean square (RMS) wavelength residuals for the lines used in the calibration. For spectral regions that are not well-covered by calibration lines, larger errors are possible. The high-resolution RMS values are averaged over all orders. To transform to per-pixel values, users should remember that the unresolved line width is always assumed to be 2 pixels.
The spacecraft velocity ranged from ±30 km/s. Observers who care about this level of precision in their wavelength calibration can add the values of the keywords VHELCORR or VLSRCORR to the spectroscopically determined velocities, in order to obtain final velocities in the Heliocentric or LSR frames, respectively.
Table 4.3: IRS Wavelength Calibration.
Module
RMS residuals of wavelength calibration lines (micron)
SL1
0.013
SL2
0.008
SL3
0.007
LL1
0.024
LL2
0.009
LL3
0.028
SH
0.001
LH
0.002
Some orders were difficult to cover with spectral lines from calibration sources (e.g., SH order 14) and the wavelength solutions for these have been updated by referencing nearby galaxies with accurate recessional velocities and strong lines.
It should also be emphasized that a pointing offset could cause a shift in the wavelength of a line. For example, a 0.5 pixel offset (due to pointing error or incorrect coordinates) in the position of a source within the slit could shift the wavelengths of that observation by a few tenths of a pixel.
We have measured the variation of spectral resolution with wavelength for the different modules. For the SH and LH modules, the resolution is constant at R~600 with wavelength. For the SL and LL modules, the resolution varies from the blue end of the spectrum to the red end as shown in
Figure 4.8 and Figure 4.9.
Figure 4.8 Measured LL resolution. The scatter is due to PSF undersampling and pointing errors.
