The steps of calibrating the MIPS/SED mode are similar to that of the MIPS 70 µm imaging mode, including (a) cosmic ray identification and rejection, (b) corrections for latency effects and other non-linearities, (c) pixel response correction using stim flashes, (d) dark correction, (e) illumination correction (i.e., flat fielding), (f) wavelength calibration, (g) aperture correction, and (h) flux calibration. Steps (a)-(d) are the same as in the imaging mode, which have been discussed in the previous chapter. We describe briefly here steps (e)-(h), for which the SED mode differs in some way from the imaging mode. For general procedures on the MIPS Ge calibration, see Gordon et al. (2005, 2007) and for a further description of the MIPS SED calibration, see Lu et al. (2008).
The flux calibration is based on multiple observations of 3 bright stars (HD108903, HD124897, and HD029139) with 70 µm flux densities of 13-19 Jy. The point-source flux calibration accuracy is estimated to be 10% or better down to at least 0.5 Jy at the blue end of the spectrum and to ~2 Jy near the red end, based on SED observations of fainter standard stars and comparison with independent data from other instruments. With additional uncertainties from the illumination and aperture corrections included, the surface brightness calibration of extended sources is accurate to ~15%. The flux repeatability in time is better than 5%.
The SED wavelength calibration was obtained on-orbit by observing the planetary nebulae NGC 6543, NGC 6826, and IC 3568. In addition, the supernova remnant IC 443 and the H II region 30 Dor were observed. For all objects, except IC 443, the [O III] 88.3 µm emission line was detected and measured. The [O III] line was the only spectral feature identified in these spectra. For IC 443, the only identified spectral feature was [O I] 63.2 µm. Combining the measurements from these nebulae yields a spectral coverage of 52 µm to 97 µm for SED mode and a dispersion of 1.71 µm/px. There is a shift of less than a pixel in the position of emission lines along the usable length of the slit, and the centroid of the spectrum on the array shifts by about a pixel from the blue to the red end of the spectrum.
The dispersion solution was determined by observing a number of bright planetary nebulae and HII regions. The main spectral lines for this purpose are [NIII]57.330µm and [OIII]88.356µm, which have high equivalent widths in these objects. As an example, Figure 4.2 illustrates the observed line profiles of the planetary nebula NGC 6543.
Figure 4.2: An SED spectrum of the bright planetary nebula NGC 6543 from an observation in MIPS Campaign 20. The spectrum is extracted with a 5-column aperture. Both [NIII] 57µm and [OIII] 88µm lines are clearly detected.
The measured line peak positions along with the assumption of a uniform dispersion across the entire wavelength range yields to scale of 1.70 µm per pixel. The first row of the array (i.e., the blue end) corresponds to 52.55 µm. Each line fitting uses 4 independent data points, the wavelength calibration uncertainty is estimated to be on the order of 0.4 µm (0.5*1.70µm / sqrt(4)) if the uniform dispersion assumption holds.
The effective wavelength coverage over detector rows 1-27 of the SED mode is roughly from 52.7 to 98.6 µm. The remaining 5 detector rows at wavelength > 98.6 µm are contaminated by shorter wavelength, second-order light and therefore are not calibrated.