For many applications, it is convenient to characterize the system spectral response in terms of standard parameters. The weighted average wavelengths, , for the 3 MIPS imaging bands are computed as
where R is the intrinsic spectral response of the detector, in uits of e- per unit energy (available on the website), is the frequency, and is the wavelength. Basic quantities of the combined detector plus filter spectral response for each of the bands are tabulated in Table 2.2. At 24 and 70 µm, short-wavelength blocking has been verified to be sufficiently effective that signals from a Rayleigh-Jeans spectrum at wavelengths less than one-third the weighted average wavelength of the filter contribute less than 1% of the total.
The response levels in Table 2.2 are given as the product of the filter transmission and spectral responsivity of the arrays, and therefore have units of responsivity, here defined as signal per Jansky falling on the telescope within the projected area of the pixel.
Blocking of UV through near-IR wavelengths at 24 and 70 µm is such that for any source that does not grossly saturate the detectors, no measurable flux passes through the blocking filter. Blocking was tested on-orbit by observing the point-source modulation transfer function of the Spitzer+MIPS optics, and checking that only terms corresponding to wavelengths within the spectral bandpasses are present.
Tests of 160 µm signals from K stars were detected to be about a factor of five stronger than expected. Review of the instrument design revealed a weakness in the stray light control that results in a short-wavelength (1-1.6 µm) light leak in this band; see section 7.2.6. Consequences are not reflected in Table 2.2.
Table 2.2: MIPS spectral response summary.
Band-integrated response (e-/sec/mJy)
* See the discussion in section 7.2.6 regarding a short-wavelength light leak at 160 µm.
Spectral Response Tracings
Tracings of the response of the three MIPS imaging bands are included as Figure 2.3, Figure 2.4, and Figure 2.5. Digitized versions of these tracings are available in the Calibration and Analysis Files section of this site. The MIPS Responses, R, posted on the web are in units of e- per energy unit.
Figure 2.3: Pre-launch response of the 24 µm band including detector and filter spectral response.
Figure 2.4: Pre-launch response of the 70 µm band including detector and filter spectral response.
Figure 2.5: Pre-launch response of the 160 µm band including detector and filter spectral response. There is a short-wavelength leak at about 1.2 µm; see section 7.2.6.
Figure 2.6: Pre-launch SED spectral response including detector, filters, and grating efficiency.
Spectral Energy Distribution (SED) Mode
The spectral response of MIPS in the Spectral Energy Distribution (SED) mode was determined pre-flight using test filters and a far-IR blackbody source. A tracing of the SED mode spectral response is in Figure 2.6; basic performance is given in Table 2.1 and Table 2.2. The slit for the SED mode is 2 pixels wide and 24 pixels long; however, 8 of those 24 pixels fall on side B (the noisy side) of the array. Moreover, another 4 pixels have incomplete spectral coverage due to the bad readout on side A. The resulting slit length giving complete spectral coverage is thus 12 pixels. The grating disperses the light parallel to the direction of motion of the scan mirror; CSMM motions are used to switch between object and sky positions, while spacecraft pointing is used to move the object between positions in the slit. The spectrum is dispersed by a reflection grating across 32 pixels, and covers the wavelength range from 55 to 95 µm, giving a nominal dispersion per pixel-pair of 3.4 µm. The resultant spectral resolution, , is 15 at 52 µm, about 24 at 80 µm, and about 25 from 85 µm to the long wavelength cutoff.