The primary flux calibrators for the SED mode are bright stars from a list compiled for the flux calibration of the MIPS 70 micron photometric mode. These are mostly supergiants of spectral type K, plus stars of other spectral types (A, G or M). The far-infrared stellar photosphere extrapolations from mid-infrared measurements for these types of stars are described by Rieke et al. (2006) and Engelbracht et al. (2006).
Because the spectral sensitivity of the SED mode decreases fairly steeply as wavelength increases, we have chosen our calibration stars in three flux ranges at 70 micron: (1) a few stars brighter than 10 Jy, (2) moderately bright stars of 3-5 Jy, and (3) ''faint'' stars of ~0.5 Jy. Our observation strategy is such that we reach S/N > 5-10 (except in a couple of early MIPS campaigns) for stars in (1) over the entire wavelength range, for stars in (2) shortward of ~85 micron, and for those in (3) shortward of ~70 micron. This tiered strategy allows for reasonable integration times at all three flux levels and provides constraints on the magnitude of any detector flux nonlinearity.
A spectral response function is derived for each observation of a calibration star. This was done on the difference image between the mosaicked on-target frames and the mosaicked off-target frames. An aperture of 5 detector columns was used for a spectrum extraction. The extracted spectrum was then divided into the model spectrum to give rise to the spectral response function in units of Jy per MIPS unit. To use the units of MJy sr-1 per MIPS unit, the result is further divided by the detector pixel size of 9.8'' x 9.8''. Note that the spectral response function derived in this way includes the spectral signature of the diffuse Galactic emission via the IC used in the data reduction. Thus, the spectral response function changes if a different IC is used in the data reduction. This IC signature is removed from the data prior to the BCD level since the same spectral response function is applied to all SED-mode targets, including calibration stars.
Table 4.4 is the mean spectral response function derived from 9 observations of the 3 bright stars: HD 124897 (spectral type K1.5III, 14.7 Jy at 70 micron; 4 observations), HD 108903 (M3.5III, 18.8 Jy; 4 observations), and HD 029139 (K5III, 13.1 Jy; 1 observation). The quoted uncertainty is the sample standard deviation of the mean, with each observation of an equal weight in averaging.
Table 4.4: Mean Spectral Response Function.
Detector Row
λ ( micron)
Responsea
σmeasuredb
1
52.556
13737
115
2
54.266
15210
161
3
55.976
17247
155
4
57.686
19221
133
5
59.395
20573
111
6
61.106
23149
120
7
62.816
26419
226
8
64.525
27963
178
9
66.236
31741
398
10
67.946
33667
301
11
69.656
36524
188
12
71.365
39683
225
13
73.076
42841
444
14
74.785
45351
289
15
76.495
48034
490
16
78.206
50939
378
17
79.916
55518
495
18
81.626
58381
396
19
83.335
63134
522
20
85.046
67530
665
21
86.756
70506
758
22
88.465
75588
653
23
90.175
77918
781
24
91.886
82470
996
25
93.596
87878
936
26
95.305
92194
1522
27
97.016
92780
1582
a Mean spectral response in units of MJy sr-1 per MIPS unit.
b Sample standard deviation of the mean spectral response.
