Spitzer Documentation & Tools
MIPS Instrument Handbook

 

4.2.3        Flux

Stars and Spectral Response Function

The primary flux calibrators for the SED mode are bright stars from a list compiled for the flux calibration of the MIPS 70 m 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 m: (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 m, and for those in (3) shortward of ~70 m. 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 m; 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 (m) 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.