Spitzer Documentation & Tools
MIPS Instrument Handbook

 

4.3.2        Flux Conversion Factors

The best conversion factors between instrumental units and MJy/sr to use for any given data set are included within the headers of the data themselves; grep on BUNIT and FLUXCONV. The factors are listed in Table 4.10. The MIPS-24 instrumental units are DN/s, but note that the instrumental MIPS-Ge units are dimensionless after the stimflash response calibration. Uncertainties in the conversion values are often limited by our astrophysical understanding of stars in the far infrared, and thus the uncertainties are likely to decrease with time. We include a history of the changes to the conversion factor of the various MIPS observation modes in Table 4.11.

 

This conversion factor at 24 microns was derived from 3 sec exposures. The difference in the conversion factor for scan mode and 10 and 30 sec exposure should not be more than 2-8% different.

Table 4.10: Conversion from instrumental units to MJy/sr.

Array Conversion factor Uncertaintya
24 0.0447 4%
70 702. 7%
70 fine scale 2808. 15%
SED 20572.7 10%
160 41.7 12%

a For optimal data reduction and flux density extraction of single sources in photometric mode, the uncertainties can be as small as 2%/5%/10% for 24/70/70fine.

 

Table 4.11: Changes in Conversion Factor with Pipeline.

Pipeline Versiona 24 m 70 m 70 m Fine 70 m SED 160 m
S10 0.04391 634. --- --- 42
S11 0.04391 634. 2536. 27093 42
S12 0.04391 634. 2536. 21835 42
S13 0.04391 702. 2808. 20572.7 44.7
S16 0.0447 702. 2808. 20572.7 44.7
S17 0.0447 702. 2808. 20572.7 41.7

a This history is meant to be suggestive of what changes occurred and when. The process of pipeline updates was complicated, with multiple versions and versions that were replaced before significant data was run through them. The data header will always contain the correct conversion factor used.

4.3.3        Magnitude Zero Points

Dr. C. Engelbracht has computed MIPS magnitude zero points using the Kurucz (1993) Vega model, which has then been scaled to the Rieke et al. (2008) 10.6m zero point of 35.03 0.3 Jy. He has further assumed that Vega is 0 magnitudes in all MIPS bands, and has computed fluxes at 23.68, 71.42, and 155.9 m. The resultant zero points are in Table 4.12.

Table 4.12: Magnitude zero points.

Wavelength (microns) Zero point (Jy) Uncertainty (Jy)
23.68 7.17 0.11
71.42 0.778 0.012
155.9 0.159 0.020