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MIPS : Sensitivity |
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The performance of MIPS in actual use depends on many factors. The
performance of the arrays themselves is important, but telescope and
optical throughput also play an important role. An important
factor for observers to consider when predicting the sensitivity they will
achieve in a given measurement is the sky background present in the field
of interest. Sources of background flux vary with wavelength and
position on the sky, and must be evaluated with care when planning
observations with MIPS. The character of the far-infrared background as it
is understood today is described in the MIPS chapter of the SOM). However, MIPS detects the
background at sensitivities and spatial scales which have not previously
been achieved, so users should be conservative when estimating a
signal-to-noise ratio. There is a resource available for professional
astronomers called "The Infrared
Compendium" which includes sections on infrared backgrounds. For a full discussion of MIPS sensitivity, calibration, and basic observation modes, see the Spitzer Observer's Manual.
* Note on confusion limits The accuracy of photometry at 70 and 160 microns will often be confusion-limited. Because MIPS provides much smaller effective beams and higher sensitivity than any previous mission, predicting the confusion limit set by such sources is difficult. Pre-launch estimates of the 1-sigma confusion limits ranged from about 0.5 to 1.3 mJy at 70 microns, and from about 7 to 19 mJy at 160 microns (Xu et al., 2001, ApJ, 562, 179; Franceschini et al., 2002, ESO conference proceedings, astro-ph/0202463; and Dole et al. 2003, ApJ, 585, 617). Early mission data from MIPS produced the following source density criterion (SDC) limits for confusion due to extragalactic sources (Dole et al. 2004, ApJS, 154, 93): 56 microJy at 24 microns, 3.2 mJy at 70 microns, and 40 mJy at 160 microns. The 5-sigma photometric confusion limit at 70 microns is 1.5 mJy (Frayer et al. 2006, ApJ 647, L9). At 24 and 70 microns, the confusion mainly results from the high density of resolved sources; at 160 microns, the confusion arises from a population fainter than the sensitivity limit; see Dole et al. (2004, ApJS, 154, 93). There is a range of values expected, and moreover it can be a function of where exactly one looks in the sky. Note that confusion due to Galactic sources is a strong function of position. Other factors may influence the effective confusion limit for a particular observation. In some instances it may be reasonable to integrate somewhat below the level of the confusion, for example when the observer has a priori knowledge of a source position. On the other hand, the presence of a nearby bright source with its diffraction artifacts will increase the effective confusion limit. Moving targets offer the possibility of taking a second "shadow" observation, allowing the suppression of confusing sources by subtracting them away. While the above values should serve as a guide for determining if a particular observing program is feasible, interested observers should consult the corresponding research papers for additional details on confusion limit determination. Observers are warned that they need to specify AORs with enough cycles to provide adequate rejection of cosmic rays and other artifacts, even if a very short integration would nominally be adequate to reach the confusion limit. See the MIPS chapter of the Spitzer Observer's Manual and the backgrounds page in the The Infrared Compendium for more information.
Sky Backgrounds for Sensitivities:Computed sensitivities are presented for three sky background levels (low, medium, and high). The assumed background levels and approximate Galactic coordinates are below. The backgrounds are for high ecliptic latitude (low background), moderate ecliptic latitude (medium background), and low ecliptic latitude (high background). The sensitivities computed for these different backgrounds can be used as a guide to the observer for how the sensitivities in in the three MIPS bands vary with background. A simple interpolation between backgrounds should provide good results for intermediate background levels.
MIPS Sensitivity by ModeThe graphs mentioned in the SOM are here:
Conversion from Point Source Sensitivity to Extended Source Sensitivity
(Copied from the SOM) Useful information
For more information, see the MIPS chapter of the SOM
Go back to MIPS Instrument Performance page,
or
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/mips/sens.html
