Spitzer Documentation & Tools
MIPS Instrument Handbook

2.5.2        Estimating Background

During the Spitzer mission background noise estimation tools could be done using Spot, the Spitzer Observation Planning Tool. An estimation of the background and the contribution of the various components to the background (e.g., zodiacal light) could be obtained from the target selection window. While the noise levels predicted via Spot were based on previous infrared space observations at lower sensitivity and spatial resolution than MIPS obtains, they were the best products available at the time for making such estimates.


The Spot model did reasonably well (see, e.g., Dole et al., 2004, ApJS, 154, 93 and Meadows et al., 2004, ApJS, 154, 469).  The only exception to this is in regions where the infrared cirrus levels are low. In these low-cirrus regions it is preferable to use a cirrus estimate based on the atomic hydrogen column density in the direction of the target.  The hydrogen column density is highly correlated with the cirrus background in regions where the cirrus background is well measured.  This correlation can be extrapolated to provide an estimate of low-cirrus regions. 


However one estimates backgrounds, the aperture size used to estimate background is important. If you need help estimating backgrounds and can not find a relevant tool on our website, please contact the Helpdesk for further information.

2.5.3        Estimating Signal-To-Noise Ratio and Integration Time

In nearly all observing circumstances, MIPS will be background-limited.  That is, the dominant source of noise in the final measurement of the brightness of an object is noise contributed by emission from the background rather than the noise due to the photon statistics of the flux from the object itself.  In the background-limited case, the signal-to-noise (S/N) ratio of an observation increases linearly with the flux from the object, and it increases as the square root of the integration time:





Where the subscript ''0'' refers to values obtained from the sensitivity plots (discussed below and on the Spitzer Heritage website), and the subscript ''obj'' refers to the desired object.  F is the energy or photon flux from the object per second, I is the integration time in seconds, and B is the background surface brightness.  (See next section for sensitivity information.)


While this equation suggests that arbitrarily high signal-to-noise can be achieved by taking long enough integrations, in practice the signal-to-noise that can be attained is finite.  The details of the confusion noise sources discussed above set a lower limit on both the fluxes that can be detected, and a corresponding upper limit on the signal-to-noise ratio that can be obtained for sources of low to moderate brightness.  The S/N that can be achieved on bright sources is limited by the reproducibility of measurements taken with the MIPS arrays (see section 4.1.3).  The reproducibility within a set of measurements is roughly 1% in relative flux; the repeatability from one set of measurements to another (e.g., photometry repeated on a source) is 8-10% rms.  Observers are reminded that timing patterns within MIPS result in ''seconds'' which are 5% longer than a real second.