5.3 Exposure Time

Armed with the characterization of the background level (see § 5.2), we can proceed to estimate the required exposure time. As before, the first stop is the webpage for the MIPS instrument sensitivity. For the latest updates, see:

http://ssc.spitzer.caltech.edu/mips/

Note: sensitivity plots are available in the Spitzer Observer's Manual (SOM), and online, at the web address given above. The latest updates for CP-4 were posted on the webpages on November 1, 2006, and should be consulted for planning observations in response to the fourth call for proposals.

The required total integration time depends on the instrument sensitivity. For the 70 micron array, the low background sensitivity is approximately $1.2 \; {\rm mJy}$ $(1\sigma)$ in 500s. Similarly, for the 160 micron array, the low background sensitivity is approximately $3 \; {\rm mJy}$ $(1\sigma)$ in 500s.

Now, especially for MIPS, one needs to consider the confusion limits. For MIPS, the confusion limits are $\simeq 1 \; {\rm mJy}$ at 70 microns and $\simeq 15 \; {\rm mJy}$ at 160 microns. See:

http://ssc.spitzer.caltech.edu/mips/sens.html

Hence, adopting a goal of imaging to the confusion limits at 70 and 160 microns, we will require integrations of $\simeq 500 \; {\rm sec}$ at 70 microns, and $\simeq 20 \; {\rm sec}$ at 160 microns.

At 24 microns, and finite observation times, the confusion limit is effectively never reached in regions of low background. In 500 seconds, with low/medium background, the $1\sigma$ sensitivity is $\simeq 20 \; \mu{\rm Jy}$. See §5.3.

Note: the confusion limits shown on the low background sensitivity plots are lower limits to the actual position dependent on-sky confusion. The lower limits shown on the low background sensitivity charts are for regions of lowest expected confusion at high Galactic latitudes and on clean sky. The observer should consider the local confusion caused by background sources when planning observations. Confusion likely will be more important in higher background regions, and can limit the sensitivity that can be achieved. Local sources of confusion, such as cirrus and the stellar background, are highly variable and can be very localized. For all MIPS observations, the sky background should be considered carefully in the observation planning. See the MIPS chapter of the Spitzer Observer's Manual for more information.

Furthermore, redundancy is vital for MIPS, and hence imaging beyond the confusion limit will often be necessary to, e.g., fill in the full FOV at 160 microns (see SOM, v7.0, § 8.2.1.2.6).