5.2 Background Estimate

We obtain a quantitative background estimate through the Spot Target entry dialog. Within the target entry/modification window, clicking on ``Background'' (see Figure 4.1), brings up the ``Background Estimate'' window.

Enter 24.0 microns as the wavelength to do the estimate, leave the ``Calculate Range When Visible'' button selected for the moment, and click ``Do Estimate.'' We repeat this calculation at 70.0 and 160 microns for all of the targets in the target list. The calculated background levels are tabulated in Table 5.1.

As with IRAC, the SSC has characterized backgrounds as ``low,'' ``medium'' and ``high,'' depending primarily upon the distance from the ecliptic, and determined the sensitivities for these quantized background levels. See:

http://ssc.spitzer.caltech.edu/obs/bg.html.

Table 5.1: The maximum background levels in the MIPS passbands as returned by Spot.

Target	$24 \; \mu {\rm m}$	Bkgd	$70 \; \mu {\rm m}$	Bkgd	$160 \; \mu {\rm m}$	Bkgd
	[MJy/sr]	Class	[MJy/sr]	Class	[MJy/sr]	Class
Abell 2218	17.194	Low	4.906	Low	4.712	Low
Abell 1689	60.157	High	17.457	Low	7.571	Low
AC 114	37.018	Med	10.083	Low	4.810	Low
Abell 665	24.974	Med	7.135	Low	7.150	Low
cl0024+16	46.638	High	13.584	Low	9.779	Low

Especially with MIPS, it is a very good idea to verify the background estimate provided by Spot with a visual check of observations provided by IRAS. To do this, we first retrieve the IRAS Sky Survey Atlas (ISSA) image of the target field (in this case, we do it for Abell 2218). To do this, select ``ISSA image...'' from the ``Images'' pulldown menu. The ISSA image retrieval pop-up window, shown in Figure 5.1, will appear.

Figure 5.1: The ISSA image retrieval entry dialog.

We select all wavelengths (12, 25, 60 and 100 micron), choose a 1 degree FOV, put all plots in a single frame, no zoom, and hit ``OK.'' When Spot has retrieved the images, you will note that there are now large black arrows and a slider bar in the upper right; see Figure 5.2. This is how you can flip between images loaded into the same window. If you load fewer than four images into the same window, then the slider only includes those images as options.

Figure 5.2: Flipping between $12$, $25$, $60$, and $100 \; \mu {\rm m}$ images.

Examining the 25, 60 and 100 micron images, we see that the maximum surface brightness at any of these wavelengths on the sky covered by the Abell 2218 observations is $\simeq 4 \; {\rm MJy/sr}$, centered on the giant, foreground spiral galaxy to the northeast of the cluster center. Note that this is different than what Spot reported for the background. For more discussion on background issues, please see §9.5.1.