DIRBE all sky maps interpolated to a solar elongation angle of 90° were used to study the distribution of sky brightness at 12, 25, 60, 100, 140, and 240 µm. The maps used were converted from the COBE nearly-equal-area pixelization scheme to and Aitoff projection by Bill Reach, with a nominal pixel scale of 0.35°x0.35°. I converted the maps from Aitoff to Cartesian coordinates. The cartesian versions were patched up to fix missing data values using the mean value of the good pixels in an 8x8 pixel boxcar. As a check I verified that the histogram resulting from the re-projected re-projected 60µm map agrees with the histogram produced from the original COBE pixelization.

Count rates were obtained from sky brightness using the conversion factors given in the table below. These assume nominal responsivity values for the Ge:Ga arrays, and so are potentially uncertain by a factor of as much as two on the 70 µm array and perhaps even more on the 160µm array. The conversion factors use the stated band-integrated detector response given in Table 8.2 of the SIRTF Observers Manual in e-/s/mJy/pixel, the pixel scales from Table 8.1 of the SOM, and the electronic gain, as summarized below.

The following table summarizes the sky brightness at MIPS-like wavelengths and gives predicted count rates for 25, 60 and 140 µms. Three typical background brightnesses are defined: Low, Medium and High. At each wavelength 50% of the sky is dimmer than the Low Background, 80% is dimmer than the Medium Background, and 90% is dimmer than the High Background. The Canonical SSC Low, Medium and High Background values are given in parenthesis where appropriate. The SSC values are about a factor of 2 lower at Low and Medium backgrounds, and are comparable at the high background.

Sky Maps and Histograms

The following table specifies the stretch of the map images and gives links to the maps and images showing the histograms. The maps are displayed with a square-root stretch, between the stretch limits stated. The coordinates are ecliptic. On histograms at 25, 60 and 140 µm the SSC Canonical Low, Medium and High backgrounds are indicated by labels. On all histograms the statitical definitions of Low, Medium and High Backgrounds (the things tabulated in the table above) are also indicated by horizontal lines and labels. The saturation limits at 160 µm are given for 3 and 10 second DCEs. At 24 and 70 µms the saturation limits fall off the top end of the scale of the histograms and so are not plotted. At 24 and 60 µms a bump on the brightness distribution is due to the Zodi and labeled as such (a similar label could be placed on the 12 µm distribution).

Lambda (µm)	Map Stretch (MJy/sr)	Map	Histogram
12	9.92 - 53.0
25	16.6 - 104.
60	2.95 - 78.2
100	0.17 - 163.
140	0.0 - 311.
240	0.0 - 211.