The two components which contribute most to the celestial background in
the infrared are the zodiacal light and the diffuse galactic emission.
Zodiacal light dominates at the shorter ISO wavelengths
m with a peak around 25 
m. The diffuse galactic
emission is more important at the longer wavelengths 
m
with a peak aorund 200 m.
The amount of zodiacal emission depends on wavelength and the ecliptic
coordinates of the object. The closer to the ecliptic plane the more
background emission is to be expected. In addition to the dependence on
celestial coordinates, zodiacal emission depends also on the
satellite orientation: The smaller the solar elongation angle, which ranges
from 120 to 60 degrees, the more zodiacal emission is received.
The diffuse galactic emission has a dependence on galactic coordinates:
Toward the galactic centre the background radiation is increasing.
While the galactic emission has a significant fraction of intensity in lines
and broad spectral features, the zodiacal light is expected to be dominantly
continuum radiation.
Both emission components can affect the detection of faint sources and it may often be desirable to determine the background flux via a reference measurement at a position in the neighbourhood of the source position. The most common methods to obtain reference measurements are beam switching and chopping, which are offered in several AOTs. For some observations the CAM field of view may be large enough to image both the source and its background in one frame, thus avoiding beam switching altogether. If a suitable reference position is relatively far from the source position (but still within 3 degrees) the proposer can also prepare two separate observations which are then concatenated (see Sect. 13.4).
A third type of background emission can occur in the vicinity of strong infrared objects. If the target object is close to, but not confused with a stronger infrared source, then the background radiation may be dominated by emission in the tail of the point spread function of the stronger object. In this case chopping and beam switching techniques are generally not working well. For these situations, recipes valid for all cases cannot be given. An observer should consider e.g.\ observations in a scan (Sect. 13.3) to obtain sufficient information of the stronger source to remove its contribution from the data at the analysis stage.
For imaging and spectroscopic observations one of the parameters an observer has to provide is the peak flux density. This includes both the emission from the target and the background. For small apertures and strong sources the background contribution is usually negligible. For weaker objects observed with larger apertures (at longer wavelengths) it is necessary to take into account the background emission as discussed in the paragraph below.
For PHT AOTs the background emission is an explicit parameter required
for the observations. At longer wavelengths IRAS maps are the best
source to obtain estimates of the galactic emission. For estimates at
wavelengths outside the IRAS wavelength range, COBE results should be
used. Table 6 gives some very rough estimates based on COBE data.
The values are relative numbers, normalized to the 100 m flux, and
should be used for expolation from the IRAS fluxes.
It should be noted that the values in Table 6 apply to
the diffuse interstellar medium only.
In molecular cloud complexes the surface brightness at 200 m may be
factors of 5 to 10 higher compared to the diffuse clouds.
|
Wavelength [ m] | Surface Brightness[MJy/sr] |
| 3.5 | 0.0016 |
| 4.9 | 0.0015 |
| 12 | 0.043 |
| 25 | 0.058 |
| 60 | 0.42 |
| 100 | 1 |
| 140 | 1.99 |
| 240 | 1.40 |
Table 6:
Typical infrared fluxes of interstellar clouds detected with COBE. The
results are averages from 10 diffuse clouds and are normalized to the
100 m flux.
A proposer should
be aware of all offset corrections made to the data products used.
E.g. IRAS maps are often provided with zodiacal emission subtracted.
As zodiacal emission may be the main contributor to the background, it
is necessary to take it into account for the total background level
estimate. This is not exactly possible as the satellite orientation is
not known prior to the actual observation. Therefore a conservative
estimate should be made to avoid saturation. Table 7
contains for various wavelengths estimates of the maximum zodiacal
light contribution as a function of the ecliptic latitude. The ecliptic
latitude 
can be obtained from Right Ascension 
and
Declination 
by equation:
 | 2.5 | 3.5 | 7.5 | 12 | 25 | 60 | 100 | 200 |
| m | m | m | m | m | m | m | m | |
 | ||||||||
 | 0.2 | 0.2 | 25 | 75 | 140 | 45 | 25 | 15 |
 | 55 | 100 | 30 | 15 | ||||
 | 40 | 70 | 20 | 10 | ||||
 | 30 | 50 | 12 | 8 | ||||
 | 20 | 35 | 10 | 6 | ||||
 | 15 | 30 | 8 | 5 | ||||
 | 0.2 | 0.15 | 4.5 | 14 | 25 | 7 | 1 | 0.26 |
with the
smallest solar aspect angle possible for ISO
(60 except for the ecliptic
pole) as a
function of the ecliptic latitude 
except for the
poles for which scans with about 90 solar aspect angle
were used). The 200