ISOPHOT was designed to measure the absolute sky brightness. As a
consequence, all routine observations included emission from the
astronomical background in the 2.5-240m wavelength range. Most
ISOPHOT observations involved discrete astronomical sources for which
the background emission had to be subtracted. The presence of background
emission contribute to
The PHT Instrument Dedicated Team together with the PHT Consortium have undertaken programmes to investigate the astronomical backgrounds. In the following sections the most important background emission components, the zodiacal and diffuse galactic emission, are discussed. In addition, confusion due to discrete galaxies is mentioned. Other components like the scattered light from zodiacal dust and the cosmic far-infrared background (Lagache & Puget 2000, [30]) are expected to give a minor contribution in most cases. More information can be found in the ISO Handbook Volume I, [20].
Confusion due to diffuse galactic emission or `cirrus confusion' became
apparent from the far-infrared maps produced by IRAS. The diffuse galactic
emission component peaks around 170m and can be approximated by a
modified blackbody of 17K. Analysis of the cirrus structure
in IRAS maps (Gautier et al. 1992, [10]),
Helou&Beichman 1990, [17]) showed that the cirrus
confusion noise
can be expressed:
![]() |
(4.8) |
This result has been confirmed with ISOPHOT (Herbstmeier et al. 1998,
[18] and references therein, Lagache & Puget
2000, [30]) at ISOPHOT wavelengths longward of
100m.
Photometry of a target in the presence of discrete background galaxies can be hampered by faint background galaxies which may lie in the beam but are not detected individually, or by the presence of a identifiable bright source close to the target.
Faint source confusion can affect photometric studies of individual
sources and dominates as long as the cumulative galaxy source
counts
rises more steeply than
with
decreasing
. Cosmological source count models predict the
turnover at
0.1mJy. Below this flux density, the bright
ource confusion limits should be included. Assuming the in-orbit
point source flux limits (after 128s integration time and S/N = 10)
in the most sensitive PHT filters at long wavelengths, the prediced
level of galaxy confusion noise is given in
Table 4.10.
For on-target integration times longer than 128 s the noise level
decreases but then galaxy confusion noise starts to become the main source
of uncertainty: in the worst case 280 mJy (for the
P3 detector, filter,
aperture). The galaxy confusion is on most regions in the sky
below the cirrus confusion noise. For ISOPHOT, galaxy confusion becomes
only important if the mean background level due to diffuse galactic
emission is
5MJysr
Bright source confusion determines the maximum amount of extragalactic
sources which can be counted in deep surveys of the darkest regions
on the sky at high galactic latitudes. Assuming an Euclidian universe
the confusion limit in sources per solid angle can be derived from
Oliver 2001, [45]:
![]() |
(4.9) | ||
![]() |
(4.10) |
where m is the diameter of the telescope and
is the
detection level above the noise in multiples of
. For
=5 and
=170
m,
sr
= 62
point sources per square degree.
Analysis of the infrared emission observed by COBE
showed the presence
of a new background component, the cosmic far-infrared background (CFIRB)
which originates from far-infrared galaxies. Using deep ISOPHOT observations
at 170m in a region with low cirrus emission Lagache & Puget
2000, ([30]) isolated spatial fluctuations
in the CFIRB. The CFIRB fluctuations are best described by a white
noise power spectrum
corresponding
to rms fluctuations around 0.07MJysr
at 170
m.
For many types of ISOPHOT observations knowledge of the emission contribution from the zodiacal dust cloud is necessary. Data from COBE can be used but have to be adapted to the higher angular resolution observations of ISOPHOT. Absolute photometric observations in a number of ISOPHOT bands at several ecliptic longitudes/latitudes were obtained to establish the zodiacal emission distribution as seen by ISO.
COBE did not cover wavelengths between 5 and 12m, in the range where
the brightness of the zodiacal light rises very steeply. The measurements
obtained with PHT-S,
PHT-P and
ISOCAM suggest that the zodiacal emission
spectrum can be well approximated by blackbodies of 260-290K, depending
on the Solar elongation and the ecliptic latitude. A method for
subtracting the zodiacal emission in PHT-S spectra using the COBE data
is presented in Ábrahám et al. 1997, [1]).
ISOCAM CVF and ISOPHOT-S
measurements have demonstrated that the spectrum
is featureless between 5 and 16
m down to about 5% of the total
brightness (Ábrahám et al. 1998, [2],
Blommaert, Boulanger & Okumura 2001, [4]).
Ábrahám et al. 1997, [1]
have searched for arcminute structure in the
zodiacal emission at low, intermediate and high ecliptic latitudes. No
structures or fluctuations were found at a level higher than 0.2% of the
total brightness. At low ecliptic latitudes (
) the
zodiacal emission includes the dust bands and cometary tails
(Ábrahám et al. 1998, [2]).