Contents of: VI/111/./abstract/KMATTILA_DGBE_CP1.abs

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SCIENTIFIC ABSTRACT
We want to investigate the IR properties of the diffuse
infrared emission of the galaxy using PHT and CAM. Different
programs are proposed depending on the accessible sky with ISO.
 
If the Galactic Centre is visible, PHT-S
spectra are measured at selected positions along and near
the galactic plane between l=-60 and l=60 degrees. An important
objective for this program is to measure intensities of the
UIR emission features in different parts of the galaxy as the
sightlines are sufficiently bright for good PHT-S spectra.
The positions are representative for the
galactic radiation away from bright discrete sources.
The PHT-S spectra will be complemented by PHT-P measurements.
 
As an additional program, LINKED observations with ISOCAM are proposed.
These observations are described in Part 2 of this proposal.
The CAM data are coordinated with the CP proposal
"Selected Area Galactic Survey with ISOCAM" (Price et al.) and are
shared between the consortia of this and Price's proposal.
 
In case Orion is visible we study the (weaker)
emission of the galactic anti-centre region. We will perform raster
scans in the C_200 and C_120 micron bands along selected pieces of 
the accessible galactic equator from l=67 to l=331. 
In addition, raster scans perpendicular to the
galactic plane will be performed centered at the same fields and reaching
from b = -0.8 to b = 2.3 degrees. These data will be utilized to investigate 
the distribution of cold interstellar dust and the ISRF throughout the outer
Galaxy and to search for small scale structures of the very cold dust 
emission. The spatial changes in dust grain populations is one central open
problem. With its 3' wide beam (1.5'x 1.5' pixels) ISOPHOT-C200 is ideally 
suited for the kind of studies suggested here. The smaller beam as compared 
to COBE provides the advantage to avoid discrete sources and to see the truly 
diffuse component of the background radiation.
 
OBSERVATION SUMMARY
 
In case of an autumn launch (hole in Orion) we want to take
PHT-S spectra with AOT P40 at regularly spaced positions along
the galactic plane at b=0.0, b=+/-1 (=on-positions), and at b=+/-5 degrees.
(=off-positions). At each ON-position a raster of 2x2 positions with 64sec 
integration time for each raster position will be observed. 
The sensitivity of PHT-S is sufficient to achieve a signal
(zodiacal background subtracted) r.m.s. of 0.8 and 1.5 MJy/sr
per pixel for SS and SL, respectivel, for the co-added signal from the four
raster positions. The observations at
b=+/-5 degrees are off-positions; therefore an 1x2 point raster map
will be performed at each OFF-position with 64 sec integration time per raster 
point. The combined PHT-S exposure time at these two off-positions will 
therefore be equal to the PHT-S exposure time spent at each one of the 
on-positions. AOT P03 measurements of 32 seconds in
P_3.6 and P_7.3 will be performed to calibrate the SS
and SL spectra. For b=0, P03 observations in the filterbands
P_25, P_60 and P100 to determine the emission in a 99" aperture.
These data will be compared and cross calibrated
with both the LWS (cf ISO proposal by Caux et al.) and
IRAS data. The P_3.6 and P_7.3 measurements will be
obtained in a 50" aperture to obtain an S/N of at least 20
and to ensure comparison with the longer wavelength data.
 
In case of a spring launch
the camera C200 and AOT P22 configured in two
filters - C_200 and C_120 - are used to
scan selected pieces of 4.5 deg length at or near the galactic equator 
in the raster scan mode. In addition, we perform also
a raster scan perpendicular to the galactic plane 
from -0.8 deg to +2.3 deg latitude at the central point of each 
galactic equator scan. The galactic coordinates of the
central positions of these cross scans are:
 
l /  67.0   90.0  118.0  165.0  240.0  275.0  301.0 degrees.
  b / 0.3    0.5    0.5    0.0   -0.5   -0.5   -0.2 degrees
 
These positions have been selected in such a way that they avoid the bright 
hot dust areas around HII regions. The positions are not exactly
at the galactic equator but follow the ridge of maximum FIR
emission as determined from the IRAS 100 micron data. This optimizes 
the detection possibilities of very cold dust.
Each set of measurements consists of ten concatenated linear raster
scans - five in each of the two filters - of 32 positions with 
180" spacing. The integration time is 10 sec per raster position.
For a typical brightness in the outer galactic plane of 50 MJy/sr
we reach a S/N of better than 500, i.e. features as weak as 0.5
can be measured with S/N = 5  in the single-pixel mode. 
 
 

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