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As part of the PHT calibration programme, we have observed an area of
10.5'x7.5' around the faint star HR 6132 at 90 um and 175 um. The star is
located in the outskirt of the Draco cloud on top of a faint cirrus filament
as revealed by IRAS. With the better resolution of ISOPHOT the central part
of the filament was resolved into three point-like sources: one is HR 6132
while the other two seem to be cirrus knots. In case they really belong to
the cirrus and such so far unidentified structures are frequently detected
even in regions of faint cirrus, the detection of faint sources with ISOPHOT
would be hampered.
Since both the 90 um and 175 um maps are very deep (t(int)=288 s and 512 s per
sky position, respectively), the field has already been used for a preliminary
analysis of the cirrus confusion noise (Herbstmeier et al. 1997, in
preparation). The Draco region is an ideal object because it is a
prototype-like cirrus cloud complex with molecular cores and atomic filaments.
An extension of the available database is however necessary as observations of
the remaining parts of the filament will
(i) improve significantly our statistics on confusion noise;
(ii) answer whether the whole filament could be resolved in a chain of point
sources;
(iii) reveal the physical nature of the newly detected point-like sources
by observations at various wavelengths.
From our experience with the previous maps we can reliably predict all the
observational parameters (expected brightness, variation of the background,
signal-to-noise).