## IRSKY Cirrus Estimation

### Estimating the Cirrus Confusion Noise

The ISSA images are of
insufficient resolution to directly compute the
confusion due to cirrus structure for typical-sized beams. Instead, we
use an empirical model of the confusion noise based on the near
infrared brightness of the target area.
An estimate of the brightness of the cirrus emission is made at the
wavelength of interest. To do this, the mean values in each of the four
bands within a 30' square aperture are computed and an interpolation to
the wavelength of interest is performed. If the background cannot be
determined due to insufficient flux, or a pathological 4-band spectrum,
an attempt is made to extrapolate the background flux from 100 um to
the wavelength lambda by scaling the 100 um flux by F(lambda)/F(100),
where F(.) is the model dust spectrum taken from Desert, 1990. If the
flux found in the 100 um ISSA image is insignificant (below the noise
level), the program will not calculate the confusion.

Next the parameters necessary for estimating the factor E0 required by
the Gautier et al. formulation are prepared. To bracket the confusion,
we chose two specific cases from the Gautier work.
Those are alpha = -2.6 and alpha= -3.2, which bracket the exponent of the
spatial power spectrum falloff of cirrus structure. The reference
subtraction method assumed is the double aperture chopping method (also
called double position-switching). The beam separations (between source
and reference beams) are preselected to be two beam diameters, but the
user may reset this. Likewise the selection of the aperture width is
predetermined by the selection of an ISO instrument, or may be set by
the user.

Computation of the confusion noise is based on Gautier, T. N. et al.
1992, AJ, and Helou and Beichman 1990.

REFERENCES:
Desert, F-X., Boulanger, F., and Puget,
J., 1990, Interstellar Dust Models for
Extinction and Emission, A & A 237, 215.
Gautier, T. N. et al. 1992, A
Calculation of Confusion Noise Due
to Infrared Cirrus, AJ, 103 1313.
[provides details of the cirrus
confusion calculation]
Helou and Beichman 1990, Proc. 29,
Liege Intn. Astr. Coll., ESA SP 314.
[provides an overview of the cirrus
confusion calculation]
This paper is online in PostScript form on IPAC's publicftp machine:
ftp.ipac.caltech.edu
in
/pub/irsky/doc
Look for the files:
helou_beichman.ps
helou_beichman_figure1.ps
helou_beichman_figure2.ps
helou_beichman_figure3.ps
helou_beichman_figure4.ps