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.

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:

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