Confusion by Infrared Cirrus

We base our discussion of confusion by infrared cirrus on the formalism of Gautier et al. (1992). To apply these results, we need to set three parameters: 1.) the power law index, , of the cirrus emission power; 2.) the power at 0.01 arcmin, (0.01/arcmin); and 3.) the observing geometry.

Analysis of the IRAS far infrared data shows that the cirrus power spectrum has a spectral index of = -2.6 to -3.2 (Gautier et al. 1992, T. N. Gautier unpublished work). The IRAS data do not extend to the high spatial frequencies that are important for future missions that will have both telescopes of larger aperture and imaging far infrared arrays. However, the power spectrum can be estimated at these high frequencies by comparison with deep CCD exposures in the visible, which can reach 1 arcsec. Cutri (private communication) finds that the power spectrum steepens slightly from the low spatial frequencies (i.e., à becomes more negative). In the following, we will assume .

The level of cirrus contamination is characterized by . As a medium level of cirrus contamination, we adopt the average for b, namely Jy/sr (Gautier et al. 1992). Other levels of cirrus emission are listed in Table II.

 
Table ii: Cirrus Confusion Noise at 100m

Because of the steep power spectrum of the cirrus emission, there is a premium in keeping the reference field as tightly coupled to the source as possible. We have therefore computed the cirrus confusion noise assuming an annular reference field that lies between 0.6 and 1.6 . For comparison with the galaxy confusion limit, we have interpolated to an effective beam diameter of 0.8 . Because of the low power at high spatial frequencies, the noise from cirrus confusion is only weakly dependent on the beam diameter but very strongly on the reference area size.

For the case of an 85 cm telescope, the resulting cirrus confusion noise limit at the average level of high latitude cirrus emission is 89Jy per beam, with these assumptions. Values for the rms confusion noise for the 85 cm telescope at other wavelengths and in other parts of the sky are entered in Table II. The confusion noise will scale as the square root of and as , i.e., as for .